Showing posts with label thermocouple. Show all posts
Showing posts with label thermocouple. Show all posts

Monday, November 25, 2019

Precision Thermocouples and RTDs for the Most Demanding Applications


Duro-Sense Corporation provides the highest quality thermocouples and RTDs to the aerospace, aviation, offshore drilling, medical, R&D, power generation, alternative energy, process control,  primary metals, high-tech and OEM industries.

Contact Duro-Sense by calling 310-533-6877 or visit their web site at https://duro-sense.com.

Wednesday, August 21, 2019

What Kind of Thermocouple Should I Use?

Temperature measurementWhat kind of thermocouple should I use? Depends on several variables related to the system to be tracked, such as its media / process environment compatibility, the frequency and precision of the necessary measurements, and the regulatory climate in your sector.

Temperature measurement in many industries, from refining to pharmaceuticals to aerospace, is a key parameter in manufacturing and processing operations. Precise temperature monitoring helps to ensure safe, efficient and optimal results.

A thermocouple is invariably the measuring tool of choice for applications above 1400° F, but the selection of the ideal industrial thermocouple also requires knowledge of the process where the device will be used.

INTERFACE WITH PROCESS

First, consider whether the thermocouple is itself in direct contact with the process media or whether it is incorporated into a thermocouple assembly that includes a thermowell. Thermowells are metal, glass, or ceramic tubes that protect the thermocouple against corrosive, fast-flowing or highly hot process media. About 75% of heavy industry thermocouples use thermowell assemblies; these industries include refining, petrochemical, the pulp and paper industry, and power generation.

JUNCTION

The thermocouple junction design depends on the applications requirements for response speed and the likelihood of electrical noise being conducted through the process. A thermocouple has three variations of sensing tip (or junction): Exposed junction, where the exposed wire tips and welded bead have no covering or protection; Grounded junction, where the welded bead is in physical contact with the thermocouple's sheath; Ungrounded junction, where the tip is inside the thermocouple sheath, but is electrical (and somewhat thermally) insulated from the sheath (no sheath contact).

MATERIAL SELECTION

Material selection is the second criteria to choose. A vast majority of industrial thermocouples are made from stainless steel, but specialized alloys such as Inconel 600, Hastelloy X, Monel,  and other unique metals are required in certain applications.

MOUNTING

Next, you have to consider the mounting arrangement. You need to determine whether a more traditional industrial thermocouple/well/head design is required, or if some sort of flexible or remote thermocouple sensor is required for use in a hard to access area.

TYPE

Last, you have to decide the "type" of industrial thermocouple you need. In the case of thermocouples, "type" refers to the composition of metal wires in the instrument whose physical properties respond to changes in temperature. Different metal compositions have different temperature ranges and other properties that make them suitable for use in special applications, or inappropriate for use.

  • Type J thermocouples use iron for the positive leg and copper-nickel (constantin) alloys for the negative leg. They may be used unprotected where there is an oxygen-deficient atmosphere, but a thermowell is recommended for cleanliness and generally longer life. Because the iron (positive leg) wire oxidizes rapidly at temperatures over 1000 deg.F, manufacturers recommend using larger gauge wires to extend the life of the thermocouple when temperatures approach the maximum operating temperature.
  • Type K thermocouples use chromium-nickel alloys for the positive leg and copper alloys for the negative leg. They are reliable and relatively accurate over a wide temperature range. It is a good practice to protect Type K thermocouples with a suitable ceramic tube, especially in reducing atmospheres. In oxidizing atmospheres, such as electric arc furnaces, tube protection may not be necessary as long as other conditions are suitable; however, manufacturers still recommend protection for cleanliness and prevention of mechanical damage. Type K thermocouples generally outlast Type J, because the iron wire in a Type J thermocouple oxidizes rapidly at higher temperatures.
  • Type N thermocouples use nickel alloys for both the positive and negative legs to achieve operation at higher temperatures, especially where sulfur compounds are present. They provide better resistance to oxidation, leading to longer service life overall.
  • Type T thermocouples use copper for the positive leg and copper-nickel alloys for the negative leg. They can be used in either oxidizing or reducing atmospheres, but, again, manufacturers recommend the use of thermowells. These are good stable thermocouples for lower temperatures.
  • Types S, R, and B thermocouples use noble metals for the leg wires and are able to perform at higher temperatures than the common Types J and K. They are, however, easily contaminated, and reducing atmospheres are particularly detrimental to their accuracy. Manufacturers of such thermocouples recommend gas-tight ceramic tubes, secondary porcelain protective tubes, and a silicon carbide or metal outer protective tube depending on service locations.
For more information about industrial thermocouples, contact Duro-Sense Corporation. Call them at 310-533-6877 or visit their web site at https://duro-sense.com.

Friday, July 26, 2019

Duro-Sense Corporation: Celebrating Our 40th Year in Business

Duro-Sense designs and manufactures temperature sensors and assemblies used in power generation, plastics production, semiconductor processing, environmental control, packaging, aerospace, medical equipment, foodservice equipment and a myriad of other industries. Duro-Sense partners with customers to optimize their temperature sensing processes, thereby assisting in improving their customers efficiency and profitability.

Since 1979, Duro-Sense has grown exponentially in product capability and market experience. The company today continues to succeed by operating under a simple core value – providing customers with superior products, meticulously engineered for their individual requirements.

Thursday, July 11, 2019

Duro-Sense: A Long History of Solving Tough Temperature Sensing Problems

Duro-Sense Temperature SensorsWith engineering, design, and support resources available at their headquarters in Southern California, Duro-Sense delivers value-added temperature sensing solutions that improve operations and increase profitability for customers in the power generation, alternative energy, plastics, medical, gas & oil, chemical, refining, mining, agricultural, food service, pharmaceutical, and aerospace industries.

Duro-Sense offers their customers products and services designed to provide outstanding value and cost savings throughout the customer's equipment life span. By integrating the highest quality standards, state-of-the-art machinery, and decades of application experience, Duro-Sense assists customers through:
  • Improved product quality
  • Optimize asset uptime and performance 
  • Lower total cost of operation and maintenance
  • Increase equipment reliability
  • Improve plant and personnel safety

ENGINEERING AND TECHNICAL SERVICES

Technical Analysis — Duro-Sense can identify temperature sensor operational issues that may be constraining output or elevating operating costs, and then recommend laser-focused solutions.

Reliability and Efficiency Services — Duro-Sense lends their decades of hands-on experience to offer practical temperature sensing solutions that improve the performance, efficiency, and reliability of your process control equipment - all while lowering your total cost of ownership.

Loop Design, Integration and Engineering Support — Duro-Sense engineers engage with their customers, providing support for grassroots project planning, system design, or project management requirements.

Equipment Life Cycle Optimization — Through a combination of assessments and technology, Duro-Sense experts help customers benchmark operational performance, define key metrics, and implement precise sensor solutions to achieve long-term operational goals.

Intelligent Product Design — By employing an array of sophisticated products, services, and software that collects, examines and understands data, Duro-Sense helps customers use predictive analytics to take action and improve asset reliability and reduce downtime.

Have a challenging temperature sensing requirement? Call Duro-Sense.

Duro-Sense Corporation
310-533-6877

Friday, May 24, 2019

Quick Comparison of Temperature Sensors


Thermocouples are commonly used because of their simplicity, reliability, and relative low cost. They are self-powered and eliminate the need for a separate sensor power supply. Thermocouples are quite durable when selected for a given application appropriately. Thermocouples can also be used for applications with high temperatures.

Resistance temperature detectors (RTDs) are attractive alternatives to thermocouples when the output is desired to be highly accurate, stable and linear (i.e. just how close the calibration curve looks a straight line). The superior linearity of relative temperature resistance enables simpler signal processing devices for RTDs than thermocouples.

Thermistors are similar to RTD because they're a resistance measurement device, but thermistors use a very cheap polymer or ceramic material as the element in lieu of the use of pure metal.

For more information on any type of industrial or OEM temperature sensor, contact Duro-Sense by calling 310-533-6877 or by visiting https://duro-sense.com.

Monday, March 25, 2019

A Pro and Con Comparison of Thermocouples and RTDs


Thermocouple Advantages 
  • Inexpensive
  • Wide temperature range
  • Various types, sizes and application methods
  • Remote read back
  • Read back electronics can be simple
  • Usable in virtually any environment
Thermocouple Disadvantages
  • Requires cold junction compensation
  • Slow response time
  • Not as accurate as many other devices without good CJC and calibration
  • Susceptible to noise
  • Connection cable/wire is expensive compared to copper conductors
  • Cable/wire length is limited
RTD Advantages
  • More linear than thermocouples
  • Cold junction not an issue
  • Special cable/wire not needed
  • Cable/wire length can be much longer than TC’s
  • Better noise immunity
  • More stable over time than thermocouples
  • Remote read back
  • Usable in virtually any environment
RTD Disadvantages
  • More expensive than thermocouples
  • More delicate than thermocouples unless encased
  • Not as wide of temperature range as thermocouples
  • Requires more conductors per device
  • Read back electronics more complex

Thursday, February 28, 2019

Theory of Thermocouple Operation

  • A thermocouple is a simple temperature measurement device consisting of a junction of two dissimilar metals.
  • Contrary to popular belief, the voltage measured (and converted to a temperature) is not a function of the junction alone. Rather it is the temperature difference (or gradient) between the junction (or hot), end and the reference (or cold), end.
  • A thermocouple circuit whose junction and reference are the same temperature will measure no temperature (0V).
  • If this were not true, we could create a self-sustaining voltage generator using a thermocouple, a resistive load and an oven, that would require energy only at start-up.
Theory of Thermocouple Operation

The temperature equation for the simplest of thermocouple circuits shown above is:

T = Tjunc – Tref

Where T is the desired measurement, Tjunc is the hot junction temperature and Tref is the reference
temperature, or cold end.

For simplicity’s sake, we use T, Tjunc and Tref here, but in reality these are voltages that are later converted to a temperature.

Cold Junctions
Theory of Thermocouple Operation

The temperature equation for this diagram is:
T = Tjunc – Tcj1 – Tcj2

A fundamental problem when using thermocouples is the fact the when connected to a measurement device (voltmeter or TC meter), a third metal is introduced (the connecting terminals), and two more thermocouple junctions are created. These adversely affect the temperature being measured. The new, (and unwanted), junctions are referred to as “cold junctions” and need some type of “cold junction compensation” in order to make accurate measurements.

In addition to the added variables in the previous equation, the temperature of the cold junctions
(reference end), is still not known. The following rule helps things out a bit:
  • If both TC connections to the meter are of the same metal or alloy, they cancel each other and have no affect on the measurement, as long both connections are at the same temperature (which can be assumed).
Since the definition of a thermocouple states that it must be of dissimilar metals, a second thermocouple must be introduced to the circuit to achieve this. This was the first of what is commonly called “cold junction compensation”

By adding a second series thermocouple suspended in an ice bath, the cold junctions at the meter are of identical metals and cancel each other. In addition, the temperature of the ice bath is known to be 0 Deg. C and becomes the reference end of the thermocouple.

The temperature equation is now simplified and once again becomes:

T = Tjunc – Tref
Theory of Thermocouple Operation
While the ice bath reference junction eliminates errors, it is clearly impractical for most, if not all applications.  Fortunately, all of today’s thermocouple read back options (meters, chart recorders, PLCs, etc.), come equipped with cold junction compensation, usually a thermistor and associated circuitry and software. By taking the cold junction worries out of the picture, the thermocouple remains one of the simplest, most robust and widely used temperature measurement devices around.

Friday, February 15, 2019

Temperature Sensing IS Rocket Science


Duro-Sense Corporation provides the precision temperature sensors to the aerospace, aviation, and space industries. Duro-Sense engineers bring proven solutions to your most difficult problems. Their R&D department is staffed with some of the industry's most qualified people, working in the most modern facilities to help advance the state of the art in temperature measurement.

Thursday, January 31, 2019

Thermocouples

thermocouple circuit
Diagram of a thermocouple circuit.
A thermocouple is a temperature measurement sensor. Thermocouples are made of two different metal wires, joined to form a junction at one end. The connection is placed on the surface or in the measured environment. As the temperature changes, the two different metals start to deform and cause resistance changes. A thermocouple naturally outputs a millivolt signal, so that the change in voltage can be measured as the resistance changes. Thermocouples are desirable because they are extremely low cost, easy to use and can provide precise measurements.

thermocouple
Typical sheathed thermocouple.
Thermocouples are produced in a variety of styles, such as sheathed, washer type, bayonet,  mineral insulated, hollow tube, food piercing, bare wire thermocouples or even thermocouple made from thermocouple wire only.

Because of their wide range of models and technical specifications, it is extremely important to understand their basic structure, functionality and range in order to better determine the right thermocouple type and material for an application.

Operating Principle

When two wires consisting of different metals are connected at both ends and one end is heated, a continuous current flows through the thermoelectric circuit. If this circuit is broken in the center, the net open circuit voltage (Seebeck Effect) depends on the temperature of the junction and the composition of the two metals. This means that a voltage is produced when the connection of the two metals is heated or cooled that can be correlated to the temperature.

Contact Duro-Sense Corporation with any questions about applying industrial and commercial thermocouples.

Duro-Sense Corporation
https://duro-sense.com
310-533-6877

Monday, January 28, 2019

Where to Mount Industrial Temperature Transmitters?

Temperature TransmittersIn an industrial plant, where there are normally long distances between the measuring points and the receiving instrumentation, some important aspects regarding the location of the transmitters can be mentioned.

There are basically three different locations for the mounting of the temperature transmitters:

  • In-head mounting - inside the connection head of the temperature sensors.
  • Field mounting – close to the temperature sensors.
  • Central mounting - in the vicinity of the control room

In-head mounting

The transmitters are mounted directly inside the connection head and are normally replacing the terminal block.  This way of mounting normally offers the biggest advantages. It is however necessary to be aware of the environmental influence (mainly the temperature) on the measurement accuracy.

Advantages
  • Maximum safety in the signal transmission. The amplified signal, e.g. 4- 20 mA, is very insensitive to electrical disturbances being induced along the transmission cable.
  • Cost savings for the transmission cables. Only two leads are required if a 2- wire transmitter is used.
  • Cost savings for installation. No extra connection points because of the transmitter.
  • Cost and space savings. No extra housings or cubicles are needed.
  • Field instruments, e.g. indicators, can easily be installed, also at a later stage without redesigning the measuring circuits.
Disadvantages
  • The ambient temperatures can be out- side the allowed limits for the transmitters.
  • The ambient temperature influence on the measuring accuracy has to be considered. 
  • Extreme vibrations might cause malfunction of the transmitters.
  • The location of the temperature sensor can give maintenance problems.

Field mounting

The transmitters are either mounted directly beside the temperature sensors or in the vicinity of the sensors. Often more than one transmitter is mounted in the same field box.

This method is more expensive than In-head mounting, but otherwise a good alternative offering most of the advantages of In-head mounting without the disadvantages mentioned above.

Advantages
  • High safety in the signal transmission. The main part of the signal transmission is made with an amplified signal.
  • No extreme temperatures or vibrations exist. This facilitates accurate and safe measurements.
  • Cost savings for transmission cables.
  • A wider selection of transmitters is available. DIN rail transmitters can also be used.Field instruments can often be installed easily.
  • Maintenance can normally be carried out without problems.
Disadvantages
  • Higher installation costs compared to In-head mounting.
  • Costs and space requirements for transmitter boxes or cubicles.

Central mounting

In this case, the transmitters are placed in the vicinity of the control room or in another central part of the plant They are often mounted inside cubicles, and/or closed rooms. The ambient conditions are normally very good and stable.

This method offers the most convenient conditions for maintenance and the best possible environment for the transmitters. There are on the other hand some disadvantages that should be considered.

Advantages
  • Convenient conditions for installation, commissioning and maintenance.
  • Minimum risk for environmental influences (e.g. temperature influence).
Disadvantages
  • Reduced safety in the signal transmission. The low-level sensor signal is rather sensitive to electrical disturbances being induced along the trans- mission cable.
  • Relatively high costs for cabling. T/C measurements require compensation or extension cables all the way to the transmitters. RTD measurements with high accuracy should be done in 4-wire connection to get rid of the lead resistance influence.
  • Costs and space requirements for cubicles or frames.
  • Rather complicated and expensive to connect field instruments, e.g. indicators.

For more information on using temperature transmitters, contact Duro-Sense by calling 310-533-6877 or by visiting https://duro-sense.com.

Tuesday, December 4, 2018

3 Bottom Line Criteria to Help You Choose Between Thermocouples and RTDs

Temperature Sensor Selection Criteria
Both RTD and thermocouple probes monitor temperature but which one is right for your application?

The first question to ask yourself is what is the temperature range you are trying to monitor?
Generally, if the temperature is above a hundred and fifty degrees Celsius, a thermocouple would be used. For anything below a hundred and fifty degrees Celsius, an RTD would be used.

The next question to ask is what is the required sensor accuracy? 
RTDs provide more accurate readings with repeatable results, this is why RTDs are typically used when temperatures are within its monitoring range.

The last question is what is the purchase budget and how many do you need?
Thermocouples can be up to three times less expensive than RTD probes making thermocouples a good choice when purchasing a large quantity or when the budget is tight

These three criteria are VERY basic, and intended just to point you in the right direction. There are many other differences between thermocouples and RTDs that need to be understood before application.  Always consult a temperature sensor application expert prior to installing or specifying a thermocouple or RTD where failure can cause harm.

Duro-Sense Corporation
https://duro-sense.com
(310) 533-6877

Wednesday, November 28, 2018

Best Practice for Mounting Thermocouples in Pipes

not recommended
Diagram 1 - Not recommended.
Immersion type thermocouples are used to measure the temperature of liquid flowing in a pipe or sitting in a vessel. The particular orientation for any installation depends on the application, whether additional hardware is required, and the relative dimensions of the thermocouple sheath and the pipe.

There are however, recommended practices for placing thermocouples in the piping flow stream.

As you can see from diagram 1, placing the thermocouple in a tee, positioned perpendicular to flow is not recommended. This is because the conduction along the sensing area may be non-negligible and could bias the measurement, depending on the liquid and ambient temperatures. In addition it may be hard to know precisely where the measurement junction is located along the cross section of the pipe in this configuration.

Ideally they should be mounted in a tee where an elbow would normally be used as you see in diagram 2. If possible the thermocouple should be oriented along the normal flow direction, and the measurement located downstream of the T-bend (the T will help mix the liquid if it is not thermally uniform).
recommended
Diagram 2 - Recommended orientation.
If there is no convenient spot where an elbow would normally be used, a u-shape can be adapted to allow the installation for the tee. See diagram 3.

U shape
Diagram 3 - Use U shape if there is no convenient placement for a tee.
For more information, contact Duro-Sense by calling 310-533-6877 or visit their web site at https://duro-sense.com.

Wednesday, October 31, 2018

Thermocouple Extension Wire

In every thermocouple circuit there must be both a measurement junction and a reference junction: this is an inevitable consequence of forming a complete circuit (loop) using dissimilar-metal wires. As we already know, the voltage received by the measuring instrument from a thermocouple will be the difference between the voltages produced by the measurement and reference junctions.

Since the purpose of most temperature instruments is to accurately measure temperature at a specific location, the effects of the reference junction’s voltage must be “compensated” for by some means, either a special circuit designed to add an additional canceling voltage or by a software algorithm to digitally cancel the reference junction’s effect.

In order for reference junction compensation to be effective, the compensation mechanism must “know” the temperature of the reference junction. This fact is so obvious, it hardly requires mentioning. However, what is not so obvious is how easily this compensation may be unintentionally defeated simply by installing a different type of wire in a thermocouple circuit.

To illustrate, let us examine a simple type K thermocouple installation, where the thermocouple connects directly to a panel-mounted temperature indicator by long wires:




Like all modern thermocouple instruments, the panel-mounted indicator contains its own internal reference junction compensation, so that it is able to compensate for the temperature of the reference junction formed at its connection terminals, where the internal (copper) wires of the indicator join to the chromel and alumel wires of the thermocouple. The indicator senses this junction temperature using a small thermistor thermally bonded to the connection terminals.

Now let us consider the same thermocouple installation with a length of copper cable (two wires) joining the field-mounted thermocouple to the panel-mounted indicator:


Even though nothing has changed in the thermocouple circuit except for the type of wires joining the thermocouple to the indicator, the reference junction has completely shifted position. What used to be a reference junction (at the indicator’s terminals) is no longer, because now we have copper wires joining to copper wires. Where there is no dissimilarity of metals, there can be no thermoelectric potential. At the thermocouple’s connection “head,” however we now have a joining of chromel and alumel wires to copper wires, thus forming a reference junction in a new location at the thermocouple head. What is worse, this new location is likely to be at a different temperature than the panel-mounted indicator, which means the indicator’s reference junction compensation will be compensating for the wrong temperature.

The only practical way to avoid this problem is to keep the reference junction where it belongs: at the terminals of the panel-mounted instrument where the ambient temperature is measured and the reference junction’s effects accurately compensated. If we must install “extension” wire to join a thermocouple to a remotely-located instrument, that wire must be of a type that does not form another dissimilar-metal junction at the thermocouple head, but will form one at the receiving instrument.

An obvious approach is to simply use thermocouple wire of the same type as the installed thermocouple to join the thermocouple to the indicator. For our hypothetical type K thermocouple, this means a type K cable installed between the thermocouple head and the panel-mounted indicator:


With chromel joining to chromel and alumel joining to alumel at the head, no dissimilar-metal junctions are created at the thermocouple. However, with chromel and alumel joining to copper at the indicator (again), the reference junction has been relocated to its rightful place. This means the thermocouple head’s temperature will have no effect on the performance of this measurement system, and the indicator will be able to properly compensate for any ambient temperature changes at the panel as it was designed to do. The only problem with this approach is the potential expense of thermocouple-grade cable. This is especially true with some types of thermocouples, where the metals used are somewhat exotic (e.g. types R, S, and B).

A more economical alternative, however, is to use something called extension-grade wire to make the connection between the thermocouple and the receiving instrument. “Extension-grade” thermocouple wire is made less expensive than full “thermocouple-grade” wire by choosing metal alloys similar in thermo-electrical characteristics to the real thermocouple wires within modest temperature ranges. So long as the temperatures at the thermocouple head and receiving instrument terminals don’t get too hot or too cold, the extension wire metals joining to the thermocouple wires and joining to the instrument’s copper wires need not be precisely identical to the true thermocouple wire alloys. This allows for a wider selection of metal types, some of which are substantially less expensive than the measurement-grade thermocouple alloys. Also, extension-grade wire may use insulation with a narrower temperature rating than thermocouple-grade wire, reducing cost even further.

Extension-grade cable is denoted by a letter “X” following the thermocouple letter. For our hypothetical type K thermocouple system, this would mean type “KX” extension cable:



Thermocouple extension cable also differs from thermocouple-grade (measurement) cable in the coloring of its outer jacket. Whereas thermocouple-grade cable is typically brown in exterior color, extension-grade cable is usually colored to match the thermocouple plug (yellow for type K, black for type J, blue for type T, etc.)

For more information on thermocouple extension wire, contact Duro-Sense by visiting https://duro-sense.com or by calling 310-533-6877



Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.


Wednesday, October 24, 2018

The MgO Thermocouple

Magnesium oxide
Magnesium oxide
Magnesium oxide (MgO), or magnesia, is a white hygroscopic solid mineral that occurs naturally. The use of compacted magnesium oxide for electrical insulation in wiring cables, heating cables, tubular heating elements, and thermocouples is well known.

While magnesium oxide is the favored high temperature insulating material, many others have been tested. Some examples are aluminum oxide, crystalline silica, and beryllium oxide. For various reasons, ranging from cost, inferior mechanical, poor electrical, and safety issues, magnesium oxide outperforms other powders in industrial applications.

MgO cable
MgO cable cutaway
The MgO thermocouple is constructed by encasing a thermocouple element inside a metal sheath, surrounded by magnesium oxide (MgO). Sheath materials are typically 304 stainless steel, 316 stainless steel, Inconel 600, and 310 stainless steel. The metal sheath is then swaged or drawn down to reduce its diameter. During the drawing process, the powder undergoes considerable compaction, and reaches a compressed density of 70-80% of the crystal density. Despite this change in density, the thermocouples remain very flexible after annealing. Impressively, an MgO thermocouples minimum bend diameter is equal to two times the outside diameter.

MgO thermocouple
MgO thermocouple
MgO thermocouples are available in a variety of sensing junctions. Grounded junctions use a thermocouple welded to the sheath and provides fast response, with good thermocouple protection. Ungrounded (isolated) junctions are insulated from sheath with magnesium oxide and are used to prevent electrical interference from affecting the signal and response is slightly slower than grounded junctions. Exposed junctions are not protected by welded end-cap, provide very quick response, but are susceptible to corrosive media. Dual element common junctions have two thermocouples with junctions welded together, and dual element isolated junctions electrically separate in the same sheath.

MgO thermocouples are known for high dielectric strength, durability, malleability and quick response to temperature fluctuations. They can be used for process applications up to 2400°F and, because the measuring junction can also be sealed from the environment, they are recommended for use in high pressure, high moisture, corrosive, and environments.

For more information on MgO thermocouples, contact Duro-Sense Corporation at https://duro-sense.com or by calling 310-533-6877

Wednesday, October 10, 2018

Overview of Thermocouple Types and Ranges

Thermocouples have been classified by the International Society of Automation (formerly Instrument Society of America) and the American National Standards Institute (ANSI), and are available for temperatures ranging from -200 deg. to 1700 deg.C (-330 deg. to 3100 deg.F). These standard tolerance thermocouples range in tolerance from ±0.5 percent to ±2 percent of true temperature. The table below presents commonly available thermocouple types and operating ranges.
Thermocouple ranges
Commonly Available Thermocouple Types and Operating Ranges
Thermocouples must be selected to meet the conditions of the application. Thermocouple and extension wires (used to transmit the voltage from the thermocouple to the monitoring point) are generally specified and ordered by their ANSI letter designations for wire types. Positive and negative legs are identified by the letter suffixes P and N, respectively. General size and type recommendations are based on length of service, temperature, type of atmosphere (gas or liquid constituents), and desired response times. Smaller wire gauges provide faster response but do not last as long under adverse conditions. Conversely, larger gauges provide longer service life but with longer response times. Thermowells and sheaths are recommended by thermocouple manufacturers for the extension of thermocouple life. Instruments used to convert thermocouple voltage to temperature scales are coded using the same letter designations. Failure to use matching thermocouples and instruments will result in erroneous readings.

Thermocouple standardsType J thermocouples use iron for the positive leg and copper-nickel (constantin) alloys for the negative leg. They may be used unprotected where there is an oxygen-deficient atmosphere, but a thermowell is recommended for cleanliness and generally longer life. Because the iron (positive leg) wire oxidizes rapidly at temperatures over 1000 deg.F, manufacturers recommend using larger gauge wires to extend the life of the thermocouple when temperatures approach the maximum operating temperature.

Type K thermocouples use chromium-nickel alloys for the positive leg and copper alloys for the negative leg. They are reliable and relatively accurate over a wide temperature range. It is a good practice to protect Type K thermocouples with a suitable ceramic tube, especially in reducing atmospheres. In oxidizing atmospheres, such as electric arc furnaces, tube protection may not be necessary as long as other conditions are suitable; however, manufacturers still recommend protection for cleanliness and prevention of mechanical damage. Type K thermocouples generally outlast Type J, because the iron wire in a Type J thermocouple oxidizes rapidly at higher temperatures.

Type N thermocouples use nickel alloys for both the positive and negative legs to achieve operation at higher temperatures, especially where sulfur compounds are present. They provide better resistance to oxidation, leading to longer service life overall.

Type T thermocouples use copper for the positive leg and copper-nickel alloys for the negative leg. They can be used in either oxidizing or reducing atmospheres, but, again, manufacturers recommend the use of thermowells. These are good stable thermocouples for lower temperatures.

Types S, R, and B thermocouples use noble metals for the leg wires and are able to perform at higher temperatures than the common Types J and K. They are, however, easily contaminated, and reducing atmospheres are particularly detrimental to their accuracy. Manufacturers of such thermocouples recommend gas-tight ceramic tubes, secondary porcelain protective tubes, and a silicon carbide or metal outer protective tube depending on service locations.

For more information about thermocouples, contact Duro-Sense Corporation by visiting https://duro-sense.com or calling 310-533-6877.

Friday, September 28, 2018

Advantage and Disadvantages of Common Temperature Sensors

THERMOCOUPLE

thermocouple
Thermocouple
Due to their simplicity, reliability, and relatively low cost, thermocouples are widely used. They are self-powered, eliminating the need for a separate power supply to the sensor. Thermocouples are fairly durable when they are appropriately chosen for a given application. Thermocouples also can be used in high-temperature applications.

Thermocouple Advantages:
  • Self-powered
  • Simple
  • Rugged
  • Inexpensive
  • Many applications
  • Wide temperature range
  • Fast response
Thermocouple Disadvantages:
  • Nonlinear output signal
  • Low voltage
  • Reference required
  • Accuracy is function of two separate measurements
  • Least sensitive
  • Sensor cannot be recalibrated
  • Least stable

RTD

RTD
RTD
Resistance temperature detectors are attractive alternatives to thermocouples when high accuracy, stability, and linearity (i.e., how closely the calibration curve resembles a straight line) of output are desired. The superior linearity of relative resistance response to temperature allows simpler signal processing devices to be used with RTD’s than with thermocouples. Resistance Temperature Detector’s can withstand temperatures up to approximately 800 C (~1500 F).

RTD Advantages:
  • More stable at moderate temperatures
  • High levels of accuracy
  • Relatively linear output signal
RTD Disadvantages:
  • Expensive
  • Self-heating
  • Lower temperature range

THERMISTOR

Thermistor
Thermistor
Thermistors work similarly to RTD’s in that they are a resistance measuring device, but instead of using pure metal, thermistors use a very inexpensive polymer or ceramic material as the element.

Thermistor Advantages:
  • High output
  • Fast
  • Two-wire ohms measurement
Thermistor Disadvantages:
  • Nonlinear
  • Limited temperature range
  • Fragile
  • Current source required
  • Self-heating

Monday, September 10, 2018

Thermowells

Thermowell
Thermowell (Duro-Sense)
One of the most important accessories for any temperature-sensing element is a pressure-tight sheath known as a thermowell. This may be thought of as a thermally conductive protrusion into a process vessel or pipe allowing a temperature-sensitive instrument to detect process temperature without opening a hole in the vessel or pipe. Thermowells are critically important for installations where the temperature element (RTD, thermocouple, etc.) must be replaceable without de-pressurizing the process.

Thermowells may be made out of any material that is thermally conductive, pressure-tight, and not chemically reactive with the process. Most thermowells are formed out of either metal (stainless steel or other alloy) or ceramic materials. A simple diagram showing a thermowell in use with a temperature sensor (RTD) is shown here:
Thermowell

As useful as thermowells are, they are not without their caveats. All thermowells, no matter how well they may be installed, increase the first-order time lag of the temperature sensor by virtue of their mass and specific heat value. It should be intuitively obvious that a few pounds of metal will not heat up and cool down as fast as a few ounces’ worth of RTD or thermocouple, and therefore the addition of a thermowell to the sensing element will decrease the responsiveness of any temperature- sensing element. What is not so obvious is that such time lags, if severe enough, may compromise the stability of feedback control. A control system receiving a “delayed” temperature measurement will not see the live temperature of the process in real time due to this lag.

Thermowell
RTD with Thermowell 
A potential problem with thermowells is incorrect installation of the temperature-sensing element. The element must be inserted with full contact at the bottom of the thermowell’s blind hole. If any air gap is allowed to exist between the end of the temperature element and the bottom of the thermowell’s hole, this will add a second time lag to the measurement system26. Some thermowells include a spring clip in the bottom of the blind hole to help maintain constant metal-to-metal contact between the sensing element and the thermowell wall.

Reprinted from Lessons In Industrial Instrumentation by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Tuesday, August 28, 2018

Temperature Transmitters

Temperature Transmitters
Temperature Transmitters (Duro-Sense)
A temperature transmitter is generally described as a device, which on the input side is connected to some sort of temperature sensor and on the output side generates a signal that is amplified and modified in different ways. Normally the output signal is directly proportional to the measured temperature within a defined measurement range. Many additional features can be added depending on the type of transmitter being used. The features of the temperature transmitter are often described by using different terms with respect to technology, mounting method, functions, etc. The following list is a brief summary of these terms.

Analog Transmitters: These transmitters are designed on analog circuit technology. They normally offer basic functions such as temperature linearization and sensor break technology. Sometimes they are adjustable for different measuring ranges, often with a fast response time.

Digital Transmitters: This transmitter type is mainly based on a microprocessor. They are often called intelligent transmitters, because they normally offer many extra features, which are not possible to realize in analog transmitters.

In-Head Transmitters: These transmitters are designed for mounting in the connection heads of temperature sensors. All Duro-Sense in-head transmitters fit into DIN B heads or larger. Special care has to be devoted to the ruggedness because of the harsh conditions that sometimes exist.

DIN Rail Transmitters: DIN rail transmitters are designed to be snapped onto a DIN rail. Duro-Sense DIN rail transmitters fit on a 35mm rail according to DIN EN 50022.

RTD Transmitters: RTD transmitters are used only for RTD sensors. (Pt100, Pt1000, Ni100, etc.). Normally they can handle only one RTD type. Most Duro-Sense transmitters can handle more than one type of RTD and are either fix- ranged or adjustable. They all have linear output.

Thermocouple TransmitterThermocouple Transmitters: Thermocouple transmitters measure a MV signal form the thermocouple and compensates for the temperature of the cold junction. The cold junction compensation is normally made by measuring the terminal temperature. Alternatively, some transmitters can be adjusted to compensate for an external fixed cold junction temperature. Pure thermocouple transmitters are often not temperature linearized due to the complicated unlinearity of the thermocouples.

Analog Output: The output signal is a current (4-20mA). Some transmitters are available with 0-20mA or 0-10mA output. The signal is normally proportional to the measured value within a defined measurement range.

Digital Output: The measured value (temperature) is presented as a binary coded message. So called Fieldbus transmitters use this technique. The Fieldbus transmitters on the market today use different standards for the communication thus creating some problems when integrating them with other instrumentation. Examples of standard available are: PROFIBUS, Interbus, Foundation Fieldbus, LonWorks and CAN-bus.

Analog and Digital Output: The HART transmitters have an analog output with a superimposed digital signal on the same wires. Typically, the analog signal is used for normal measurements and the digital signal only for temporary measurements, because of the low communication speed. The digital signal is mainly used for configuration and status information.

Isolated Transmitters: Isolated transmitters have no leading connections between circuits that are isolated from each other. The isolation effectively eliminates the risk for circulating currents and facilitates the connection of transmitters to control systems with grounded inputs.

Non-Isolated Transmitters: These transmitters have leading connections between, for instance, input and output circuits. They should be used with care.

For more information on temperature transmitters, visit https://duro-sense.com or call 310-533-6877.