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

Friday, April 19, 2019

Thermocouples: Proper Use, Recommended Practices, and Avoiding Problems

High temperature thermocouples

Proper use and maintenance of thermocouple systems begin with good system design based on the strengths and weaknesses of various thermocouple types. Because these sensors contain sensitive electronics, general good practice includes use of shielded cases and twisted- pair wire, use of proper sheathing, avoidance of steep temperature gradients, use of large-gauge extension wire, and use of guarded integrating voltmeters or ohmmeters, which electronically filter out unwanted signals. The signal conditioner should be located as close as possible to the sensor, and twisted copper-wire pairs should be used to transmit the signal to the control station. To minimize electromagnetic field interference, sensor system wires should not be located parallel to power supply cables. The primary causes of loss of calibration in thermocouples include the following:
  1. Electric “noise” from nearby motors, electric furnaces, or other such electrically noisy equipment;
  2. Radio frequency interference from the use of hand-held radios near the instrument.
  3. “Ground loops” that result when condensation and corrosion ground the thermocouple and create a ground loop circuit with another ground connection in the sensing circuit.
Most problems with thermocouples are aggravated by use of the thermocouple to measure temperatures that approach or exceed their upper temperature limits. Careful recording of events that could affect measurements should be kept in a logbook. Any adjustments or calibrations should also be recorded. The logbook should contain the names of individuals performing maintenance and calibrations as well as defined procedures. In systems monitoring many locations, such a log is especially useful for fault diagnosis.

Thermocouples sometimes experience catastrophic failures, which may be preceded by extreme oscillations or erratic readings. In such cases, all connections associated with the thermocouple should be checked for loose screws, oxidation, and galvanic corrosion. In many cases, drift may be a more serious problem because it can go unnoticed for long periods of time. The most common causes of loss of calibration are excessive heat, work hardening, and contamination. Work hardening generally is due to excessive bending or vibration and can be prevented with properly designed thermowells, insertion lengths, and materials. Contamination is caused by chemicals and moisture, which sometimes attack wiring by penetrating sheaths, and can result in short-circuiting. A simple test to check for this problem is to disconnect the sensor at its closest connection and check for electrical continuity between the wires and the sheath using a multimeter. If the meter indicates continuity, the sensor should be replaced. Because the electromotive force (EMF) produced by thermocouples is so small, electrical noise can severely affect thermocouple performance. For that reason, it also is very important that transmitters be isolated. Thermocouples used in the vicinity of electrostatic precipitators must be shielded to avoid electrical interference. If the potential electrical interference is high, an RTD or other type of sensor may be preferred to thermocouples. With respect to thermocouple and protection tube selection, the following should be noted:
  1. Type J thermocouples particularly should not be used in applications in which they might be exposed to moisture because the iron in the thermocouple will rust and deteriorate quickly;
  2. Type K thermocouples should not be used in the presence of sulfur, which causes the element to corrode; because cutting oils often contain sulfur, protection tubes should be degreased before being used; stainless steel sheaths should be used to protect Type K thermocouples in stacks where SO2 emissions are significant;
  3. Platinum thermocouple elements (Types R, S, or B) should not be used with metal protection tubes unless the tubes have a ceramic lining because the metal will contaminate the platinum;
  4. Ceramic, silicon carbide, and composite (metal ceramic, Cerite-II, Cerite-III) protection tubes are subject to thermal shock and should be preheated prior to inserting in high temperature process environments; and
  5. Molybdenum - or tantalum-sheathed thermocouples will fail rapidly if placed in oxidizing atmospheres.
During one study of thermocouple performance, 24 combinations of thermocouple and sheath material types were tested at temperatures up to 1200 C (2200 F). The results indicated that above 600 C (1110 F) thermocouples are affected by complex chemical interactions between their components; even though wires and sheaths were physically separated, exchange of constituents occurred. The study concluded that thermocouples maintain calibration better if sheath material is similar in composition to thermocouple alloys. By using similar alloys longer performance can be expected for sensors subjected to temperatures above 600 C (1110 F), and the use of similar alloys is essential for temperatures above 1000 C (1830 F).

For more information on the proper use of thermocouples, contact Duro-Sense Corporation by calling (310) 533-6877 or visit their website at https://duro-sense.com.

Reprinted from CAM Technical Guidance Document courtesy of EPA.gov.

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, August 17, 2018

Temperature Sensors for the Toughest Applications

Duro-Sense Corporation
Click for larger view.
Duro-Sense Corporation manufactures the finest quality temperature sensors available. The company has a long history of supplying thermocouples and RTDs to the top manufacturers in aerospace, medical equipment, semiconductor processing, plastics processing, and heavy industry.  From large industrial thermocouples used in primary metal production, to miniature, discreet sensors used in military aircraft, Duro-Sense products are proven to be ultra-reliable, accurate, and of extremely high value.

All Duro-Sense customers benefit from years of tackling difficult applications. By implementing stringent quality practices and advanced manufacturing processes, Duro-Sense continues to solve the most challenging temperature sensing applications.

Duro-Sense is a one-stop, full service provider of anything related to temperature sensing. Service. Quality. On-time delivery.

Rely on the Duro-Sense difference.
www.duro-sense.com  |  310-533-6877.



Tuesday, June 19, 2018

Thermocouple Types, Materials of Composition, and Temperature Ranges

Thermocouple Types
Nickel-Alloy Thermocouples


Type E: 
(pos) 90% Nickel / 10% Chromium;
(neg) 55% Copper / 45% Nickel (Constantan)
32 to 1600°F, 0 to 870°C

Type J:
(pos) 100% Iron
(neg) 55% Copper / 45% Nickel (Constantan)
32 to 1400°F, 0 to 760°C

Type K:
(pos) 90% Nickel / 10% Chromium
(neg) 95% Nickel / 2% Aluminum / 2% Manganese / 1% Silicon
32 to 2300°F, 0 to 1260°C


Type M:
(pos) 82% Nickel / 18% Molybdenum
(neg) 99.2% Nickel / 0.8% Cobalt
-58 to 2570°F, -50 to 1410°C

Type N:
(pos) 84.5% Nickel / 14% Chromium / 1.5% Silicon
(neg) 95.4% Nickel / 4.5% Silicon / 0.1% Magnesium
32 to 2300°F, 0 to 1260°C

Type T:
(pos) 100% Copper
(neg) 55% Copper / 45% Nickel (Constantan)
-328 to 700°F, -200 to 370°C

Platinum/rhodium-Alloy thermocouples


Type B:
(pos) 70% Platinum / 30% Rhodium
(neg) 94% Platinum  / 6% Rhodium
1600 to 3100°F, 871 to 1704°C

Type R:
(pos) 87% Platinum / 13% Rhodium
(neg) 100% Platinum
1000 to 2700°F, 538 to 1482°C

Type S:
(pos) 90% Platinum / 10% Rhodium
(neg) 100% Platinum
1000 to 2700°F, 538 to 1482°C

Tungsten/Rhenium-Alloy Thermocouples


Type C:
(pos) 95% Tungsten / 5% Rhenium
(neg) 74% Tungsten / 26% Rhenium
32 to 4200°F, 0 to 2315°C

Chromel–Gold/Iron-Alloy Thermocouples


Type P:
(pos) 55% Palladium / 31% Platinum / 14% Gold
(neg) 65% Gold / 35% Palladium
32 to 2543°F, 0 to 1395°C

Friday, May 25, 2018

Power Plant Temperature Sensors

Power plants (generating facilities) transform the mechanical energy of a spinning generator into electrical energy. Heat (from flame, nuclear reaction, or chemical reaction) is used to create steam that, in turn, produces the mechanical energy to drive turbines. There are many areas where precision temperature measurement and monitoring is critical to keep power plant systems running. Thermocouples and RTD sensors provide accurate, repeatable, and reliable  measurement.

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


Wednesday, May 16, 2018

RTD and Thermocouple Selection and Location for Optimal Control

Loop diagram
Loop diagram *
A perfectly designed temperature loop would precisely balance the power required to heat the media to it's desired temperature while compensating for system losses. In the real world however, there are many external variables that upset the balance between energy input and desired temperature. To offset these external variables and ensure adequate power is available to do the work, energy calculations with liberal safety factors are coupled with temperature controllers that throttle or proportion the amount of energy added to the process.

Most temperature control loops have (5) four major components:

1) The media to be heated (e.g. metal platen, a tank of liquid, a stream of gas)
2) A energy source (e.g. electric heater, steam, hot oil, flame)
3) A temperature sensor (thermocouple, RTD)
4) A controller (e.g. electronic thermostat, PID controller)
5) Control element (e.g. control valve, SCR, SSR)

Temperature controllers provide sophisticated functions that "learn" or understand the relationship between available power and sensor temperature. They then adjust the amount of energy (heat) added, based on the current reading of the sensor and the desired temperature setpoint.  Unfortunately, temperature controllers are often relied upon to overcome the oversights and inadequacies of poor control loop design.

Lag time
Lag time *
In poor control situations the controller usually takes the blame, when in actuality,  the problem lies in the system design. The controller's actions are a function of the difference between setpoint and sensor reading, the availability of energy to eliminate that error, and the sensor lag time. The further the sensor or energy source is located from the process media, the wider the swings in energy input will be, and therefore produce a more difficult loop to balance. Considering this, it is important to optimize the sensor (thermocouple or RTD) placement. Any distance or barrier between sensor and process media introduces lag which is an impediment to close control.

In the most ideal situation, temperature sensors, the energy source and process media would all be at the same physical location. Since it's virtually impossible to accomplish this, compromises have to be made to allow for the mechanical, physical, and electrical realities of the application. Here are some practical recommendations for sensor selection and placement to improve temperature loop performance:
  • Thermocouples, because of their low mass generally have a response advantage over RTDs.
  • Exposed junction thermocouples are the fastest responding (least lag) sensor choice, but they are also the most prone to physical and chemical damage.
  • Narrow, sheathed, grounded junction magnesium oxide insulated thermocouples are nearly as fast as exposed junctions, and provide protection from the process media.
  • Applications that require protection sheaths and thermowells for RTDs and thermocouples increase sensor lag time.
  • An immersion length of at least 10 times the diameter of the thermowell or sensor sheath should be used to minimize heat loss along the sensor sheath or thermowell wall from tip to process connection.
  • Where possible, insert the temperature sensor in a pipe elbow into the oncoming flow.
  • Tapered, swaged, or stepped thermowells are  faster responding
  • Always make sure the fit between sensor outer diameter and thermowell inner diameter is tight, and that the tip of the sensor is in direct contact with the thermowell. 

* Images courtesy of Tony Kuphaldt and his book "Lessons In Industrial Instrumentation"

Thursday, May 3, 2018

Advantages and Disadvantages of Thermocouples

Industrial thermocouples
Industrial thermocouples in ceramic
protection tubes (Duro-Sense)
Temperature sensors, inherent to their design or operating principle, are often the weakest link in the control loop. Fragility, noise sensitivity, and material deterioration have to be considered when applying temperature sensors. Generally speaking, when it comes to temperature sensing technology, RTDs are more accurate and stable, but are also fragile; thermistors have greater temperature sensitivity, but their electrical properties can change overtime; thermocouples (TC) are rugged, inexpensive, and operate over a wide temperature range, but they do have drawbacks that need to be understood for successful implementation.

The thermocouple's primary disadvantage is their weak output signal and their susceptibility to electrical noise. The mV signal generated is so small it requires conditioning, namely amplification and linearization.  The good news is this conditioning is built-in to and provided by the TC's corresponding controller, indicator or transmitter.  Calibration drift due to oxidation, contamination, or other physical changes to the alloys is another problem associated with thermocouples, requiring a facility to implement periodic recalibration procedures. Lastly, thermocouples require alloy-matching thermocouple extension wire when running lead wires any distance from sensor to instrument. This adds cost, as thermocouple extension wire is more expensive than standard wire.

Despite these concerns, thermocouples are widely used in industry because of their relative low cost, ruggedness, wide selection of operating ranges, and versatility in size, shape, and configuration. They have been used in millions of process control applications for over a generation, and provide an excellent balance of performance, cost, and simplicity. Successful application depends on knowing thermocouple's strengths and weaknesses, and consulting with an applications expert prior to specifying or installing any temperature sensor is always recommended to ensure the sensor's longest life and best performance.

Monday, March 5, 2018

Industrial Thermocouples

Industrial thermocouples are used to sense temperature in industrial processes. They come in a wide variety of types. The type of thermocouple, defined by the combination of dissimilar metals used, determines the functional temperature range and should be matched to the requirements of the application.

Magnesium Oxide (MgO) thermocouples consists of a thermocouple junction encased in a metal sheath, surrounded by compressed magnesium oxide insulation. The thermocouple junction, also known as the sensing junction, is the point where the two dissimilar metals meet and are usually welded together. Since the sensing junction is sealed from the environment, there is reduced potential for contamination and corrosion. MgO thermocouple sheaths are annealed and can be formed into different shapes and diameters.

Industrial thermocouple usually are constructed by inserting the thermocouple element into a metallic thermowell or ceramic protection tube. Not only does this protect the sensing element from the process, it accommodates easy removal and replacement.  Industrial thermocouples can be designed with virtually endless combinations of elements, wells, protection tubes, junction boxes, wiring terminals, and process connections.