Advantage and Disadvantages of Common Temperature Sensors

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

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

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

Duro-Sense - Kind Words

Thermowells

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:

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.

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.

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 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.

Temperature Sensors for the Toughest Applications

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.

Video: Comparison of Thermocouples and RTDs

The video below describes the basic differences between industrial thermocouples and RTDs.

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

https://duro-sense.com
310-533-6877

Reliable, Robust, and Affordable Process Heating Sensors and Controls

The ability to effectively measure, monitor, and control process heating operations is essential to minimize product variability and maintain product quality. This level of control requires reliable and affordable sensors and control systems that can withstand harsh environments and not require recalibration for at least one year. Process heating could become far more effective with access to more reliable, robust, and affordable sensors and process controls. There is a need for reliable, cost effective sensors for harsh environments and for the real-time measurement of the chemical composition of the fuel, oxidant, and flue gas in combustion processes. Real-time combustion controls for multiple fuel applications could help maximize fuel flexibility, while improved sensors as part of smart control systems could increase efficiency, safety, and reliability. In electromagnetic processes, low cost, robust, and reliable sensors are needed to measure field strength, as well as sensors that can measure process parameters but are immune to direct excitation by the electromagnetic energy.

Technology opportunities for sensors and process controls to improve the overall control and
performance of process heating systems include the following:

Sensors for Harsh, High-Temperature Environments: Technologies and methods are needed to reliably monitor and control critical product parameters (temperature, chemistry, pressure, etc.), especially robust sensors to measure critical parameters in harsh combustion environments. This includes direct process measurement sensors, and more accurate and reliable thermocouples and other sensors. The development of sensors that can provide accurate readings in high-temperature environments could enable opportunities to optimize heat transfer and containment systems in those conditions.

Furnace Control: In fuel-fired equipment, reliable sensing and control technologies can provide better fuel utilization, energy savings, temperature control, and system performance over time. This includes sensors that can accurately measure compositional characteristics of fuels and oxidant; low-cost, highly reliable flame monitoring systems to control flame quality and stability; and continuous flue gas analysis. By regulating and stabilizing internal furnace pressure, pressure controllers can eliminate cold air infiltration, maintain uniform temperatures, and reduce wear that would require more frequent and costly maintenance.

Advanced Control Strategies to Optimize Process Heating: Cost-effective smart process controls that can be integrated with the overall manufacturing system are needed. Analysis of flue gases can be used to optimize the inlet fuel/air ratio. By using sensors to measure oxygen and carbon monoxide in the flue gas stream, conditions can be created for ideal combustion scenarios.