Temperature Sensors Selection Criteria

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In the process industry Resistance Temperature Detectors (RTDs) and Thermocouples (T/Cs) are most common temperature sensors in use. When exactly should you use an RTD or a Thermocouple? Are their applications where a thermocouple is more suitable than an RTD or vice versa? Does accuracy level of my application determine which sensor to use? These and many other questions will be answered when you finish reading this article.

In the table below are some application questions that needs to be answered before you can successfully apply the right temperature sensor technology. When you are  able to provide the right answers to these application questions /selection criteria required, you will be in good stead to choose the right sensor for your application. The advice given below are only for guidance. Always consult the vendor for a specific temperature sensor application.

Application Question(s)/ Selection Criterion	Application Advice
What process are we measuring the   temperature?	To be able to select the appropriate temperature sensor, you must understand the process thoroughly
What is the process fluid concerned?	In most cases, temperature measurement is required for either gas, liquid, steam or granular fluids. Understanding the nature of the fluid helps to select the best temperature sensor required
What is the operating pressure? What is the maximum pressure in the system?	Knowing what the pressure level in the system where we need to measure temperature will give us the an idea of the right sensor to select.
What is the normal, maximum, and minimum fluid flow rate?	Flow rate is a key requirement in selecting a temperature sensor.
Will the measurement be taken in a pipe or vessel?
Is the pipe or vessel  full or partially filled?	The level of fluid in the pipe or vessel with enable the right sensor with the right probe length to be selected
What is the ambient temperature range around the measurement point?	Ambient temperature range will impact on sensor accuracy as we can easily predict the ambient temperature effect on the measurements taken from the sensor
Where will the measurement be taken? Ground level or elevated?
Is it desirable to have a local display of the temperature ?	Local display capability is good for operators in the field who can easily determine the temperature.
Are you monitoring temperature trend or an actual controlled value?	Trend monitoring requires little or no accuracy. An RTD or a Thermocouple will do the job in this type of application
What is the temperature range of the application? What is the control point? What is the maximum and minimum temperature required for the application	If the maximum temperature to be measured is above 850°C, then a thermocouple is the only choice. For most other applications, an RTD or a thermocouple would be fine.
Do you have established plant or company preferences that may influence your choice of temperature sensor?	If you use specific sensor types on specific applications based on successful experience, it will be wise to follow this trend. However do not always follow this trend of we have always done it this way. Asking a few more questions and probing deeper will not hurt. Most plants always use an RTD as the first choice temperature sensor unless  they are limited by high temperature requirement where a thermocouple becomes the obvious choice
Do you have certain temperature sensor types kept in your inventory?	Use only a normally stocked temperature sensor type if it meets the requirement for your application.
What is the stability and control precision requirement?	If accuracy requirement is far  better than ±20F, use an RTD. If long term stability is required, an RTD is a better choice than a thermocouple Spring  loaded wire wound designs for temperature sensors provide the best accuracy
What is the speed of response to temperature change requirement?	Spring loaded temperature sensors and stepped thermowells provide good speed of response. Fill the voids between the temperature sensor and the inside bore of the thermowell to increase the speed of response.
Is there a significant return on investment (ROI) for best possible accuracy and stability?	If you are dealing with an application such as custody transfer where improved accuracy can yield significant ROI, consider using an RTD. Use an RTD with sensor-transmitter matching option for system accuracy as good as 0.015 °C
What costs are associated with temperature measurement failure?	Measurement failure most often results in production down time costs, off-spec products that may require re-processing or selling at a reduced price; energy inefficiency; dangerous runaway reactions etc. To avoid the above costly consequences, use a high quality transmitter integrally mounted with a quality sensor A high quality RTD will suffice where measurement failure could trigger serious financial consequences
What is the frequency and severity of the piping and vessel vibration? What is the typical and maximum vibration trend?	High vibration requires the use of a thin film spring loaded RTD sensor. In some applications, a helical coil wire wound RTD will perform better. A heavy gauge thermocouple is a possible alternative. Where in doubt, consult vendor product data sheets for specifications.
Is the temperature measurement part of a safety instrumented system (SIS)?	Where an SIS system is involved, use the highest possible quality of sensor and reliability system

The Callendar Van Dusen Equation for Platinum Based RTD Thermometers

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The Callendar-Van-Dusen equation describes the relationship between the Ohmic Resistance and Temperature of Platinum based temperature instruments. This equation is commonly used in commercial applications in RTD thermometers and RTD Transmitters. In its original form, the Callendar-Van-Dusen Equation is given by:

Rt = R0 + R0α[t – δ(0.01t – 1)(0.01t) – β(0.01t – 1)(0.01t)3]

Where:
Rt = Resistance at Temperature t (°C)
R0  = Resistance at t = 0°C
α  = Sensor specific contant
δ  = Sensor specific constant
β  = Sensor specific constant (0 at t > 0°C, 0.11 at t < 0°C)

The above equation has been used in the EN/IEC 60751 standard in the format below:

Common RTD Application Problems and their Remedies

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Resistance Temperature Detectors(RTDs) are widely used across several industries – power, food and beverage etc – for temperature measurement. In using them, different types of application problems occur once in a while. To help resolve these problems, the table below shows a list of the common problems encountered in RTD applications and possible remedies or corrective measures. This list is by no means exhaustive:

How to Calibrate an RTD Transmitter

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The RTD transmitter is usually factory calibrated to the temperature range shown on the device name plate. When the performance deteriorate and the transmitter needs recalibration, the transmitter is normally calibrated by using a resistance decade box. 

Materials Required for Calibration

To calibrate the RTD transmitter, the following equipment will be required:

1.  Voltmeters (digital) of suitable accuracy and very high resolution – 1mv

2.  A 24VDC power source

3.  A 5 dial Resistance Decade Box with high precision providing 100Ω steps 

Calibration Steps:

Connect the above equipment as in the setup below:

How to Convert RTD Resistance to Temperature

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An RTD resistance can be converted into temperature using standard tables that gives values of temperatures for any given resistance value of the RTD.

The table below shows temperature versus resistance data in degree celsius with temperature coefficient of resistance of: 0.003916 ohm/ohm/°C.

How to Specify an RTD Sensor

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When a Resistance Temperature Detector (RTD) is required for a given application, many parameters need to be accurately documented for the particular RTD to be procured from the manufacturers. Since there are many different manufacturers of RTDs, there will be several different styles of RTDs in the market. Each manufacturer has their own way of specifying their product. In any case, when specifying an RTD you will always be required to select the following:

Resistance Temperature Detectors(RTDs): Application limitations, Comparison of types and Failure mode

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Application Limitations of RTDs:
RTDs can be quite bulky, which can inhibit their use in applications.Self heating can be a problem with RTDs. In order to measure the resistance of an RTD device, we must pass an electric current through it. Unfortunately, this results in the generation of heat at the resistance according to Joule’s Law:

 P = I² R 

                                                                 

This dissipated power causes the RTD to increase in temperature beyond its surrounding environment, introducing a positive measurement error. The effect may be minimized by

RTD Construction and Lead Wire Configurations

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Platinum RTD elements are available in two types of constructions:

(a) Thin film and

(b) Wire wound.

Thin Film

Thin-film RTD elements are produced by depositing a thin layer of platinum onto a substrate. A pattern is then created that provides an electrical circuit that is trimmed to provide a specific resistance. Lead wires are then attached and the element coated to protect the platinum film and wire connections.
Thin film elements are available in the.

European standard (0.00385 Ω/Ω/°C), and in a special version, used primarily in the appliance industry, that has a temperature coefficient of 0.00375 Ω/Ω/°C. Thin film elements are not available in the American standard.

Wire Wound:
RTD elements also come in wire-wound constructions. There are two types of wire-wound elements:

(a)Those with coils of wire packaged inside a ceramic or glass tube(the most commonly used wire-wound construction), and

(b)Those wound around a glass or ceramic core and covered with additional glass or ceramic material (used in more specialized applications).

Wiring Arrangement of RTDs:

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In order to measure temperature, the RTD element must be connected to some sort of monitoring or control equipment. Since the temperature measurement is based on the element resistance, any other resistance (lead wire resistance, connections, etc.) added to the circuit will result in measurement error. The four basic RTD element wiring methods according to the IEC/ASTM color  codes are:
(a) 2 Wire configuration
(b) 3 Wire configuration
(c) 4 Wire configuration
(d) 2 Wire configuration with compensating loop.

2 Wire configuration RTD:

This wire configuration provides one connection to each end of the RTD sensor. This construction is suitable where the resistance of the run of lead wire may be considered as an additive constant in the circuit, and particularly where the changes in lead resistance due to ambient temperature changes can be ignored. This wire configuration is shown below:

  

Note that the resistance of probe and extension is added to the RTD resistance and will increase the measured value. This could be a source of error in applications where high accuracy is required.

 

3 Wire Configuration RTD:
This is the standard wire configuration for most RTDs. It provides one connection to one end and two to the other end of the RTD sensor. Connected to an instrument designed to accept three-wire input, compensation is achieved for lead resistance and temperature change in lead resistance. This is the most commonly used configuration.

  

4 Wire Configuration RTD:
This wire configuration provides two connections to each end of the RTD sensor. This construction is used for measurements of the highest precision.

2 Wire Configuration RTD with Compensating Loop:
This is similar to 4 wire configuration RTD except that a separate pair of wires is provided as a loop to provide compensation for lead resistance and ambient temperature changes in lead resistance. 

For more information on RTD Sensors, check out:

• How to Specify an RTD Sensor
• Fundamentals of RTDs
• Application Limitations, Types and Failure Modes of RTD
• How to Convert RTD Resistance to Temperature

Basics of Resistance Temperature Detectors (RTDs)

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Resistance temperature detectors (RTDs) are based on the principle that the electrical resistance of a metal increases as temperature increases. Thus, a temperature measurement can be inferred by measuring the resistance of the RTD element.. A basic construction of an RTD is shown below: