Temperature Sensors Selection Criteria

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