How to Calibrate a Thermocouple Transmitter

Custom Search

To calibrate a thermocouple transmitter will require a thermocouple simulator with an accuracy of at least four times greater than the thermocouple sensor we desire to calibrate. 

Equipment and Materials Required

The following equipment/materials are required to successfully calibrate a thermocouple transmitter:

1. Thermocouple Simulator (of at least four times the accuracy of sensor)
2. Two Digital Voltmeters (Five-digit readout) with accuracy of at least ±0.01% with resolution 1mV
3. 24 VDC Power Supply of at least 35 – 40mA current output
4. Thermocouple wire of the same type of wire the thermocouple transmitter is constructed of.

Equipment Setup

Below is the equipment set up for the calibration

Calibration Procedure

1. Remove the thermocouple transmitter terminal housing cover
2. If the transmitter is already connected, remove all the thermocouple lead connections.
3. Determine the base and full scale temperatures. Read: How to convert thermocouple millivolt to temperature.
4. Turn power supply on.
5. Consult the thermocouple simulator manual for instructions on setting the thermocouple type and engineering units.
6. Set the simulator to the base (zero) temperature and adjust the zero pot until the output is 4mA or 40mV at the test terminals
7. Set the simulator to the full scale temperature and adjust the span pot until the output is 20mA
8. Repeat steps (1-7) above until both the 4 and 20mA readings are obtained without re-adjusting the span and zero pots.

Temperature Sensors Selection Criteria

Custom Search

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

Thermocouple Concepts: Cold Junction Compensation(CJC); Thermocouple Loop Resistance; Thermocouple Degradation

Custom Search

Thermocouple Cold Junction
A thermocouple has two junctions. The difference in temperature of these junctions is what is used to measure temperature. One is called the hot junction which is inserted in the process whose temperature is required to be measured while the cold junction also known as the reference junction is the termination point outside of the process where the temperature is known and where the voltage is being measured. Typically the cold junction is located in a transmitter or signal conditioner.

Thermocouple Cold Junction Compensation (CJC):
The voltage measured at the cold junction correlates to the temperature difference between the hot and cold junctions; therefore, the temperature at the cold junction must be known for the hot junction temperature to be accurately determined. This process is known as cold junction compensation.

How to Determine Thermocouple Accuracy with Initial Calibration Tolerances

Custom Search

Thermocouple accuracy is the amount of error which exists in its temperature measurement. It indicates how close the measured temperature value of a given thermocouple is close to the true temperature value. It is called the tolerance or error. A table called the “Initial Calibration Tolerances” tells us what accuracy or tolerance we can expect from a given thermocouple.

Thermocouple Wire Color Codes

Custom Search

Thermocouple (T/C) lead wires consist of two individual wires (positive and negative), enclosed by coloured insulation sheath:

Thermocouple Wire Insulation Colour Coding

Due to the Seebeck effect, thermocouple wires have a set polarity such that positive and negative wires must be connected to the correct terminals. A variety of standards exist for lead wire insulation colours to identify each thermocouple type.The different standards utilize unique wire colours to differentiate

Common Thermocouples Application Problems and their Remedies

Custom Search

Just like RTDs, thermocouples when applied for temperature measurement present their own peculiar application problems. To understand and be able to solve these problems requires a comprehensive understanding of thermocouples as temperature measurement device. You can start with the basics of temperature measurement with thermocouples here.

To help resolve some of the problems you might need to tackle when using thermocouples, here is a list of the common problems encountered. This is not a comprehensive list anyway:

How to Use a Thermocouple: Practical Application Tips

Custom Search

A thermocouple is said to be a ‘’simple’’ temperature measurement device. With a difference in temperature between its cold junction and hot junction, you have a voltage reading that gives you an indication of the temperature being measured. But is this really a simple device?

Factors to Consider When Selecting a Thermocouple for Temperature measurement Application.

Custom Search

There are many different types of thermocouples. Each has its advantages and disadvantages over other types of thermocouples that you may find in the market. Some of the factors to guide your selection of thermocouple for any given applications are discussed below.

How to Convert Thermocouple Milivolts to Temperature

Custom Search

The voltage generated by thermocouples is very small. They are in the order of milivolts. In the application of thermocouples to measure temperature, it is often required to convert the milivolts signals of thermocouples to temperature values. To aid this conversion, several milivolts voltages for the different types of thermocouples are tabulated against known standard temperatures. With these thermocouple reference tables, it is then easy to determine any given temperature for known milivolts values.

Thermocouple reference tables are based on a reference junction of 0 degree C. If the reference junction is not at 0 degree C, then a correction factor must be applied.

Calculating Temperature from Voltage (reference junction = 0 degree C)
The steps involved are:

• Select the correct reference table for the thermocouple type in use. e.g. J,S,T etc

• Locate the milivolt reading in the body of the table, and read from the margins the temperature value.

Note that the temperature determined from a particular thermocouple reference table only gives accuracy to that of the increments on the scale in the table. For more accurate measurement,

Reducing Noise In Thermocouple Installations

Custom Search

As you may already know, a thermocouple is formed by joining two different metal alloys at a point called a junction. This junction is called the measuring or hot junction. The thermocouple leads are usually attached to a temperature indicator or controller. This connection point is called the reference or cold junction.

When the measuring junction is heated, a small DC voltage is generated in the thermocouple wires. The temperature controller measures the small voltage signal and converts it to a temperature reading. However, the voltage generated in the thermocouple is so small that

Temperature Measurements With Thermocouples

Custom Search

Temperature sensors:
Temperature is the measure of average molecular kinetic energy within a substance. This follows that as the kinetic energy of the substance increases so does the temperature. Temperature measurement relies on the transfer of heat energy from the process material to the measuring device. The measuring device therefore needs to be temperature dependent.

There are two main types of industrial temperature sensors namely: