Skip to main content

Thermocouple Calibration

 Thermocouples

Thermocouples measure temperature and are used quite often in process controls.


How They Work

When wires of two different thermoelectrically homogeneous materials are joined at one end and placed in a temperature gradient, a thermoelectric voltage (EMF) is observed at the other end. The connection is called the measuring junction.

On all thermocouples, the red lead is negative. The color of the other wire indicates the thermocouple type.

For example, on a J-type thermocouple, the positive wire is white. Tables for each type of thermocouple list the voltages produced at various temperatures.

Thermocouples should be checked whenever there are indications that the output is not accurate.

It may also be necessary to check a thermocouple that will be used for a measurement standard.

Input and Output Measurement Standards

A temperature bath provides controlled temperatures for testing a sensor. A well in the temperature bath is used to hold the sensor during the accuracy check.

Another well is used to hold a measurement standard thermometer, it is used to confirm the actual bath temperature.

A second measurement standard thermometer is used to read the ambient temperature at the reference junction.

The sensor output signal and ranges determine the thermocouple output measurement standard.

Since the output is measured in millivolts, a millivolt meter is used to read the output.

Connections

Set up the temperature bath as a temperature input standard to the thermocouple.

Select the output standard with the appropriate range for reading millivolts.

Connect the red lead to the negative millivolt meter input and the white lead to the positive millivolt meter input.

Three Point Checks

Because no adjustments are possible, we can only check the calibration of a thermocouple sensor.

This check is generally done at three test point input values: ambient temperature, mid-range temperature, and upper value of the application range.

Recall that in a thermocouple, it is the difference in the temperature between the reference and the measuring junction that produces a millvoltage output.

Before inserting the thermocouple into the bath, determine the ambient temperature, which represents the temperature at the measuring junction of the thermocouple.

When using look-up tables that are referenced to 0 Deg., you must compensate for ambient temperature.

The millivolt value in the tables for the ambient temperature is added to the value from the sensor.

This compensated millivolt value is used to determine from the tables the correct temperature.


Also Read : Thermocouple Tables for millivolt to Temperature conversion


Thermocouple Transmitters

Calibration of temperature transmitters should be checked on a periodic basis.


Input and Output Measurement Standards


The temperature transmitter discussed here receives an input signal from a thermocouple.


A millivolt input signal will be needed for calibration, so a millivolt source can be used for the input standard.


A milliammeter can be used to measure the transmitter output. Use a standard thermometer to calculate the input signal compensation for the ambient temperature.


Finally, a power supply for the transmitter is necessary.


To calibrate a temperature transmitter with a millivolt meter as an input standard, you must compensate for any reference temperature other than 0 Deg. C ( 32 Deg. F.).


Connections


To make the input connections, the location of the reference junction must first be determined.

When thermocouple wires are used to connect the millivolt source to the transmitter, the reference junction is at the transmitter connection.

So, ambient temperature is measured in the transmitter housing. If copper wires are used, the reference junction is at the connection to the millvolt source, so measure ambient temperature at the millivolt source.

Always observe the polarity of the leads. Connect the negative output from the millivolt source to the positive transmitter terminal.

Connect the milliammeter in series with the transmitter and the power supply.

Setting the Equipment

Adjust the millivolt source and the milliammeter to the proper values as required, turn on equipment and begin calibration.

Five Point Check

Perform a five point check to determine if the transmitter is accurate according to specifications.

Accuracy of the Instrument

Adjust the zero shift first. It should be set with an input value of 10%.

With the zero properly set, a 10% input results in a 10% output. Adjust the span using a 90% input.

The zero and span may interact, check and readjust as required.



Labels:

Comments

Post a Comment

Popular posts from this blog

PLC Program for Mixing Tank

 Create a ladder diagram for controlling a batch mixing process. Implement a PLC program for mixing tank or Mixing Process using PLC Ladder Logic. PLC Program for Mixing Tank Fig : Mixing tank A tank is used to mix two liquids. The required control circuit operates as follows: A. When the START button is pressed, solenoids A and B energize. This permits the two liquids to begin filling the tank. B. When the tank is filled, the float switch trips. This de-energizes solenoids A and B and starts the motor used to mix the liquids together. C. The motor is permitted to run for 1 minute. After 1 minute has elapsed, the motor turns off and solenoid C energizes to drain the tank. D. When the tank is empty, the float switch de- energizes solenoid C. E. A STOP button can be used to stop the process at any point. F. If the motor becomes overloaded, the action of the entire circuit will stop. G. Once the circuit has been energized, it will continue to operate until it is manually stopped. Solution...
1 comment
Read more

What is Relay? How it Works? Types, Applications, Testing

 We use relays for a wide range of applications such as home automation, cars and bikes (automobiles), industrial applications, DIY Projects, test and measurement equipment, and many more. But what is Relay? How a Relay Works? What are the Applications of Relays? Let us explore more about relays in this guide. What is a Relay? A Relay is a simple electromechanical switch. While we use normal switches to close or open a circuit manually, a Relay is also a switch that connects or disconnects two circuits. But instead of a manual operation, a relay uses an electrical signal to control an electromagnet, which in turn connects or disconnects another circuit. Relays can be of different types like electromechanical, solid state. Electromechanical relays are frequently used. Let us see the internal parts of this relay before knowing about it working. Although many different types of relay were present, their working is same. Every electromechanical relay consists of an consists of an Elect...
1 comment
Read more

Chlorine dioxide Analyzer Principle

 Chlorine dioxide measurement Chlorine dioxide (ClO2) is an instable, non-storable, toxic gas with a characteristic scent. The molecule consists of one chlorine atom and two oxygen atoms – represented in the chemical formula ClO2. It is very reactive. To avoid the risk of spontaneous explosions of gaseous chlorine dioxide or concentrated solutions, it is generally handled in dilution with low concentrations. ClO2 is soluble in water, but tends to evaporate quickly. Typically it is prepared on site, for example from hydrochloric acid and sodium chlorite. The procedure provides solutions with approx. 2 g/l ClO2 that can be safely handled and stored for several days. Image Credits : krohne Sensor Parts : Reference electrode Applied chlorine dioxide specific potential Current needed to maintain the constant potential Counter electrode Measuring electrode The disinfection effect of ClO2 is due to the transfer of oxygen instead of chlorine, so that no chlorinated byproducts are formed. C...
6 comments
Read more