Hoe thermocouple calibrationis done

Thermocouples calibration is not the same as instruments because they cannot be calibrated as they are non adjustable.However its functioning can be validated by using simple process.

At home or lab we can do it by following procedure

We use Standard temperature source large enough so that the thermocouple can be inserted into it like ice bath.We can use  the boiling and/or freezing point of a series of pure solvents as those temperatures are well known at STP.We measure the voltage output of thermocouple.Plot that on the Voltage temperature graph.And see if it fits the graph obtained from the thermocouple chart.Since thermocouple chart  use reference junction to be Zero degree.We need to find the room temperature(Reference junction) using thermometer etc and covert it to voltage using charts  and add that to voltage being measured by voltmeter.It comes in temperature range of about 100 to 1200 degC as thermocouple are more accurate in higher range temperatures.

Another such heat source is thermocouple calibration furnace.They have fast heating time.Accurate temperature display about+-0.5decC.Also they can be used to calibrate multiple thermocouples at the same time.Thermocouple calibration furnace

 

Calibration at Industries.

Temperature of heating equipment,furnace ,solvents etc shown by Thermocouple can be read directly by SCADA.SCADA stands for (supervisory control and data acquisition.)These are systems which are used to collect data from various sensors locates at different areas of the factory.It then sends them to main computer which moniters and control them.There is a seperate hardware interface (Controllers like PLC Input output cards etc),which is used is used for controlling of various parameters .It can also be controlled manually through HMI(Human machine Interface) or the main computer with the necessary software installed.

And to validate the temperature we measure the millivolts at thermocouple end using voltmeter .Voltage is measured at two ends of thermocouple near the main probe.
Now  using ASTM E230-03 (Standard Specification and Temperature-Electromotive Force (EMF) Tables for Standardized Thermocouples”) find the temperature in degree centigrade  for the measured voltage or use a thermocouple calibrator (a digital thermometer with type setting)which will tell the temperature of according to the voltage measured for the type of thermocouple set in the meter.For ex,if it is set to Type K it will read temperature according to type K thermocouple.The same temperature should be shown by SCADA instruments.While using reference tables Diiference in hot and cold junctio is considered. To deter mine the temperature at thermocouple probe ,junction temperature where voltmeter is used  can be considered to be room temperature.

 

Some of the top SCADA instruments manufacturers are

Intellution,Rockwell Software,Wonderware,Citect,GE/Fanuc,Siemens,Iconics
USDATA,National Instruments,Indusoft.

 

How to read the chart

It consist of rows of values showing millivolt.

The reference junction(cold junction) is Zero degree C for all data.

Each row consist of  eleven values

Voltage value is for temperature corresponding to the row containing that voltage value plus the temperature corresponding to column containing that voltage value.

Typical Thermocouple table

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