Learn how to read thermocouple tables
Wow, we had a lot of temperature content in March, didn’t we? I wrote an article listing the main temperature sensors we have in instrumentation, like thermocouples and RTDs.
When I wrote about thermocouples, I mentioned needing tables to translate your data from millivolts to temperature. You probably think that if you connect the sensor to the transmitter, then you don’t need a table to find the temperature. True, but you need to think of different situations. What if the operator tells you the transmitter isn’t reading correctly? How can you solve that?
You have a number of ways, and reading the millivolts from the sensor is one. Learning how to read the table might help you and others in the future.
You have a few concepts you should know before you pull your numbers and scan the tables. When you scale out a thermocouple, you’ll always see the junction as 0 degrees Celsius. So what does that mean?
When you have 0 degrees Celsius in the cold junction, you don’t have additional voltage because 0 degrees Celsius is 0 millivolts, right? Now, in reality, you usually have a higher temperature in the cold junction. Therefore, if you don’t compensate, then you’ll get the wrong value.
Okay, some of you just thought, “I don’t have a cold junction. I can just go in with my voltmeter and check the voltage directly!” Nope. When you connect the voltmeter, you create a cold junction. You need to read up on the law of intermediate metals, or law of thermocouples. Go ahead.
We’ll wait. Welcome back! Let’s move on.
Okay, down to the nitty gritty. In case you missed it earlier, you need a voltmeter or a multimeter to check the thermocouple voltage. Next, you need to know the type of thermocouple in your process. And last but not least, you need to download the table of reference that matches your thermocouple.
Okay, but what does a reference table look like? Good question! Have a look, and don’t panic. I’ll teach you how to read it.
Here we have the table for a type K. The first column (in red) on the left shows temperatures in increments of 10. Okay, so if you have 14, then where do you find 14 on the table? Easy. Start by finding 10 on the left. Then slide right to find the column with 4 above it (row in blue). Yup, that gives you 14. It works the same for 105, 66, 92, and so on.
The numbers in the yellow box are the millivoltages. If you know the millivoltage, then find it in the table and follow the column and row to find your temperature.
Let me give you an example based on this table. You have the millivoltage of 3.474. That number sits in the row of 80 and the column of 5. That makes 85 degrees Celsius. Tada!
So say you have an ambient temperature of 24 degrees Celsius, and you check a type K thermocouple with your voltmeter. How do you find your temperature?
First, remember the cold junction. The 24 degrees Celsius applies here. If you check the reference table, then you’ll see the value for 24 degrees Celsius, 0.960 millivolts. Now that you have these numbers, you need to do some simple math.
So let’s say the voltmeter gave you 4.409 millivolts. Just add this number to the millivolt value you found for the ambient temperature, and you should get 5.369 millivolts.
Next, look at the table. The value you found equals 131 degrees Celsius. Simple, right? Hope this information helps in your daily activities.
Check out our other content on temperature:
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