Thermocouple reference table – how to read one
So what exactly is a thermocouple reference table, and how do you read it? If you studied heat transfer at some point in your degree, then you know what I’m talking about. If you didn’t, then I’ll catch you up.
Reference tables tell you the material properties of a certain pressure or temperature. A thermocouple chart works the same way. And don’t worry. Thermocouple tables are way easier to read than many other thermodynamic tables!
In this article, we’ll show you how to check your thermocouple temperature with a multimeter. For that, we’ll use two examples, one with a type K thermocouple table and the other on a type T thermocouple chart.
Before you start
You have a few concepts you should know before you pull your thermocouple voltage with your multimeter and check the voltage to temperature conversion table. When you scale out a thermocouple, you’ll always see the junction as zero degrees Celsius (0° C). So what does that mean? That means you don’t have extra voltage because 0° C equals zero millivolts (0 mV), right? Nope.
In reality, you usually have a higher temp in the cold junction. Therefore, if you don’t compensate for that, 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 again. When you connect the voltmeter, you create a cold junction! You need to read up on the law of intermediate metals, also known as the 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 what type of thermocouple you have. And last but not least, you need to check the reference tables that match 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.
Type K thermocouple reference table
To start things off, let’s check out this type K thermocouple chart.
The first column on the left (in red) 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 easy example based on this type K thermocouple table. So say you have 3.474 mV. Now scan the table, and you’ll find that number in the row of 80 and the column of 5. That makes 85° C. Tada!
Cold junction compensation
Okay, say you have an ambient temperature of 24° C, and you check a type K thermocouple with your voltmeter. How do you find your temperature?
First, remember the cold junction. The 24° C applies here. If you check the thermocouple reference table, then you’ll see the value for 24° C, 0.960 mV. Now that you have these numbers, do some simple math.
So let’s say the voltmeter gave you 4.409 mV. Just add that number to the value you found for the ambient temperature, and you should get 5.369 mV.
Next, look at the table, and you’ll see that value equals 131° C. Simple, right? Now let’s move on to the example with the type T thermocouple chart.
Type T thermocouple chart
No secrets here. The procedure to read the type T thermocouple table is exactly the same as the one from the type K thermocouple table. We’ll make this example a bit more complex though.
Now suppose you measure the temperature of a process in a nice controlled room temp of 18° C. Since you know the ambient temp, go to your type T thermocouple chart and look for the temperature to millivolt conversion.
Column 8 and row 10 will give you 18’s value, 0.709 mV. So far so good, right? Now you get a thermocouple voltage of 2.759 mV from 0° C, so do the reference junction compensation: 2.759 + 0.709 = 3.468 mV. Basic math. Then with this result, you have one more step to go. So look on the table for the temperature value which matches 3.468 mV.
But then you see that the table doesn’t have 3.468! Because our voltage falls between 82 and 83° C, we’ll need to interpolate the value from the table. So how do we do it?
To save some time, instead of explaining it step-by-step, I’ll show you the final formula.
- Tm = temperature measured (°C)
- Vm = thermocouple voltage measured
- Tl = lower temperature on the thermocouple reference table
- Th = higher temperature on the thermocouple reference table
- Vl = voltage referent to Tl
- Vh = voltage referent to Th
With this formula, you just throw your numbers in. Here we have Tl at 82 and Th at 83. Then we have 3.448 and 3.494 as their respective temp-to-voltage conversions. Vm comes from our measurement plus the cold junction compensation, 3.468.
My calc app says Tm = 82.43° C. Anyone else get a different answer?
So there you go. Now you know how to measure the temperature of a thermocouple with a multimeter. Did this article help? Do you have any questions? Please leave them in the comments below, and we’ll do our best to answer them to your satisfaction!
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