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Anonymous Poster #1

Thermocouple Measurement

08/08/2016 1:21 PM

We do have k type thermocouple on our boiler while measuring on the thermocouple directly we get around 480 F on the multimeter . Upon conecting on 3 mtr k type cable getting and measuring on the cable with multimeter I am getting 580 F . 100 more than direct measurement

We are measuring . Why this is happening and how can I suppress it .

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#1

Re: Thermocouple

08/08/2016 1:41 PM

One possibility is that your multimeter measurement isn't taking into consideration ambient temperature, and the effects of the "reference junction" created by clipping copper test leads onto thermocouple wire. This will cause it to register colder than it actually is, by whatever the ambient temperature happens to be.

If the ambient temp close to the thermocouple is a lot hotter than 3 meters away, that could explain it.

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#4
In reply to #1

Re: Thermocouple

08/08/2016 2:20 PM

Hi

Ambient temperature is around 50 C . The sensor is inside the boiler . And the conection is just outside the boiler . If I conect the cable even 1 mtr I am getting 100F more . I can't understand if I can trust the thermocouple itself.Is there any other stuff that I should check

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#5
In reply to #4

Re: Thermocouple

08/08/2016 2:23 PM

How are you interpreting the multimeter's voltage measurement? Are you simply looking up the temperature for that millivoltage in a type K thermocouple table? If so, then you're skipping the important step of manually compensating for the ambient temperature's effect on the reference (or "cold") junction formed between your multimeter's test leads and the thermocouple wires.

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#7
In reply to #5

Re: Thermocouple

08/08/2016 2:30 PM

I just check the milli volt directly to the cable and directify to the thermocouple . And it is fluke there is an option to check the temp direct by chossing mv then F or C

I check on another boiler for reference . It came only around 7 F difference between thermocouple and end of cable

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#9
In reply to #7

Re: Thermocouple

08/08/2016 2:45 PM

That explains it.

By directly referencing a thermocouple table with the meter's millivoltage reading, you are not taking into consideration the voltage produced at the junction between your meter's test leads and the thermocouple wires.

Setting the Fluke meter to register in degrees won't work, because your meter's reference junction compensation is built into the meter itself, and therefore is basing that compensation on the temperature of the meter which is substantially cooler than where you're connecting copper wires to the thermocouple leads.

In order to properly measure the thermocouple signal, you are either going to have to carefully measure the ambient temperature where copper test leads meet chromel/alumel wires (and do the manual compensating math) or use a internally compensated meter with thermocouple extension wire all the way up to the meter's input jacks and give that meter time to settle at that ambient temperature.

In summary, the thermocouple is likely just fine -- the problem is how you're measuring its signal.

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#2

Re: Thermocouple

08/08/2016 1:49 PM

unless

- the multimeter is designed for a K Thermocouple input (meaning it has cold junction reference capability (most multimeters do not), and

- the multimeter's cold junction reference has been allowed to stabilize or equilibrate for 10 minutes so it is reporting the correct cold junction temperature, and

- the thermocouple is connected to the meter using the correct type extension or thermocouple wire (not copper wire), and

the mV reading on the multimeter must be corrected for cold junction error.

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#6
In reply to #2

Re: Thermocouple

08/08/2016 2:23 PM

Will Try and update

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#3

Re: Thermocouple

08/08/2016 1:52 PM

Get an IR thermometer, they're idiot proof. I've got one.

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#10
In reply to #3

Re: Thermocouple

08/08/2016 4:11 PM

Well, that ought to just about prove your statement.

Actually, I think the AP/OP did not make the first connection correctly, as there is only one correct way to connect to a thermocouple, and anything that uses two copper leads connected directly to Chromel and Alumel (type K), is a really bad idea if accuracy of any nature is desired.

There are compensating blocks that can be used, many time people just don't understand the physics enough behind thermocouples to make them work correctly, or they don't know that they don't know they need more parts (such as the special connector that goes on the end of these if they are to be directly connected to a meter).

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#11
In reply to #10

Re: Thermocouple

08/08/2016 5:14 PM

I think the OP has not a clue.

Worse yet, he doesn't know how to find out.

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#8

Re: Thermocouple

08/08/2016 2:32 PM

How were the connections made?

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#12

Re: Thermocouple Measurement

08/09/2016 4:52 AM

A thermocouple works based on the principle of the voltage produced when two dissimilar metals are connected together.

Every junction of two dissimilar metals will produce a voltage that varies with the temperature of the junction.

So every time you splice leads to a themocouple,the leads must be the same type and polarity. Likewise with the meter leads.

If the polarity is reversed at the splice,the voltages will be inaccurate,so consider all of the connections to be a potential source of an error.

Check polarity at all junctions carefully.

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#13
In reply to #12

Re: Thermocouple Measurement

08/09/2016 8:12 AM

Currently checked . As furnished I have doubt with the conector . Trying to change to conector. And check

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#14
In reply to #13

Re: Thermocouple Measurement

08/09/2016 12:49 PM

They make terminal blocks for the application.

Ceramic base

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#15

Re: Thermocouple Measurement

08/09/2016 10:26 PM

Here are few advises

1. Use Type-K connector and remember that they have marking for which wire goes where. Both wires are not identical.

2. Use Type-K compensating cable and again watch out to match the cable and don't interchange them.

3. If Multimeter has Type thermocouple input option then read it there and not in mV scale option else room temperature compensation won't work. Check the Multimeter with small type-k thermocouple to see if it reads the room temperature correctly.

4. Give time for the Multimeter / type K temperature reader to stabilise itself and then read the temperature.

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#16

Re: Thermocouple Measurement

08/10/2016 12:00 AM

A friendly reminder: In the thermocouple world, the red wire is negative.

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