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Products => Test Equipment => Topic started by: neslekkim on March 10, 2014, 12:16:53 pm

Title: Measuring Thermocouple resistance
Post by: neslekkim on March 10, 2014, 12:16:53 pm
I have been trying to measure resistance on some soldertips with built in heater and thermocouple (JBC 245 and 210 tips), but since an thermocouple also gives some voltage, it seems like it's not that easy to measure this correct?
I have been using Fluke 87-v, Fluke 289 and Agilent 34461A

Both fluke's are just rolling the numbers, impossible to take a read, but the Agilent it's easier to get something, but not steady always.
Someone said that the Agilent U1272A have an smart ohm function which can measure even if you have voltage..

How can i do this?, Do I need 4wire probes for my Agilent?, and is the Flukes unusable for this?
(I'm also considering getting an DE5000 LCR meter, does those meters understand this type of resistance?)
Title: Re: Measuring Thermocouple resistance
Post by: macboy on March 10, 2014, 12:32:39 pm
You need a meter that can measure "offset compensated" resistance. This is intended to compensate for the thermoelectric/thermocouple effects of the dissimilar metals used in the the wires, connectors, solder joints, etc. in all the connections between the meter and the device under test, but it will work fine for the thermocouple effect of a ... thermocouple.

A question that needs to be asked though, is why do you need to know the resistance of the thermocouple?
Title: Re: Measuring Thermocouple resistance
Post by: neslekkim on March 10, 2014, 01:44:48 pm
Quote from: macboy on March 10, 2014, 12:32:39 pm
A question that needs to be asked though, is why do you need to know the resistance of the thermocouple?

Trying to find out where the thermocouple is, and where the heater is :)
On the C245 tips, there are bigger difference in the resistance so it's kinda easier to see it, but on C210 there are very small resistance on the heater, so it was difficult to find out what is what..
But since my meters behaved so strange I got a little caught up with this..

Can't any of my meters handle this?, do I need to buy an "cheap" agilent handheld to get this to work?
Title: Re: Measuring Thermocouple resistance
Post by: ruffy91 on March 10, 2014, 03:34:49 pm
You could just measure the Voltage on the 4 leads to find out where the thermocouple is, the heating element will read 0V, the thermocouple a few mV if you heat it with a lighter.
Title: Re: Measuring Thermocouple resistance
Post by: cyr on March 10, 2014, 05:37:08 pm
You should be able to do offset compensation manually on any DMM with 4-wire ohms.

https://www.youtube.com/watch?v=l_BclZUWgDU (https://www.youtube.com/watch?v=l_BclZUWgDU)
Title: Re: Measuring Thermocouple resistance
Post by: neslekkim on March 10, 2014, 11:32:25 pm
So, then I have to find some decent kelvin probes?