Testing a thermocouple using Mvolts

[onlyfitidealboilers]

what reading would you expect ? & how do you go about connecting your multi meter to it? I have never in my life had to do this but would like to know..

Thanks as always

 

[Gas2Air]

It's very simple. Have one end of the meter on the TC branch the other on the gas valve connection. Heat it up and measure in Mv. I have only done this once years ago on a practical test at interview, otherwise it's either shot or breaking down so no need to bother. If you do it i think it gives 22Mv from memory. Resistance is even too low to measure and can give misleading results.

 

[Agile]

Unfortunately measuring a T/C open circuit voltage is pretty irrelevant but would be about 22-23mV.

What is relevant is the voltage ON LOAD. Either use a specific TC test meter which should have a load inside or take a magnetovalve from an old GV and connect that and measure the on load voltage which should normally be about 8-13 mV.

When you have an interupter you should measure the voltage to chassis from each connection going to the o/h stat.

A high resistance o/h stat is a common fault on older boilers. It should not drop more than 1-2 mV as measured at the interrupter.

Most GVs need at least 8 mV to reliably hold the magneto valve on although with a few its 7 mV. In most cases with no interrupter its about 11-13 mV with a new TC.

Best discussed in the CC.

Tony Glazier

 

[Weedrip]

Interesting point of view... I was taught that open voltage is important, or to be more precise the relationship between open and load voltage will assist in finding what is failing.

Here is a table of voltages that will help to understand what might be going on. When testing a TC. it is also important to test with the fitted load to properly identify the possible fault.

View media item 32616
Or just bung a new TC on and if it cures the problem... its sorted.