Type-K thermocouples - any users out there?

[Eric The Viking]

I'm refurbishing our solar heating system (oh the joys of lockdown!), and I would like to neaten-up the thermocouple wiring.

The annoying bit goes up to the sensor on the roof. The cable is far too long, and a lot of it is in a (literally) tacky coil "secured" to a rafter with insulating tape by the original installer (who also used phono plugs/sockets to extend it!).

I checked and reinstalled the actual thermocouple into one of the panels a while back and now can't easily get access to it. I think it's type K, as from memory on the bench I got it to track nicely with the other ones I have (have a digital voltmeter that is sold as a temperature readout, into which type K ones plug, with a direct temperature readout).

My assumption is that, however it's "read" by the heating controller, it must be a high resistance/impedance input, and therefore I can shorten the cable without causing problems. Am I correct, or are there other common ways of doing this that might give pause?

TIA for any advice on the thermocouple cable.

E.

In case anybody is interested, the solar controller is a Resol DeltaSol B - a bit old but elegantly simple and functional. I see no need to upgrade it at present.

 

[sunnybob]

I used to work with lots of varied heating machinery, we fitted new and maintained them. I have no technical grounds for this but our tech manager would never let us shorten sensor cables. We always had to coil or just plain hide them neatly.

 

[Eric The Viking]

I used to work with lots of varied heating machinery, we fitted new and maintained them. I have no technical grounds for this but our tech manager would never let us shorten sensor cables. We always had to coil or just plain hide them neatly.

I have just found the documentation for our system - Googling indicates that they are actually Platinum Resistance Thermometers (PT-1000). The installation guide talks about extending the cables (using co-ax, which might explain the phono plugs!).

There doesn't seem to be any calibration routine, but it has a table at the back that gives a resistance of 1kOhm at 0 deg C (might explain the "1000" bit). I must have mis-remembered checking a different thermocouple!

Crucially it should be 1385-1404 Ohms at the upper usable temperature of around 104 deg C (the system can be slightly pressurised - in high summer the panels do get right up to around 104). The amount of extra cable I want to remove would only amount to perhaps 0.5-1 Ohm, and I can probably get a thermocouple to the back of the same panel close to the sensor (the actual sensor is inaccessible now), so can double check actual temp before recommissioning.

I think I can work with that!

Many thanks.

E.

 

[Sideways]

This might be worth a quick look.
https://www.sciencedirect.com/topics/en ... hermometer
A PT-1000 unit by definition, is designed to have a resistance of 1000 Ohms at 0 degrees C,
To use it for precision measurement at a distance, correction has to be made for the resistance of the connecting cables, and that their resistance also changes with temperature.
This is typically done by using a cable with extra conductors and the article explains.
As yours is set up using coax (2 core cable) I guess the circuit / temp sensing doesn't need super precision and they either omit the compensation for the cables entirely (the resistance of the coax will be quite small, especially compared to 1000 Ohms plus of the platinum resistor) or the circuit is designed allowing a fixed resistance for the connecting cable.
That leaves us unable to give clear advice. If the circuit has no compensation at all, shortening the cables should improve accuracy. If it's designed for a fixed cable resistance, then as Bob suggests, best to leave well alne as it it could make it less accurate.

Oh, and last point is that longer cable will increase the overall resistance and make the system think the temperature is (maybe a degree or two) higher than it really is. So there shouldn't be a safety risk at high temperatures as the system will over read.
Assuming the system has antifreeze atc, having it think that it is 2 or 3 degrees when it's actually zero outside may not be an issue.

 

[Eric The Viking]

This might be worth a quick look.
https://www.sciencedirect.com/topics/en ... hermometer
A PT-1000 unit by definition, is designed to have a resistance of 1000 Ohms at 0 degrees C,
To use it for precision measurement at a distance, correction has to be made for the resistance of the connecting cables, and that their resistance also changes with temperature.

Yup, I realised once I twigged it wasn't a thermocouple (what a twerp - still don't know where that came from - I do remember testing it on the kitchen table though...).

This is typically done by using a cable with extra conductors and the article explains.

Will read the article but I assume the other pair are shorted at the sensor and used on the other side of a Wheatstone bridge or similar, which would null out the cable resistance.

As yours is set up using coax (2 core cable) I guess the circuit / temp sensing doesn't need super precision and they either omit the compensation for the cables entirely (the resistance of the coax will be quite small, especially compared to 1000 Ohms plus of the platinum resistor) or the circuit is designed allowing a fixed resistance for the connecting cable.

I suspect it's the latter, if at all. I've re-read the manual and there is nothing about distance compensation etc. And, although the manual says use coax, the actual installation has been extended with fairly thin mains flex (2-core, rated at around 5A at a guess - I'll measure its resistance with an AVO as my handy DMM is not much good on low resistance ranges. He did use coax on the short run to the tank though! The sensor tails themselves aren't screened.

That leaves us unable to give clear advice. If the circuit has no compensation at all, shortening the cables should improve accuracy. If it's designed for a fixed cable resistance, then as Bob suggests, best to leave well alone as it it could make it less accurate.

Oh, and last point is that longer cable will increase the overall resistance and make the system think the temperature is (maybe a degree or two) higher than it really is. So there shouldn't be a safety risk at high temperatures as the system will over read.

Good point, that!

Assuming the system has antifreeze atc, having it think that it is 2 or 3 degrees when it's actually zero outside may not be an issue.

It's no biggie. It's a "drainback" system, meaning that the water returns under gravity to a tank in the warmer part of the house whenever the pump goes off. This prevents ice causing damage and similarly any risk of boiling on hot summer days.

I did wonder about putting a load/compensation resistor in (there is a lot of loose cable I'dlike to remove), but it occurred to me that too would have a positive coefficient, so the cure might be worse than the disease (and anyway it would need to be very small).

As you point out, over-reading is inherently failsafe, so I suspect it has no compensation. I think it's worth me sending a quick email to the manufacturer to check.

Thanks both - really appreciative!

E.
PS: just read the article Wheatstone bridge arrangement as I suspected, and of course it wouldn't be a thermocouple, as they really need a second junction at a known temperature for accuracy, whereas resistance measurement doesn't. I get my 'wake yer brain up' *** next Friday!

 

[Just4Fun]

In case anybody is interested, the solar controller is a Resol DeltaSol B

Mine is a Resol DeltaSol BS Pro which I assume is basically the same thing. As an electrical ignoramus I just extended one of the four sensor cables on my system without regard to the technical sensitivities identified by the more tech savvy folks who have posted. Everything has worked OK despite that, so I don't think you are likely to cause yourself any major issues by shortening your cables.

 

[sunnybob]

All of the controllers we worked with had calibration allowances built in.
Using a separate thermometer we could adjust the controllers to agree with the true readings.