Thermocouple + Thermowell? Questions

[aamcle]

When inserted onto a thermowell can the bare metal wires that form the junction be allowed to touch the metal thermowell?

Can any type of heat transfer paste be used to speed up the response time of the thermocouple/thermowell combination?

Any other way of reducing the response time?


Many Thanks. Aamcle

 

[Markdon]

Im not speaking from brew experience, but from work, we use k type thermocouples (two ires, crimped at the end, with a two pin plug on the end) and if you are using something similar then whatever point where the two wires touch, thats your reading. So if metal is introduced, it messes things up a bit. Putting a bit of clear selotape over the end of the wires may help, but then again if the thermowell is in the liquid, it shouldnt matter. I should have read this before writing the reply!

 

[keith1664]

Is this a brewing related question? The reason I ask is that thermocouples are designed to be used at far higher temperatures than we brewers are interested in, the probes we use at brewing temperatures use the principle of varying resistance with temperature, not generating an EMF.

So no, you wouldn't want a thermocouple junction touching a thermowell. However I don't see a problem with using a thermal paste with a resistive (ie PT100) probe in a thermowell.

 

[Markdon]

Just to add, depending on the thermocouple, we use them in a range from -10 to +220c at work, calibrated across the range.

 

[aamcle]

Its a type k and is fully use able for brewing. So I need to keep the junction from touching the stainless.

Aamcle

 

[winnywood]

A thermocouple is formed by two disimalar metals forming a junction as long as the stainless from the thermowell is not part of the circuit then there should be no problem with it touching.
A thermal conductive paste will help in a thermowell.
Type k thermocouples will be ok for brewing temps but pt100 would be better.

 

[aamcle]

Do the Pt100s respond slower than thermocouple? I'd like to keep the response time as low as I can.

ATB. Aamcle

 

[winnywood]

Thermocouples will have a better response time but pt100 will be more accurate.
Pt 100 are used when pricise temp measurment is required.
If your process can tolerate a tolerance of + - of 2ºc then thermocouples will be fine.
Just bear in mind that you cannot just extend thermocouple with ordanary cable it has to be of a compensating type ensuring that the joints are of the same metal.

 

[winnywood]

Just out of intrest what device are you using to read the sensor signal.

 

[aamcle]

An ebay PID N2006P so it may be that the probe is the most accurate part of the system.

I will have both a 3Kw heating element and the probe in the malt tube under the bottom filter of a Spiedal clone so should the system block and the flow stop temp change could be rapid.


ATB. Aamcle

 

[Vossy1]

Also worth noting that you have to factor the accuracies/inac of both the PID and probe to get the overall accuracy/inac.
Keeping in mind that high accuracy PIDS are really expensive I chose Class A PT100's/RTD's throughout my system to remove as much variance as possible.. Response time can be set from the PID, so if important look at the tech figures of the PID 1st.
RTD's have a linear response unlike K type which means that RTD's should be relatively accurate with only 1 calibration across our brewing range (I use ice bath) type K will require more based on my early experiments with K.
Some good info on PT100/RTD's here

 

[aamcle]

Reading up on pt100/thermocouples it seem that the latter might benefit from calibrating at 5C intervals across the working range.

I only need 1 probe so if Santa is good to me I'll get a Class A Pt100.


Atb. Aamcle