Acceptable level of temp probe resistance drift?

[Sean Monaghan]

Hi HBT,

Apologies if I'm not using correct terminology, but I'm putting together my first electric rig and have some concern around the resistance my multimeter is reading between the red and white wires at the end of my temp probe.

I'm working with a RTD probe with a 13' cable going to a small three prong disconnect on my control panel. The solder work isn't pretty, but no cold joints so I don't think it's adding much resistance to the system. Right with my work space at about 80 degrees F, the ohm measurement between the white and red wire where it attaches to my EZBoil controller is about 111 ohms. I know that the default resistance should be about 100 ohms at 32 degrees F (please correct me if I am wrong). I estimate that the ohm value at my room temp should read about 108 ohms, but the additional +3 gives me pause.

Wondering if this is a normal level of added resistance that I would be able to correct with the Pb value on my PID? Or if this is not a difference that can be corrected using the offset value. Was curious to hear about your experiences in this area?

 

[Vale71]

That's a thermocouple and not an RTD. Resistance measurements on a thermocouple are meaningless as thermocouples do not produce variable resistance but variable voltage with changing temperature.

 

[Sean Monaghan]

That's a thermocouple and not an RTD. Resistance measurements on a thermocouple are meaningless as thermocouples do not produce variable resistance but variable voltage with changing temperature.

I was mistaken, thank you for highlighting. I am using an RTD PT-100 temp sensor. There is no K element, I was confused. The original post has now been amended to prevent further confusion, thank you.

 

[Vale71]

Then the resistance reading you should be getting is aroun 110.5 (assuming 80°F) which is very close to your actual measurement.

 

[Sean Monaghan]

Then the resistance reading you should be getting is aroun 110.5 (assuming 80°F) which is very close to your actual measurement.

Many thanks Vale71! To clarify, you feel that this would be within a tolerable range? Something that could be corrected using the offset value? I wonder at what point it could be considered unacceptable or bad to use, for reference.

 

[Vale71]

As long as it's a fixed offset your controller should be able to correct for it without any issues.

 

[Sean Monaghan]

As long as it's a fixed offset your controller should be able to correct for it without any issues.

Thank you, when you say fixed offset, do you mean having the probe in a place that is independent of flow rate? For example, I have mine mounted inside the bottom of my kettle, and not in a RIMS tube with worth flowing past.

 

[doug293cz]

As long as it's a fixed offset your controller should be able to correct for it without any issues.

The reason there are three wires on an RTD is so the controller can do some corrections for lead and contact resistances (that you cannot do with a two lead ohmmeter.) I suspect your probe + wires are just fine. Check temps in an ice bath (container FULL of crushed ice w/ just enough water to cover the ice) and boiling water (if you are at high altitude, you need to correct the boiling temp for that.) Set the offset in your controller based on the calibration measurements. For example, if you read 33°F in the ice bath, and 213°F in boiling water, you would want to set your temp offset to -1°F. You should repeat the calibration after you have adjusted the offset.

The intrinsic accuracy of the temp probe has nothing to due with probe placement or flow. Placement and flow can affect whether or not the temp at the probe represents the temp at the important places in your process.

Brew on

 

[Sean Monaghan]

The reason there are three wires on an RTD is so the controller can do some corrections for lead and contact resistances (that you cannot do with a two lead ohmmeter.) I suspect your probe + wires are just fine. Check temps in an ice bath (container FULL of crushed ice w/ just enough water to cover the ice) and boiling water (if you are at high altitude, you need to correct the boiling temp for that.) Set the offset in your controller based on the calibration measurements. For example, if you read 33°F in the ice bath, and 213°F in boiling water, you would want to set your temp offset to -1°F. You should repeat the calibration after you have adjusted the offset.

The intrinsic accuracy of the temp probe has nothing to due with probe placement or flow. Placement and flow can affect whether or not the temp at the probe represents the temp at the important places in your process.

Brew on

This is an extremely helpful response, thank you! I had planned to take advantage of the phase change temperatures of water in order to help calibrate as it seemed the most reliable method - i'm at a low altitude. Now my only fear is that i will come up with a greater offset at the high temp vs the low temp, but one bridge to cross at a time..

 

[doug293cz]

This is an extremely helpful response, thank you! I had planned to take advantage of the phase change temperatures of water in order to help calibrate as it seemed the most reliable method - i'm at a low altitude. Now my only fear is that i will come up with a greater offset at the high temp vs the low temp, but one bridge to cross at a time..

If you get wildly different offsets at freezing and boiling, then maybe the best thing to do is figure the offset at 150°F (linear interpolation), and use that, since the most accurate measurements are needed during mashing.

Brew on

 

[Sean Monaghan]

If you get wildly different offsets at freezing and boiling, then maybe the best thing to do is figure the offset at 150°F (linear interpolation), and use that, since the most accurate measurements are needed during mashing.

Brew on

Was hesitant to post about something so trivial, but the communities responses have made it well worth it. Thank you! This make a lot of sense haha.

 

[Vale71]

Thank you, when you say fixed offset, do you mean having the probe in a place that is independent of flow rate? For example, I have mine mounted inside the bottom of my kettle, and not in a RIMS tube with worth flowing past.

Fixed offset means stable in time. You used the word drift in the title and that implies that there are changes with time, if that is the case then the reading becomes unreliable as you cannot compensate for an error that is changing all the time.

 

[Sean Monaghan]

Fixed offset means stable in time. You used the word drift in the title and that implies that there are changes with time, if that is the case then the reading becomes unreliable as you cannot compensate for an error that is changing all the time.

Understood, thank you for the explain! Learning a lot today