http://en.wikipedia.org/wiki/Thermocouple
The voltage established across the terminals of a bi-metallic thermocouple is proportional to both the temperature at the temp measurement point (the hot junction) and the temperature at the voltage measurement point (the cold junction). If you calibrate the PID to read the TC at a given ambient temp for the cold junction and then alter that temp during operation, the calibration will drift, producing inaccurate readings. I've noticed the PID in my HLT drifts out of calibration by 4-5 degrees between the summer and middle of winter, since mine is located in the garage. Having an extra analog/mechanical thermometer in place is a good way to guard against this effect's influence on your brew process. Alternatively, brewing in a temp controlled environment will help diminish this effect.
Shank question
Do RTD probes suffer from this as well?
I guess my understanding was that RTD worked by correlating resistance and thermocouples worked by micro-volts. The high end pH meter we use at work all the time is very temperature sensitive and also works by a micro-volt measurement. I know RTDs can get shifted with lengthy wiring runs or crappy connections.
At work pretty much only RTD types of probes are used...probably thousands of them in the plant. I guess I had never heard of a shift or drift with changes in PLC cabinet temps. I can ask the programmer or engineer.
I guess I thought the RTD price premium was supposed to make up with higher stability.
What kind of probes and PIDs are you electric guys using? I am using the standard Auber PID for SSR with the RTD probes. I can always double check with the thermapen...but maybe that is off even. I suppose I could mount a thermometer on the control box so I can note the different ambient temps....but then what if that isn't calibrated right? AHHHHH!!!
I'm sure Dr. Lee can help here... or I can get some clarity at work since we rely on them in every step of our processes.
Paul
I guess my understanding was that RTD worked by correlating resistance and thermocouples worked by micro-volts. The high end pH meter we use at work all the time is very temperature sensitive and also works by a micro-volt measurement. I know RTDs can get shifted with lengthy wiring runs or crappy connections.
At work pretty much only RTD types of probes are used...probably thousands of them in the plant. I guess I had never heard of a shift or drift with changes in PLC cabinet temps. I can ask the programmer or engineer.
I guess I thought the RTD price premium was supposed to make up with higher stability.
What kind of probes and PIDs are you electric guys using? I am using the standard Auber PID for SSR with the RTD probes. I can always double check with the thermapen...but maybe that is off even. I suppose I could mount a thermometer on the control box so I can note the different ambient temps....but then what if that isn't calibrated right? AHHHHH!!!
I'm sure Dr. Lee can help here... or I can get some clarity at work since we rely on them in every step of our processes.
Paul
http://en.wikipedia.org/wiki/Resistance_thermometer
It's my understanding that RTDs do not suffer from the same effect because they do not require any sort of bi-metallic junction, whose voltage will change as a function of temperature. If you scroll halfway down the wiki page linked above, you'll see an example of a 3-wire configuration. If your RTD probe has 3 wires, it's probably very precise, but will still need to be calibrated for accuracy.
I double-check the temps I care about with a thermapen. I suspect it should suffer from the same cold junction effect, but since I leave it in the warm house or in my pocket when not in use, the cold junction temp shouldn't change much with the seasons. In general, I trust the thermapen to always be right. I don't trust my PIDs to always be right.
I use TCs with all my PIDs, but only because that's what I've dug out of the trash at work. My PIDs are pretty ancient, but are worth their price. When/If they finally die, I plan to upgrade to one of the lower end models from Auber and get RTD probes. If nothing else, I shouldn't have to double-check the calibration as often with an RTD probe.
I try not to worry too much about the exact value of my HLT or MLT temps, since I have multiple ways to measure them and I believe that the time the mash spends in a temperature region is more important than the specific temperature. Just so long as my dough-in temps are within 5 degrees and my mash-out isn't too high, I don't worry about it. If I did, I suppose I would also have to keep track of ramp rates between temperature steps and I'm way too lazy to write down all that info every time.
It's my understanding that RTDs do not suffer from the same effect because they do not require any sort of bi-metallic junction, whose voltage will change as a function of temperature. If you scroll halfway down the wiki page linked above, you'll see an example of a 3-wire configuration. If your RTD probe has 3 wires, it's probably very precise, but will still need to be calibrated for accuracy.
I double-check the temps I care about with a thermapen. I suspect it should suffer from the same cold junction effect, but since I leave it in the warm house or in my pocket when not in use, the cold junction temp shouldn't change much with the seasons. In general, I trust the thermapen to always be right. I don't trust my PIDs to always be right.
I use TCs with all my PIDs, but only because that's what I've dug out of the trash at work. My PIDs are pretty ancient, but are worth their price. When/If they finally die, I plan to upgrade to one of the lower end models from Auber and get RTD probes. If nothing else, I shouldn't have to double-check the calibration as often with an RTD probe.
I try not to worry too much about the exact value of my HLT or MLT temps, since I have multiple ways to measure them and I believe that the time the mash spends in a temperature region is more important than the specific temperature. Just so long as my dough-in temps are within 5 degrees and my mash-out isn't too high, I don't worry about it. If I did, I suppose I would also have to keep track of ramp rates between temperature steps and I'm way too lazy to write down all that info every time.
Shank question
Cool. Nice to have a Doc in the club to help with the tech side :)Thanks Lee!From: "DrPaulsen" <brew-equipment@crbeernuts.org>
Date: Sat, 07 Apr 2012 16:58:43 -0500
To: <brew-equipment@crbeernuts.org>
ReplyTo: brew-equipment@crbeernuts.org
Subject: Re: Shank question
http://en.wikipedia.org/wiki/Resistance_thermometer It's my understanding that RTDs do not suffer from the same effect because they do not require any sort of bi-metallic junction, whose voltage will change as a function of temperature. If you scroll halfway down the wiki page linked above, you'll see an example of a 3-wire configuration. If your RTD probe has 3 wires, it's probably very precise, but will still need to be calibrated for accuracy. I double-check the temps I care about with a thermapen. I suspect it should suffer from the same cold junction effect, but since I leave it in the warm house or in my pocket when not in use, the cold junction temp shouldn't change much with the seasons. In general, I trust the thermapen to always be right. I don't trust my PIDs to always be right. I use TCs with all my PIDs, but only because that's what I've dug out of the trash at work. My PIDs are pretty ancient, but are worth their price. When/If they finally die, I plan to upgrade to one of the lower end models from Auber and get RTD probes. If nothing else, I shouldn't have to double-check the calibration as often with an RTD probe. I try not to worry too much about the exact value of my HLT or MLT temps, since I have multiple ways to measure them and I believe that the time the mash spends in a temperature region is more important than the specific temperature. Just so long as my dough-in temps are within 5 degrees and my mash-out isn't too high, I don't worry about it. If I did, I suppose I would also have to keep track of ramp rates between temperature steps and I'm way too lazy to write down all that info every time.
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Post generated using Mail2Forum (http://www.mail2forum.com)
Date: Sat, 07 Apr 2012 16:58:43 -0500
To: <brew-equipment@crbeernuts.org>
ReplyTo: brew-equipment@crbeernuts.org
Subject: Re: Shank question
http://en.wikipedia.org/wiki/Resistance_thermometer It's my understanding that RTDs do not suffer from the same effect because they do not require any sort of bi-metallic junction, whose voltage will change as a function of temperature. If you scroll halfway down the wiki page linked above, you'll see an example of a 3-wire configuration. If your RTD probe has 3 wires, it's probably very precise, but will still need to be calibrated for accuracy. I double-check the temps I care about with a thermapen. I suspect it should suffer from the same cold junction effect, but since I leave it in the warm house or in my pocket when not in use, the cold junction temp shouldn't change much with the seasons. In general, I trust the thermapen to always be right. I don't trust my PIDs to always be right. I use TCs with all my PIDs, but only because that's what I've dug out of the trash at work. My PIDs are pretty ancient, but are worth their price. When/If they finally die, I plan to upgrade to one of the lower end models from Auber and get RTD probes. If nothing else, I shouldn't have to double-check the calibration as often with an RTD probe. I try not to worry too much about the exact value of my HLT or MLT temps, since I have multiple ways to measure them and I believe that the time the mash spends in a temperature region is more important than the specific temperature. Just so long as my dough-in temps are within 5 degrees and my mash-out isn't too high, I don't worry about it. If I did, I suppose I would also have to keep track of ramp rates between temperature steps and I'm way too lazy to write down all that info every time.
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i have both k type thermal couples and rtd. i run the auber pid controls. the only reason i noticed something was up i had an inconsisten offset for the k type. ince i vented the box, i am with in a few degrees for my HLT, and boil kettle it doesn't matter. Also i use auber pid for temp control for 2 fermentation chambers, work well so far.
Brandon Franklin - The other Franklin
I was curious about the technology in the somewhat standard reference, the wife's thermapen. That's the arrangement in my house anyway. According to their website, the probe is a "Type K Thermocouple", "High Performance" nonetheless. (before looking it up, I was guessing an RTD.) So why doesn't it suffer from errors based on the ambient temperature? I'm guessing they're using the Cold-Junction Compensation trick (which wikipedia just taught me) : http://en.wikipedia.org/wiki/Thermocoup ... mpensation. I don't think this is something built into a PID controller. For my money, I'm spending the extra few $ for the platinum RTD for my brewhouse, since I have to source all the parts separately anyway. For fermentation temp control, the convenience of a prebuilt Ranco or similar (controller, relay, temp probe) makes up for a degree or so error.
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middleTspeer
- Posts: 201
- Joined: Wed Feb 16, 2011 10:14 pm
- Location: cedar rapids, ia
I don't knew about the controls your using, but the on that i have and use at work has a built in cold junction confiscator. With this it does not make a deferens what the temperature of the cold junction is. At work we have type K and at home i'm using a type T for now. For the most accrate you would want to go to a 4 wire RTD, but i think that is a bit overkill.