FORUM ON BEER, HOMEBREWING, AND RELATED ISSUES
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Contents:
Thermocouples ("A. J. deLange")
RE: Electric PID control question (Bob Sheck)
Re: Electric PID control question... (mossview5)
Re: Electric PID control question... ("Jeff Dieterle")
Re: Electric PID control question... (David Huber)
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Date: Tue, 06 Mar 2012 23:47:10 -0500
From: "A. J. deLange" <ajdel at cox.net>
Subject: Thermocouples
Thermocouples are not as accurate as RTD's but even with RTD's there are
potential problems so that an RTD installation needs to be calibrated as
well.
The big issue I have had in my experiences with PID controllers is
having a good tuneset for each application. For heating water in the HLT
the load is always the same - an HLT full of water. It's easy to come up
with a tuneset and as the load is always the same it always works. In
mash and decoction vessels the load is quite variable depending on the
amount of malt being mashed and the amount of water being used to mash
it. Thus one needs a library of tunesets and each one needs to be
established. The only way to do this is during a mash and the autotune
feature of your controller is interested in auto tuning - not mashing so
you will probably have to waste the grain used to obtain the tuneset.
Tuning for water in the mashtun has not worked for me. As a consequence
I control the mash and decoctions manually (pedally actually - I have a
pedal switch I step on to turn on the steam). All the controller does is
take and display temperature readings.
A.J.
------------------------------
Date: Wed, 7 Mar 2012 00:32:07 -0500
From: Bob Sheck <bobsheck at gmail.com>
Subject: RE: Electric PID control question
Mike Eyre asked about RTDs and PIDs. Here ya go, Mike:
RTDs - or Resistance Temperature Detectors are one type of sensor.
There are other types of sensors, thermocouples and they
have other names like K, J, etc
Best to point you to:
http://en.wikipedia.org/wiki/Thermocouple
Some other (maybe) useful links are:
http://www.omega.com/temperature/pdf/rtd_gen_specs_ref.pdf
and
http://en.wikipedia.org/wiki/Resistance_thermometer
for a really good review of how these things work.
That saved me a lot of writing! (and was a good review for me, too!)
PIDs have a set-up program and you must tell them (some way or
tuther) which type of sensor is attached.
Used to be that the HBD was the best place to get information,
but now I go straight to google.com when I really want to find
something fast. Which in a great way is a troubling thing, as I
have been gleaning helpful tips from the HBD since somewhere
around 1993 or so. Of course, I still "monitor" it, in the hopes
that I may learn something new (which I sometimes do) and
also a good friend is a sponsor. Plus, Pat is one helluva good guy for
keeping this thing online these many years. Not to mention the rather
large group of learned folk who are subject-matter-experts in many
areas. Now I'm going to have to send Pat a check, which I have
been remiss in for some too-long a time. You should too.
Cheers~
- --
Bob Sheck // Down East Alers //
Greenville, NC
------------------------------
Date: Wed, 7 Mar 2012 08:56:50 -0500
From: mossview5 <mossview5 at gmail.com>
Subject: Re: Electric PID control question...
I have PID control for both my HLT and RIMS. I thought about moving
up to RTD for the sensors, but I already had K-type thermocouples in
use and the additional precision did not seem warranted. As mentioned
by the OP, thermocouples provide a measurement precision within about
a degree C. Not great, but acceptable. In a HLT, a degree or two off
is not going to mean anything. A degree or two off for the RIMS is a
little more consternating. Since I use additional dial thermometers
(all calibrated to a NIST reference thermometer), I have other checks
of the temperature in the wort circuit to assure that the temperature
is within spec.
I have to admit that the PID control of the RIMS can be problematic.
The sensor has to be as close as possible to the heat source. In
addition, the tuning on the PID has to be right. Since the heater
control and performance at low flow rate is the most critical
scenario, that is what I tuned the system to. It took a couple
Auto-tune sessions on the Auber PIDs to get their parameter settings
sensitive enough to avoid overshoot.
Now, with proper offsetting of the temp readings from the
thermocouples, the PID readout is typically within a degree of the
calibrated thermometers. As far as I'm concerned, the thermometers
are my most precise measurement and the PID system just needs to avoid
overheating the wort in the RIMS. A momentary excursion of a degree
does not concern me too much. That is about as good as it gets for
me.
I still wonder if its worth moving to a RTD for the mash, but that
will have to wait until I find a bargain someday.
Martin Brungard
------------------------------
Date: Wed, 7 Mar 2012 09:46:32 -0500
From: "Jeff Dieterle" <jddieterle at hughes.net>
Subject: Re: Electric PID control question...
Hi Mike,
A t/c is very simple, two dissimilar wires silver-soldered together, i.e.
chromel & alumel(type K) or iron & Constantine(type J) and can be packaged
in a variety of ways. So with that said a T/C is probably more robust and
versatile than a RTD. Not so sure about T/C's having a +/- 1dg C error. They
can be calibrated easily to .1dg F, however they do require periodic
re-calibration to maintain accuracy.
RTD's maintain accuracy with less drift than a T/C but are more expensive. I
use T/C's in my mashtun, hlt and fermentor because I have a standardized
instrument & T/C to calibrate against and T/C's are cheap and relatively
indestructible. I read the T/C's with Honeywell UDC controllers and do the
temp correction in the controller s/w so I read true temp (that is I don't
have to make a manual correction to the instrument reading) If I didn't have
a way to maintain calibration, I would have went with RTD's.
Date: Tue, 6 Mar 2012 11:02:21 -0500
From: Mike Eyre <mikeeyre74 at gmail.com>
Subject: Electric PID control question...
Anyone have any experience with PID controlling your HLT or Mashtun? If so,
or, hell.. even if you have any experience with these things in general, I'm
wondering if there's a thermocouple of the various types that will meet my
needs or if I should go with an RTD instead. The RTDs appear to be super
accurate, to within a fraction of a degree, but I'm not sure that's really
necessary. The Thermocouples seem to be +/- a degree C, so that's sorta wide
ranging for a mashtun.. but they're a fair bit cheaper, it seems. Anyone
have any practical experience to shed on this for me?
Mike Eyre
mikeeyre74 at gmail.com
------------------------------
Date: Wed, 7 Mar 2012 10:03:16 -0500
From: David Huber <n3uks.dave at gmail.com>
Subject: Re: Electric PID control question...
Mike Eyre asked about temperature sensors. I don't have experience
with automatic control of mash tuns, but I have had experience with
thermocouples and thermistors, but not RTDs. Ultimately what you care
about is relative accuracy because absolute accuracy you calibrate
out, just like you'd do when you calibrate your thermometer or
hydrometer. I think any of the three temperature sensors would serve
you well for what you want to use it for. I've used thermocouples for
sub-degree (a few tenths) relative accuracy, but you need to be aware
that they can pretty easily pick up electronic noise so the shorter
you can keep your leads the better (don't have a long run of wire
routed next to your running March pump, for instance).
Whichever sensor you use, you're probably not just going to shove it
unprotected into your mash, so I would choose whichever one mounts
best where you are going to use it. If you're going to use a
thermowell, maybe one of them won't fit, etc. I don't know how RTDs
are specified, and I don't remember how thermistors are either, but
you can also help yourself out by selecting the sensor with the
narrowest temperature range, such as a type-T thermocouple (-270C to
400C) instead of a type K (-230C to 1370C).
All the big science supply companies (Omega, Newark, NI, etc.) will
usually have a whitepaper or two on temperature sensor selection, such
as:
http://www.newark.com/pdfs/techarticles/Accurate_Temperature_Measurement-Sen
sors.pdf
Good luck,
Dave Huber
Jessup, MD
------------------------------
End of HOMEBREW Digest #5910, 03/07/12
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