Thermistors
for Brewery Temperature Measurement
C.D. Pritchard
New, 2/02. Added link to controller page instead of crude schematic
on 4/02.
The thermistors I use look a bit like
and are about the same size as a kitchen match head and have a couple of
leads attached. Their resistance varies in a precise but nonlinear
manner with temperature. The ones I use: RadioShack cat. #
271-110A- 10 kOhm @ 25 degC.
Advantages:
-
The are cheap- about $1 at Radio Shack and
less from places like All Electronics.
On the low end, only a $10 or less digital volt-ohm meter and a look-up
table or graph is required.
-
A bit better than 1degF accuracy when I calibrated
them against a 1/2 degF or better thermometer. Better yet, their
resistance to temperature (R-T) response from thermistor to thermistor
is dead-on so they are interchangeable.
-
There are small- a bit smaller than the head
on a kitchen match. This results in them being very easy to mount
in a probe and allow them to respond very rapidly to temp. changes.
-
A probe can be any length you want- a long
one is great for mash probes for assessing temperature gradients
in a mash tun while a short one is ideal for a fixed mounting in a pipe
run or to a mash tun or boiler so you're less likely to whack it while
stirring. (A distinct problem with longer stemmed thermometers.
-
They are fairly rugged. The only way
I've managed to damage one is by dropping a completed temperature probe
so the exposed thermistor at its business end hit on the concrete floor.
-
They are immune to electrical noise so you
can and can use about any kind of wiring with them.
Disadvantages:
-
Their resistance vs. temperature response
is not linear. This is a very big complication if you want to build
a direct reading digital thermometer. For this sort of application,
a diode or a sensor like the LM23 is a better choice. Linear thermistors
are made, but they have temperature range limits and are more expensive.
Info on designing and building you own is in the Links
section below.
-
For use in liquids, they have to be mounted
and sealed in a probe.
-
A "maybe"- I don't know what the coating or
paint is that covers the bead of the thermistor. I don't worry about
it- very little of it is exposed to wort and I've not noticed any degradation
of the coating in several years of use.
Making
and Using a Probe
A drawing:
I mount them in 1/4" copper or stainless
steel tubing so that a lot of the business end of the thermistor is exposed.
This makes them very responsive to changes in temperature.
Here are some very
detailed Instructions.
I'll avoid insulting your intelligence
by detailing how to connect the probe to a volt-ohm meter (VOM).
BTW, a digital VOM is much better and an analog one for this application.
Calibration/Checking
Since the thermistors I've gotten from
Radio Shack have been very uniform (within < 1degF), I treat them as
interchangeable and hence there's no need to calibrate each probe.
What I did initially was to put the probe and an accurate darkroom thermometer
in an big insulated glass which also had a slowish stirrer in it.
I stated with hot water- around 150 degF and periodically recorded the
temp. and resistance as the temperature of the water dropped
The R-T values were essentially the same as those on the back of the Radio
Shack pack. I also dunked the probe in boiling water and in an ice
bath and, again, got the resistances on the back of the pack.
Using
a Probe
After connecting a probe to a digital
VOM set to measure kOhms, it'll display the resistance of the thermistor.
The back of the RadioShack package their thermistors come in have a table
of resistance vs. temperature (R-T) values but it's only in 5 degC
intervals. Not good enough for most homebrewing applications, so
make one of the following:
A Table
of R-T values
One approach is a table like that is the
tab delimited text file Table of resistance vs.
temperature (Hint: save link as a text file on your disk).
It contains the resistance values for the range 23 to 212 degF, in
1 degF intervals. Edit/pretty it up it to your
liking with your word processor and print it out or stuff it into a spreadsheet
and play around with it. The NTC app. notes in the Links
section below has info on R-T equations which are perhaps more precise
than the one I used (a fifth order polynomial).
Graphs
of R-T values
1. Here's a graph
that goes from 70-212 degF (on the horz. axis). (Sorry
about the gray background and your waste of toner should you print it,
but, I got tired and hence very pissed trying to find where ^&$#@$@
Bill Gate$ hid the background color setting for it in Excel.)
2. OK, here's a
better graph (via a very old version Lotus 123!)
Mounting
a Probe
I use 1/4" compression fittings for mounting
probes to brewing equipment after drilling through the fitting so the 1/4"
OD probe will pass through it. A tip: If you want to be able
to later adjust the protrusion of the probe into a vessel (say, to reuse
it in another application), use a nylon ferrule instead of brass one that
comes with most compression fittings.
A slick way to mount a proble in a run
of 1/2" copper tubing is to hack-saw and grind down one end of 1/4" brass
compression union coupling to leave a short nub and shoulder on one end
and a compression end on the other. Drill a hole in the 1/2" tubing
to accept the nub and solder the hacked fitting to it. Plenty strong
and cheap.
Going
Further
Learn
more about thermistors- Links
The Thermometrics
web site has much useful info. Recommended stuff:
Connect them to
a computer
I use thermistors in conjunction with
on my RIMS, boiler
and HLT as well as in fermenters and in the beer fridge. All but
the fridge and the boiler use thermistors hooked to a basic Stamp 2 (basically
a computer on a $40 chip) for automatic control. Basically, a thermistor
and a capacitor are connected to one of the Stamp's in/output ports (it
has 16 of them...). The stamp charges up the capacitor, stops charging
and starts a counter. Stopping the charging allows the capacitor
to discharge through the thermistor. When the capacitor's voltage
falls to a reference level, the counter is stopped and the count is read
by the Stamps program. It only takes a 3 or so lines of code to to
all of this. More details are available on my RIMS
page under the heading "Controller".
Make a simple
controller
As much as I like 'puters and Stamps,
I'm still an engineer so I try to chose the most appropriate tool for the
job at hand. Since maintaining a beer fridge at a constant temp.
is a very easy task, I built a simple
temperature controller circuit.
Questions or comments? Email
me.
