DIY Temperature Sensors
by Steve Spence
sspence@green-trust.orgI needed a way to check the oil temperature on my vegetable oil powered diesel conversions. Many of the canned remote temperature sensors quit at 150 F, and I needed something that could go to 212F. The following is what I came up with.
First, I looked around for a sensor. Radio Shack caries a thermistor (Catalog #: 271-110) for $2.49 that fits the bill. All i needed to do was plug it into a digital Multimeter, set for measuring resistance (Ohm's) and read the resistance in k ohms. The resistance changes in proportion to temperature with a -50 to +110 degree C range.
Then I needed a cheap digital multimeter. Again, Radio Shack came to the rescue with Catalog #: 22-810 . For $15, I had my basis for my monitoring system. For about $20, I can monitor the temperature of the vegetable oil, engine coolant, even indoor and outdoor temperatures. Just add a $3 2 pole, 6 position rotary switch to select sensor (Catalog #: 275-1386).
Now, it's not quite that easy. The meter is not displaying temperature, it's displaying resistance, in Ohm's. A chart is provided with the thermistor that converts resistance to Celsius. I'm also going to provide you with Fahrenheit equivalents.
Resistance (kohm's) Temp C Temp F 320.2 -50 -58 247.5 -45 -49 188.4 -40 -40 144 -35 -31 111.3 -30 -22 86.39 -25 -13 67.74 -20 -4 53.39 -15 5 42.45 -10 14 33.89 -5 23 27.28 0 32 22.05 5 41 17.96 10 50 14.68 15 59 12.09 20 68 10 25 77 8.313 30 86 6.941 35 95 5.826 40 104 4.912 45 113 4.161 50 122 3.537 55 131 3.021 60 140 2.589 65 149 2.229 70 158 1.924 75 167 1.669 80 176 1.451 85 185 1.366 90 194 1.108 95 203 0.9375 100 212 0.8575 105 221 0.7579 110 230 So, there you go, quick and dirty (and cheap) temperature sensing. With a Basic Stamp you could even control devices based on these values.
Steve Spence
If you play with it any more, look at the National Semiconductor
LM34/LM35. They output in mv/degree C (LM35) or F (LM34), and range to
150C. That way you wouldn't have to do the conversion.
We used to make fishin' thermometers out of them by encasing the
sensor in epoxy and connecting it to a cheapo 2v scale panel meter.
Set the decimal in the proper place, and it reads directly.Steve Cothran
For a simple "up to 212F" measurement, an LM34 (300F) ($14.07 from
DigiKey in quantity of 1) or LM34C (230F) ($6.03-11.67 from DigiKey in
quantity of 1, depending on package) is cheap, simple and accurate for a
lot less money, and without the nonlinearity of the thermistor. Add a
$10 DMM (measuring voltage) and you get a direct temperature reading
(you'll have to shift the decimal point yourself) for $16 or so. 2.12
volts is 212 Fahrenheit, 2.00 volts is 200 Fahrenheit. If you prefer
Celsius/Centigrade, use the LM35. Or you can save a few more bucks and
stress an LM34D ($1.89-2.33 from DigiKey in quantity of 1, depending on
package) to its max operating temp of 212F.
As Steve did with his thermistor, add a switch and one DMM can read
several different sensors, or you can wire the sensors to data
collection and not have to change tables when you change sensors
(thermistors are notoriously individualistic).
Cheaper yet in larger quantities, might be cheaper from other suppliers,
Digi-Key is simply the first place I think to look that I know sells in
quantity 1, as well as 25, 1000, 5000, etc.
You do need to supply voltage, but that's anything from 5-30 V, so the
vehicle battery supply should do, or a small battery (like the 9v that
may be in your dmm) will do for a standalone system - disconnect when
not measuring.
With the bottom end part the price is the same, and even with the
top-end part it's not so high, while the result is quick, easy and not
so inscrutable as the resistance of a thermistor.
Ecnerwal
Ah, a subject of way too much experience here. For twenty years I was
continually replacing the thermistors used by the old analog solar DHW
controller. It appears that the hotter they run on average, the "cooler"
their output value becomes, until the differential controller only turns
the pump on when the panels are extremely hot, and there isn't enough hot
water at the end of the day.
When I installed a full-time digital computer for other reasons, I
transferred the solar DHW control there. My first sensor iteration was the
LM334 analog device, which is cheap ($1 to $2), accurate, and simple (add
one resistor and measure the current it allows through, and the output is
directly proportional to degrees K). You could put your RS multimeter in
series with the output and read temperature, and if you pick the right
resistor your display could be directly in K. I believe there is also one
now that shifts the reading to Centigrade. The disadvantage for my purposes
is that each sensor requires a separate wire from the sense location back
to the computer, and an A to D channel of my computer. Not as difficult to
use as a "real" thermocouple, though - the output is higher, and linear,
and you can use any old wire.
The second generation was the Maxim/Dallas 6575. It is a more complex IC
that converts temperature to a proportional time between a trigger pulse
and a response pulse. The advantage is that eight of them can share one
wire, and it only takes one TTL output and one TTL input to read any or all
of them. But you need an accurate PC clock (as if...) to judge the timing,
the programming is trickier, and the only physical form is a tiny surface
mount SOT23 that is not easy to use "by hand". I have several still in use,
but the killer is that noise on the lines, like when I'm putting 150A into
my batteries from the genset, prevents accurate retrieval of the exact
pulse edges and spoils the temperature readings.
The latest iteration is the Maxim/Dallas 18B20 - one of their "one-wire"
bus devices. They are closer to $5 each, the electrical interface to the
bus is much more complex, requiring three buffered TTL outputs (pull down,
weak pull up, and strong pull up to provide power to the sensor chips over
the same wire) plus a TTL input, and the programming is definitely not
trivial - but can be pretty much cribbed from their publications. (If you
have a spare serial port, you can buy a ready-made bus adaptor with a ready
made program, and using "one-wire" devices becomes much simpler.)
The plus side of the 18B20 is that once it works you can have 128 sensors
on the same single wire (plus ground, of course), each sensor has an
internal serial number you can read remotely, and there are all manner of
other devices, including memories and output drivers (and complete weather
stations) that can share the same bus. They are by far the most accurate in
temp reading, and it is inherent digital accuracy that doesn't depend on
your hand-picking resistors or individually calibrating the results. And
they can bust through all the interference I've thrown at them (if the
reading is not exactly valid, it is one of two recognizable values that can
easily be discarded).
Probably more than anyone wanted to know about remote temperature sensing,
but all learned by direct experience. One note about all of these IC
devices, though - they (officially, at least) top out at 125 C, which is
fine for hot water applications but might not meet your needs for hot oil.Loren Amelang <loren@pacific.net>
The temperature probe on my multimeter reads -70 to 600 degrees "C" total
cost $99 and it reads temperature! (+/- 2%)
Radio Shack Thermistors are not always available (Catalog number retrieves
message "not available on line") and a part similar (The detailed
specifications were not posted either) to the one you bought are available
elsewhere for between $0.60 and $1.80 each (10 each)
Wouldn't it be better to use a probe shielded against physical damage for
the measurements? The cost is more, but it would be less likely to begin
reading wrong when the epoxy coating (good in air) cracks!
I even use metal cased probes in water (HVAC controllers) and glass
encapsulated thermistors in Satellite applications (70 to 340 degrees K)Roger Gathright
You can buy the exact same sensor - already potted in a highly stable,
adaptable and corrosion resistant package - at any auto parts store.
You can also get them extremely cheap at any junk yard. All you need
to do is pick one type (i.e. for Americans it would be 90's model Ford
or GM because they're cheapest in the US) and stick to it.
http://www.painlessperformance.com/manuals/90518m/90518Sec06.htm#Figure%206.4
Just one example of thousands of possibilities. In the sensor on the
right (air charge temp) you can even see the element inside the
protective "cage." This isn't needed or desirable for coolant temp
sensing, thus the more durable model on the left.
You can also get the connectors (cheap and easy) at the junkyard - or
you can order them brand new from Pioneer Standard, Hamilton Avnet, or
any other distributor that reps Berg or Packard connectors.Scott <scott@f2o.org>
I have had some success using a simple diode, or you could use a cheap
transistor as diode. The forward voltage at a fixed current is a function of
temperature. Maxim makes a bunch of little sensors based on this simple
principal. If you have a good thermometer you can borrow for a few hours,
you just calibrate the voltage points around where you want to measure, and
you are set.Fred B. McGalliard <frederick.b.mcgalliard@boeing.com>
I was in COMPUSA this week and they had a neat dual digital LCD
readout temperature panel designed to fit in an open 5 1/4" computer
disk drive bay. It came with about 18" of wire with tiny thermisters
and some heat-proof tape. You place the sensors on devices like the
CPU and VIDEO chip(s) to monitor their temperature(s). Kind of a
waste and more of a gimmick for computer "geeks" IMHO. I work part
time for a company making large vats of jams & sauces. It's real
important to keep the temperature above 150 degrees while filling the
jars. Before I came along they used to stick a thermometer into every
50th jar or so. I just bolted this panel to the filler and used
Velcro to hold one sensor on the filler tube and one on the hopper.
They are good for only 199.9 degrees but for us they worked great as
we fill at 150-190 degrees. At $29 that's only $15 per readout. Just
my 2c and FYI.Jim Parish <jparish@j-com.net>
Having seen your excellent web page on the subject, I now understand -- thanks!
I was an EE in a former life, and set about converting the scale to something directly readable, but came across the following part:
"National Semiconductor #LM34DZ, precision integrated circuit temperature sensor whose voltage is linearly proportional to the Fahrenheit temperature. Operates on 5-30 VDC. Requires no external calibration to provide +-1 degree F accuracy at room temperature. Operates on +32 to +212 degrees F. Spec sheet available."
This is a three-lead device: ground, power, and a lead to the voltmeter, which will read out directly in degrees Fahrenheit. $2.50 each from All Electronics Corp. <http://www.allelectronics.com>. I just ordered 5 of them.
Spec sheet at <http://www.allelectronics.com/spec/LM34DZ.pdf>
Jan Steinman <Jan@Bytesmiths.com>