Alternative designs for the Thermistor Probe
The basic thermistor probe design is intended to be fairly robust and simple to fabricate. It is adequate for its intended purpose, which is to measure the temperature of water in the fish tank project. The thermistor probe is not the most elegant design and it may not be the best design for other applications.
Here is a list of considerations for design modifications
- Alternative sensor elements
- Alternative encapsulation
- Four wire resistance measurement
Alternative sensor elements
The thermistor is Digikey part number 317-1258-ND, which is a 10 kOhm, NTC thermistor made by Cantherm (P/N MF52A103J3470). This is an inexpensive part -- $0.33 each for ten or more in May 2010 -- which gives students an opportunity to explore thermistors. It has a stated resistance tolerance of plus or minus five percent.
As of May 2010, here is a comparison of 10 kOhm thermistors manufactured by Cantherm and sold by Digikey
| Digikey PN | R tolerance | Price (May 2010) |
|---|---|---|
| 317-1305-ND | 0.5% | $1.73 each (10 or more) |
| 317-1355-ND | 1.0% | $1.38 each (10 or more) |
| 317-1258-ND | 5.0% | $0.33 each (10 or more) |
Of course there are many other manufacturers of thermistor elements. At a higher cost per element, you can purchase thermistors that can be interchanged with small errors, e.g. 0.5 C. Those types of thermistors are usually provided with calibration curves by the manufacturer.
Alternative encapsulation
There are other ways to seal and insulate the outer layer of the probe. For example, one could use the hot glue alone, without the outer layer of heat shrink. Epoxy or expoy-based nail polish could be used to seal the outer layers if the probe is not going to be immersed for long periods of time -- epoxy tends to absorb water. Marine varnish and urethane are also options.
One could also use a conformal coating for printed circuit boards, e.g. one of the following.
- Acrylic lacquer coating from MG Chemicals
- Sprayon electrical lacquer
- Turbo-coat from techspray.com
- One of the coatings from HumiSeal Corporation.
Regarding the use of the outer heat shrink layer, further experimentation is needed. The advantage of using an outer layer of heat shrink tubing is that the heat shrink forms a tight (diametrically) bundle. The disadvantage is that it adds to the thermal mass of the sensor, which increases the sensor response time, i.e. makes the sensor respond more slowly to changes in its ambient temperature.
Four wire measurements
Anyone wishing to make precise temperature measurements with thermistors should at least consider a four-wire resistance measurement. For details, consult the Wikipedia entry on resistance thermometers or this web page from allaboutcircuits.com or the instruction manual for your precision multimeter. Note that increasing the precision and accuracy of the electrical measurement of resistance, does not necessarily guarantee an accurate temperature measurement. Any sensor will disturb its environment and the temperature of the sensor may not be the true temperature of the undisturbed environment.