Fabricating a (Reasonably) Waterproof Thermistor Probe

This document provides instructions for fabricating a thermistor probe used to measure the temperature in the fish tank for EAS 199B. The design is fairly robust, fairly simple and it provides opportunities for students to learn how to solder and insulate the extension leads of a sensor. A separate page provides a brief discussion of design trade-offs and suggestions for improvement.

The bare thermistor is Digikey part number 317-1258-ND, which is a 10 kOhm, negative temperature coefficient (NTC) thermistor made by Cantherm (Part Number MF52A103J3470).

The twisted pair extension wire can be scavenged from a piece of scrap Cat-5 cable. The extension wire does not need to literally be a twisted pair. Twisted pair from ethernet wiring (CAT-5, CAT-5E or CAT-6) is convenient because the pairs are already twisted.

1. Obtain a thermistor and a pair of extension wire leads

Strip and tin approximately 1 cm of one end of each of the twisted pair leads

Before starting any permanent wiring, use a multimeter to measure the resistance of the thermistor. Hold the thermistor between your thumb and index finger and watch whether the resistance increases or decreases. Do you have an negative temperature coefficient (NTC) or positive temperature coefficient (PTC) thermistor?

2. Obtain pieces of poly tubing and heat shrink tubing

Materials for this step are

Tygon Micro Bore PVC Tubing .020" ID x .060" OD. SmallParts.com Part Number B000PHCDEY
1/16" (nominal) Polyolefin heat shrink tubing. Digikey Part Number Q2F018B-ND
3/32" (nominal) Polyolefin heat shrink tubing. Digikey Part Number Q2F332B-ND

An alternative to the Tygon tubing is 24 Gage PTFE tubing, Smallparts.com Part Number B000F0V02S

Cut one piece of small poly tubing, one piece of small diameter shrink wrap, and one piece of large diameter shrink wrap. Use the following photo to estimate the required length of tubing. Only take what you need.

3. Slide the large diameter heat shrink tubing over the twisted pair

Slide the large diameter heat shrink tubing over the end of the twisted pair leads. Make sure the shrink tubing is a good distance from the tinned ends so that heat from the soldering does not prematurely shrink the tubing. You decide how far "far enough" is.

Large diameter heat shrink over the plain leads

This step could be delayed until the very end because the heat shrink can be slid along the full length of the twisted pair leads. However, it is a good idea to get in the habit of planning ahead with shrink tubing. Sooner or later you will finish a complex soldering job only to discover that the heat shrink tubing cannot be slid onto the conductors because there is no free end. For example, when splicing wires in bundles, there is no free end.

4. Solder the first thermistor lead.

Slide the clear tubing over one of the thermistor leads. The small diameter plastic tubing is needed for only one of the thermistor leads. It doesn't matter which one.

Use an alligator clip as a heat sink. Gently clamp the bead-end of the thermistor wires in the jaws of an alligator clip to provide a heat sink during soldering. The heat sink will help prevent overheating of the thermistor bead.

Adjust the vise holding the extension wire and the vise holding the thermistor probe so that one leg is parallel to and touching (or nearly touching) each other. After the vises are adjusted, manually adjust the extension wire (slightly bending it as necessary) so that the extension wire and thermistor leg are touching. Solder these wires together.

Note that in the following photograph, a soldering heat sink clip, not an ordinary alligator clip is used. (Alternative products here, and here. ) The heat sink has flat jaws, which makes it easier to grip the wire without realigning it. Other than a slightly less convenient gripping action, an alligator clip works just as well as a specialized soldering heat sink for this application.

First completed solder joint for the thermistor

After the joint is soldered, verify the continuity from end of the newly soldered lead to the remaining lead of the thermistor.

5. Slide heat shrink over the second twisted-pair lead.

This step must be done before soldering the other pair of lead wires.

Slide the small diameter heat shrink tubing onto the (remaining) unsoldered lead of the extension wire. Make sure that the heat shrink tubing is far enough from the area affected by the heat from soldering. You man need to untwist the lead wires to provide enough distance between the solder joint and a safe, temporary location for the heat shrink.

Intial location of small diameter heat shrink from away the solder joint

6. Solder the second thermistor lead.

Reposition the alligator clip if it was moved. Gently clamp the bead-end of the thermistor wires in the jaws of an alligator clip to provide a heat sink during soldering. The heat sink will help prevent overheating of the thermistor bead.

Solder the remaining (unsoldered) end of the twisted pair to the free (unsoldered) lead of the thermistor.

Second completed solder joint for the thermistor

After the joint is soldered, verify the continuity from the two ends of the extension wire.

7. Shrink the heat shrink over the second twisted-pair lead.

Slide the small diameter heat shrink tubing down to cover the second solder joint. The purpose of this heat shrink tubing is to isolate the two leads, not to isolate the second lead from the environment. Make sure that the heat shrink tubing covers the newest (second) solder joint and that it provides electrical isolation from the opposite lead.

Position heat shrink after completing second solder joint

With the heat shrink tubing in proper position, use the heat gun to shrink the tubing.

Note: The heat gun can raise the temperature of the solder enough to loosen, and possibly de-solder, the joints you have just made. If that happens, you can use the heat gun to melt the solder and re-join the parts -- an extra pair of hands may be helpful in this operation.

8. Shrink the heat shrink over the probe.

While keeping the poly tubing snug against the bead end of the thermistor, slide the large diameter shrink tubing down to cover the two solder joints. Shrink the tubing with the heat gun.

Before:

Large diameter heat shrink over the completed assembly

After heating:

After completing the assembly, verify the continuity from one leg of the leads to the other leg.

9. Dip the probe in hot glue or lacquer, or spray with lacquer

To make the probe waterproof, dip it in the can of melted hot glue on the workbench near the sink. Carefully hold the probe over the tin of melted hot glue until the excess glue drips off and the remaining glue solidifies (about a minute).

After dipping in hot glue:

[Click for larger image]

As the preceding image shows, the hot glue forms a thick layer over the thermistor probe. The amount of glue shown in the image should not be a problem. It will only slow the response time of the probe.

To obtain a slimmer coating of glue on the probe, you can remove excess glue by careful reheating it with the heat gun. This will cause the excess glue to flow into large dripping gobs that can be removed with a stick or a flat screwdriver.

After re-melting and removing the excess hot glue:

Thermistor probe after hot glue is remelted with heat gun