How to Mount a Thermometer in the Cryostation

There are many instances where a user may need to mount a sample or user thermometer in the system. A few notes of caution:

!! Always use caution when handling the thermometers, the Cernox thermometer wires are extremely fragile and can break! Be especially careful at the thermal clamps.

!! Always clean the surface the sensor will be mounted on with isopropyl alcohol or acetone.

!! It is very important that the thermometer has good contact with the surface where it will be mounted. Use the least amount of N-grease or GE varnish.

To unmount a thermometer, first remove any GE varnish, which is the adhesive which lags the thermometer wire. The GE varnish can be removed with isopropyl alcohol or acetone. Then clean off the thermometer and any remaining GE varnish on the thermometer.

The thermometer wire should always be lagged properly, without proper thermal conductance of the wire, the thermometer will read an inaccurately high temperature as the wire heats the sensor. Please see the Basic Thermometry article for more information on this.

MOUNTING THERMOMETER WITH A CLAMP

When mounting with a clamp, the thermometer sensor contacts the metal surface through N-grease. Use a very thin layer of N-grease, when the clamp is tightened down minimal grease should press out the sides. The thermometer should be placed flat against the surface and when clamped down, avoid clamping down hard such that part of the thermometer springs up, while another part of the surface is mounted hard to the surface. The thermometer should be evenly contacting the surface, as shown below.

Figure 1: Thermometer with clamp

Once the thermometer is in place, lag the wires for about a centimeter with GE varnish. The wires must contact the metal surface, they cannot be encased in GE varnish. The wire can be tied in place with unwaxed floss to hold the wire, and then the GE varnish applied. After the GE varnish is cured the floss can be clipped, or left in place if it aids in keeping the wire positioned.  

Figure 2: Lagging of thermometer with GE Varnish

To ensure that the wire made direct contact with the metal, in Figure 3, Kapton tape was used to keep the wire in place while the GE varnish was applied. Once cured, the Kapton tape was removed.

Figure 3: Another clamp mounted thermometer example

MOUNTING THERMOMETER IN A PLUG

In several instances the thermometer can be mounted in a plug, in this instance it should come pre-mounted to a thermometer holder. Most Montana Instruments supplied sample mounts come with a hole for the sample thermometer to be installed in. The thermometer is glued to the holder with GE Varnish and the wire is lagged with GE Varnish to the end of the holder, but the user will have to additionally lag the wire. To install the plug, first apply a thin layer of N-grease around the thermometer holder. Then install into the hole. Take care not to break the fragile wires of the thermometer. The thermometer should be installed by pressing on the holder, and if it needs to be removed, press on the side opposite of the wires with an allen wrench to remove the thermometer.

Figure 4: Installing thermometer plug into sample mount

When lagging the thermometer wire, again the wire can be kept in place by tying it with unwaxed floss or nylon string, or taped in place with Kapton tape. GE varnish can be applied to the wire (Figure 6), or it can just stay lagged with the unwaxed floss as shown in Figure 5.

Figure 5: Thermometer installed in sample mount plug and tied in place

Figure 6: Thermometer mounted in sample mount and lagged with GE varnish

If the thermometer remains unlagged however, it may read a falsely high temperature.

Figure 7: Unlagged sample thermometer

OTHER MOUNTING TECHNIQUES

If neither a plug nor a clamp is available to mount a thermometer, then the user must use GE varnish to hold the thermometer in place. The wire should be tied or taped in place first. The user should have some way to keep pressure on the sensor while it cures, ideally with a small weight that will not harm the wires of the thermometer. The GE varnish should be thinned with a little acetone or isopropyl alcohol, and then applied to the surface of the mount, within 10 seconds the sensor should be put in place. Then add the weight and allow the sensor to cure for 2 hours before cooling down. GE varnish can be added to the wire also or it should be tied in place.