1
1. Introduction ______________________________________________________________
Thermistors are semiconductors, which behave as thermal resistors – that is, resistors with a
high (usually negative) temperature coefficient of resistance. The thermistor beads are made
from a mixture of metal oxides encased in epoxy or glass. The beads are small in size and
extremely robust with a high degree of stability over a long life span. Because their resistance
change is so great, it is unusual for cable effects to be significant. However, for high accuracy
work the cable resistance can be taken into account. Accuracies of the types of thermistor
beads used here is,
+/- 0.5
°
C
, (Model 3800-1-1-1), or
+/- 0. 2
°
C
( Model 3800-1-2-1). Standard
temperature ranges are -50 to 150 degrees C . High temperature versions are -30 to 230
degrees C.
2. Installation _______________________________________________________________
Model 3800-1-1, and Model 3800-1-2 thermistors are supplied inside a housing already potted
on the end of a cable ready to be attached to a structure or buried in the ground or in concrete.
The potting chamber is made from PVC for low temperature models or from stainless steel for
the high temperature models. Model 3810 Thermistor strings are made by removing a short
length of the outer jacket of a multi-pair conductor cable, at pre-determined depths; splicing a
Model 3800-1-1-1 into one of the pairs of conductors, then waterproofing the splice. Before
Installation read and record all the thermistors to check that they are functional and to establish
ambient temperature readings. Read and record once again as soon as the thermistor string has
been deployed and again after a certain length of time has elapsed to allow the temperatures
stabilize so as to establish good base line temperature readings.
3. Readout _________________________________________________________________
Thermistors can read using either a GK 403 or GK 404 readout box, which display the
temperature directly in degrees Celsius. Alternatively, for standard thermistors, rated at –50 to
+150 degrees Celsius a digital ohmmeter can be used in conjunction with the Table B1 shown
on the next page. High temperature thermistors can be read in a similar manner using Table B2
Note that if long cables are use it may be necessary to correct for the cable resistance. Standard
cable is 22 AWG with a resistance of 16 ohms per 1000ft. Remember, when calculating the
cable effect to count the distance there and back.
4. Datalogger Connection _____________________________________________________
A Multiplexer (Model 8032) is required to connect the Model 3810 Thermistor string to a
Datalogger (Model 8021/8025). The Common connection of the Thermistor String limits the
ability to connect the string directly to a Datalogger. A jumper is required on each channel of the
Multiplexer in order to split the Common between all of the Thermistors in the string.
