© Isothermal Technology
Page 23 of 102
923 milliK manual - issue: 1.10
the extension connections and therefore minimise any error from the imbalance in their
resistance.
3.2 Using a Thermistor with milliK
The milliK can be used with resistance thermometers based on thermistors. These are
typically NTC (negative temperature coefficient) devices which have well defined
characteristics and can offer accuracies down to 25mK or less. Compared with PRTs/SPRTs,
thermistors have a much higher resistance (typically measured in tens of k
Ω
), operate over a
more limited temperature range (typically < 150°C) are highly non-linear, but they are lower
cost and can be more robust. The non-linear resistance-temperature characteristics make
thermistor sensors very sensitive over a limited temperature range. Their relatively high
resistance means that 4-wire measurement is not as important as with PRTs/SPRTs (although
the millik still uses 4-wire measurement). However, their high resistance makes them more
prone to electrical interference so greater care is required when connecting them to the
milliK, in particular the use of screened cables is highly recommended (see section 4.2).
Unlike PRTs/SPRTs, the resistance-temperature characteristics vary considerably between
different types of thermistor. There are, therefore, no nominal or generic conversion
algorithms. The millik can be used to monitor the resistance of a thermometer declared to be
a Default Thermistor, but in order to measure temperature, the thermistor and its calibration
details must be entered into the milliK’s thermometer database.
Connect your thermistor to either of the Lemo 6-pin circular connectors on the front panel
using a 2-wire or 4-wire connection arrangement (see section 4.2 for pin-out and connection
details). Ensure that the miniature thermocouple connector for the same channel is unused
(since it is connected in parallel with the Lemo connector). Your milliK is supplied with two
Lemo connectors for you to use with thermistors. The part number for this connector is:
Lemo part number: FGG.1B.306.CLAD62Z
Additional connectors are readily available internationally from Farnell Electronics and can
be ordered via their website
www.farnell.com
(Farnell stock code 3817325).
Channels 1 and 2 are not isolated from each other, but are isolated from Channel 3 and from
the digital interfaces in order to optimise immunity to external noise sources.
