©
Page 49 of 97
Issue: 1.04
PRTs and thermocouples means that in order to achieve uncertainties at the mK
level, we need to make electrical measurements that rival those of a good
electrical metrology laboratory. For example, for a 25
Ω
SPRT a 1mK
temperature uncertainty corresponds to 100µ
Ω
resistance uncertainty. With a
1mA sense current, this corresponds to a voltage uncertainty of 0.1µV.
The microK has been optimised for electromagnetic compatibility (minimising
emissions and maximising immunity). However, since the microK is capable of
measuring to such low signal levels it is worthwhile adopting good electrical
measurement practices. Here are a few guidelines:
The most sensitive points are the inputs to the microK (measuring to
better than 0.1µV). Whilst the microK will work satisfactorily with just a
four wire connection to the PRT or reference resistor (using the voltage
sense and current terminals marked “V” and “I” respectively), it is better
to use a screened cable and to connect the screen to the measurement
ground terminal above the input terminals:
V
I
SPRT or
Reference Resistor
Screen (Measurement Ground) Terminal
The screen should also be connected to the outer sheath of the SPRT (if it
is metal clad) or the case of the reference resistor. The use of screened
cables is more important when the microK is used with thermistors, which
have a higher resistance than PRTs or thermocouples.
Keep the cables to the microK input terminals away from other cables
that might be sources of electrical noise (for example electrical supplies
to furnaces).
The insulation in high temperature furnaces (any that ‘glow’) begins to
conduct at higher temperatures. This can cause high common-mode
voltages on any thermometer in the furnace. Whilst the microK is
designed to reject common-mode DC and AC signals (at both 50 and
