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© Isothermal Technology
Page 58 of 102
923 milliK manual - issue: 1.10
5.1 Standards Required for Calibration
The following standards are recommended for checking or adjusting the calibration:
A 4-terminal short circuit (see section 5.1.1 for details of how to make one)
A calibrated 100
Ω
resistor of suitable quality.
A calibrated 400
Ω
resistor of suitable quality
A calibrated 500k
Ω
resistor of suitable quality
A 0mV short-circuit connected to a miniature thermocouple plug (see section 5.1.2 for
details of how to make one)
A 10mA precision current reference, such as the Metron Designs I-REF2. This is used
to calibrate the 4-20mA input but it can also be used in calibrating the 115mV input
by connecting it to a calibrated 10
Ω
resistor of suitable quality to generate a nominal
100mV calibration signal. A 100mV signal source can be used to check and adjust the
voltage measurement system, but since we are trying to calibrate at the <1µV level,
suitable calibration sources are expensive. Since you already need a 10mA source to
calibrate the 4-20mA input, using a calibrated 10
Ω
resistor to generate the signal
offers a reliable and inexpensive solution.
A calibrated 10
Ω
resistor for use with the 10mA source to generate a 100mV signal
A thermocouple of adequate quality. A type T thermocouple is suitable since it has a
high sensitivity near ambient temperatures and is readily available.
All the calibrated resistors used should be either a Wilkins resistance standard or a high
stability foil resistor such as a hermetically sealed Vishay Z-foil resistor (or the H series for
the 10
Ω
resistor). The resistors should be maintained in a stable temperature environment
(such as a silicone oil bath) whose temperature is near to the resistor’s calibration temperature
and that is monitored so that compensation can be applied for its temperature coefficient of
resistance (TCR).
5.1.1 Making a 4-Terminal Short-Circuit
A 4-terminal short circuit is a connection that measures as 0
Ω
when measured as a 4-terminal
resistance. It can be fabricated using standard copper wire by shorting together the current
connections and the voltage connections separately and then linking together these two short-
circuits with a single connection: