ASL INTERCOM F100, User Manual

Page: 12 / 57

Setting up the F100
Page 1-12 F200-14-001.2
 ASL 2005
1.3 Principles of measurement
The F100 measures the voltage (V
t
) developed across the unknown sensor resistance (R
t
) and the
voltage (V
s
) across a stable internal reference resistance (R
s
). The voltages are proportional to the
resistances so the thermometer resistance is derived from –
s
t
s t
V
V
R R
× =
This technique achieves immunity from slow moving time and temperature drifts in the electronics, as
it is not affected by voltage measurement gain variations or current source fluctuations.
In the same way that AC resistance measurement eliminates thermal EMFs, switched DC achieves a
similar advantage. Switched DC works by reversing the current flow on alternate measurement cycles
and taking the average value, thereby minimising any thermal EMF offsets from the measurement.
For PRTs, the relationship between resistance and temperature varies slightly from one PRT to
another. Therefore, no matter how accurately the F100 measures the PRT resistance, if the
relationship between resistance and temperature for a particular PRT is not known, accurate
temperature measurement is not possible. For thermistors, the relationship depends totally on the
thermistor type and specifications.
The F100 uses PRT and thermistor calibration data to overcome this problem and calculates the result
from temperature conversion functions stored in either the sensors ‘SMART’ connector or the F100’s
internal non-volatile memory. This method enables the F100 to convert resistance to temperature,
uniquely for each sensor used.
It is very important, therefore, that a sensor without a ‘SMART’ connector is used on a properly
configured input channel and that the probes’ coefficients are correctly entered into the instrument.
Note
For non-Smart probes, always check that the coefficients are
correct set for the probe being used.
Failure to do so, may lead to incorrect measurements.
System accuracy is a combination of the F100 accuracy in measuring sensor resistance and the
calibration uncertainty placed on PRTs and thermistors by the calibrating laboratory.