WIKA operating instructions, model CTH7000
EN
12
08/2020 EN/BG
4. Design and function
4. Design and function
4.1 Principles of measurement
The CTH7000 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.
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 cancelling 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 CTH7000 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 CTH7000 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 CTH7000’s internal non-volatile memory. This method enables
the CTH7000 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.
System accuracy is a combination of the CTH7000 accuracy in measuring sensor
resistance and the calibration uncertainty placed on PRTs and thermistors by the
calibrating laboratory.
