Wahl DSTCAL Software User Manual
21
9. CALIBRATION
– Thermocouple Models
9.1. Theory of Calibration
Meter Calibration is performed to ensure that the meter will measure mV signals
accurately. This is accomplished by using known, DC millivolt signals of a specified
accuracy. Wahl recommends using mV signal generators with a known accuracy equal
to or better than ± 6.0µV and thermocouple emissions of ±0.30°C (includes CJC) at
25°C.
The calibration routine also allows entry of the exact millivolt signal. This will allow the
technician to enter the calibrated mV value from a NIST traceable laboratory data sheet
or by measuring a less accurate mV signal with a precision voltmeter, such as an
Agilent 3458A, and
entering the measured value. See “METER CALIBRATION” below
for details.
Cold Junction Compensation (CJC) Calibration is required to calibrate the cold junction
circuit to provide the proper amount of correction for the specific thermocouple type
being used. It is essential that this calibration be performed after changing the meter’s
thermocouple type. While the millivolt calibration is performed with copper wire
connected to the meter, the CJC calibration must be performed with the specific
thermocouple type wire. The correction factor used by the meter is the mV equivalent
of the temperature offset measured in accordance with the selected thermocouple
type’s mV to temperature table.
9.2. Meter Calibration
mV Meter Calibration Window
Selecting Millivolt Screen
1.
Prompts user to “Connect 0 Millivolt Standard to Meter Input”, user
may optionally enter exact value of millivolt signal
into the “Measured
Millivolts (mV)
” box.
2.
Click “Next” to continue (Clicking “Cancel” will return to main screen).
Waiting for Stabilization Screen
3. Unit waits 10 seconds for stabilization of capacitors
Averaging Samples Screen
4. Unit takes 10 readings to average for calibration
Selecting Millivolt Screen
5.
Prompts user to “Connect 75 Millivolt Standard to Meter Input”, user
may optionally enter exact value of millivolt signal
into the “Measured
Millivolts (mV)
” box.
6.
Click “Next” to continue (Clicking “Cancel” will return to main screen).
Note: When performing the Millivolt Calibration, during the calibration of the 75mV, on occasion the
calibrated value will show as the maximum mV value of 74.999996 mV. If this occurs, the unit is at its
maximum value and may show inaccuracy at higher temperatures. To remedy this potential problem,
simply recalibrate the unit and when DSTCAL prompts you to input 75mV, set the mV source to 74.0mV
and type 74.0 into the DSTCAL window for the actual value entered. This will likely correct the problem.
