12
13
Keep decreasing the supply voltage until 5.5 volts is reached, as measured
with a digital DVM. The meter should maintain correct operation even though
the small “L” or “Lo Bat” is on, indicating a low battery.
Now raise the voltage back up until the small “L” or “Lo Bat” turns off. The
voltage at which this happens should be from 5.50 to 5.55 volts.
Under normal operation, an additional 20 minutes will elapse from the time
the small “L” or “Lo Bat” appears until the battery voltage decreases to 5.5
volts, assuming a normal 5-pack of AA NiCad batteries is being used. Below
5.5 volts, the computer may introduce some errors or not operate the display
at all.
6.4 Zero Adjustment Using Wahl Calibrator
1. Insert the Wahl calibrator and adjust the calibrator to the
32°F/0°C position. Apply power to the meter and make an initial adjustment
at R5 for a reading of 32.0°F or 0.0°C depending on your model.
NOTE: For the version 392HP, which has a 4-wire probe configuration, the
microprocessor averaging process is slightly slower. The moving average for
the 392HP may be made faster by turning the power off and then back on
again, any time an input resistance or pot setting is changed.
2. Make the best adjustment you can at this point, and then proceed
to the span adjustment.
6.5 Span Adjustment Using Wahl Calibrator
1. Set Wahl Calibrator for 284°F/140°C and apply power to the
meter. Adjust R35 (R16 on HP.) clockwise for a display of 284.0°F or
140.0°C.
2. Repeat the zero span adjustment until the meter reads
32.0°F/0.0°C exactly, and reads 212°F/100°C or 284°F/140°C within accept-
able limits of error.
Since only one span adjustment covers both low and high scales, any error
that must be tolerated should be pushed to the high scale where percentage
wise it will be less of an error.
6.6 Adjustment Using Decade Resistance Box
Basic Theory: Platinum has a well defined resistance versus temperature
curve. Accordingly, if a decade resistor is put in the place of the probe to
simulate the standard platinum element, a quick and accurate calibration can
be achieved.
6.6.1. Setup and Calculations
The decade resistor box may be used as the source to perform a standard
calibration. A coiled cord cable with connector for the Wahl probe should be
used (Model Number 10434-01). This has an approximate resistance of 0.36
ohms. The actual resistance should be determined for your setup and this
value used in the calculations. The total resistance to be simulated is .94
ohms above the sensor resistance specified in the ITS90 Platinum Resistance
Tables for 100 ohm, .00385 ohm/ohm/degree C sensors. Therefore, the
decade resistance setting should be a table value plus 0.94 ohms minus the
actual cable resistance measured above i.e. the decade box setting for 0°C
should be: 100.00 + 0.94 - 0.36 = 100.58 ohms.
Note: for HP models do not subtract cord resistance in calculation steps. Use
coiled cord Model Number 10434-02.
Step 1: Set the Decade Box for a total resistance (decade box + lead
resistance) for 100.94 ohms and adjust zero control R5 for a display reading
of 0.0°C or 32.0°F, ±0.1°.
Step 2: Set the Decade Box for a total resistance (decade box + lead
resistance) for 154.52 ohms and adjust span control R35 (R16 for HP mod-
els) for a display reading of 140.0°C or 284.0°F, ±0.1°.
Step 3: Repeat Steps 1 and 2 as necessary for optimum calibration.
6.7 Peak Hold Circuit
1. To enable the PEAK HOLD mode, depress the forward/ upper
position of the Power Switch. Adjust calibrator or decade box for an upscale
temperature reading allowing the display to stabilize.
2. When display has stabilized to a temperature reading appropriate
for the calibrator/decade box setting, move the calibrator/decade box to the
position or resistance that represents a lower temperature, and verify that the
peak temperature of Step 1 is held.
NOTE: This is a digital peak that is computed by the peak hold circuit, and
as such will not drift up or down as may happen with analog circuits. What
may happen is that the calibrator or decade box can momentarily “go open”
as it is switched between settings. The meter may see this and jump up scale
and retain a higher reading. When this happens, switch meter off and back
on to peak hold setting, and the reading will be restored.