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6.8 Fahrenheit to Centigrade Conversion
The HEAT-Prober can be ordered as a °F or °C unit. The 392 will read out in
°F unless a jumper is in place. When jumped to equivalent reading of what
was displayed in °F will be displayed in °C. The computer makes the calcula-
tion and there is no adjustment. "D" versions will have a front power switch
for selecting between °F and °C.
6.9 Open Probe Display
Remove the calibrator or decade box from the meter while the power is on.
The display should show -OL-.
If power is turned on while probe is absent, the same display should occur.
6.9.1 "Help" Display (Appears on 392E or 392M only, 392EX or 392MX
does not display "Help")
The microcomputer is programmed to display "Help" when span adjustment
outside the normal range is attempted. If "Help" continues to be displayed, it
indicates that an incorrect adjustment is being attempted or a faulty meter.
7.0 THERMAL CALIBRATION PROCEDURE
For thermal calibration of temperature, baths are needed for immersion
probes, and surface plates are used for surface probes. However, remember
that calibrating with a specific probe will introduce the error of the probe into
the meter system. Other probes may therefore introduce greater or lesser
error, because the meter is moved away from its nominal curve. DO NOT cali-
brate a surface probe by immersion. There will be an error because compen-
sation is made at the factory for heat loss in surface probes.
Insert the probe in the unit and into well stirred, controlled ice baths at 0°C.
(Be sure it is well stirred). The temperature can vary 1°C, depending on the
layer of water the probe is in, or proximity to a piece of ice.
With the top cover removed (as explained in Section 6.2) and instrument
open, adjust until the 0.0 reading is obtained.
For span, a 250°C or higher bath, should be used. Adjust R35 until the tem-
perature reading is achieved.
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8.0 392M and 392HP OPTIONS
8.1 392HP
The "HP" option allows probes to be calibrated and interchangeable over the
entire temperature range. This also results in superior system accuracies at
all temperatures. The "HP" option utilizes a Noise Rejection program which
minimizes transient noise effects.
In this version a special 4-pin receptacle is installed at the probe connection,
and the four pins are wired to the printed circuit board from the probe con-
nector side as shown in Figure 8-1. A resistor with a nominal value of 5K is
installed in each individual probe via the extra pin. 5K resistance indicates to
the microprocessor that nominal curve values in the memory shall be used.
Other values of this resistor tell the microprocessor to initiate corrections to
the nominal curve.
Close matching with a probe is achieved as follows: The probe is tested at a
specific bath temperature, and a decade box is attached where the R33 resis-
tor would be in the probe. The resistance is adjusted until the 392HP reads
the precise temperature of the bath, for example 100°C, taking out the toler-
ance of the probe. A value of 4K, for example, may be needed in the decade
box to adjust the reading by 0.2%. The new resistor value of 4K tells the
microprocessor to make an adjustment for that specific probe. The decade
box is removed, and a resistor with the same value as the adjusted box is
potted inside the 4-pin probe connector between pins 3 and 4 (See Figure 8-
2).
8.2 Degree F to C Conversion (Specify Suffix D)
The microprocessor provides true digital conversion for temperature
throughout the whole range of the instrument. This switch is a convenience
where dual measurement scales are needed.
8.3 Intrinsically Safe for Hazardous Operations (Order -S Models)
The standard 392M instrument is Factory Mutual classified for Class I and II,
Division I, Group D (for methane and propane), and Group G (for grain dust).
To achieve the higher level Group C rating (for ethylene gas), Group F (for
coal dust), and Group E rating (for metal dust), the instrument is required to
have an additional current limiting resistor in the NiCad battery pack.
Therefore -S Models use battery pack Model Number 11681-2, which
includes the current limiting resistor. In all other aspects, it is the same as
the standard 392M.
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