(b) Thermal Coupling. So that the inserted sensor is thermally coupled to
the well, a close-fitting liner and non-corrosive oil should be used to fill
the well so that all air spaces are removed. In the absence of a suitable
liner, use oil to fill the well. Light mineral oil is suitable for this
application. Coat the outside of the liner with oil before installing it into
the well.
(c) Humid Environment Conditions. When operating in a humid
environment, moisture will condense in the well and over a period of time
displaces oil from the well. If noninsulated sensors (bare thermocouples)
are inserted into the well, leakage resistance problems in the measuring
circuit may arise. The use of a non-conductive insulator or the use of an
oil with a specific gravity greater than one will correct this situation.
Water in the cell will not damage or otherwise harm the cell.
(6) OPERATION, AND SHUT-DOWN. This instrument has been designed for
continuous duty operation (24 hours a day, seven days a week). It is, therefore,
unnecessary to turn the unit off at the end of each working day. When testing is
complete, shut-down is accomplished merely by turning the switch to the "OFF"
position.
f. INSTRUMENT CALIBRATION.
(1) INTRODUCTION. The recommended basis for calibration is that of a
comparative technique between a freshly made well-maintained ice bath and the
reference unit.
(2) DIFFERENTIAL THERMOCOUPLE. A continuous lead differential
thermocouple, copper-constantan-copper, is a suitable sensor for use in calibrating
this instrument as the sensor itself does not need calibration, as long as the gross
wire characteristics have been substantiated. Initially, place both thermocouples
into the freshly made ice-bath. Allow for stabilization and note any off-set on the
read-out instrumentation.
NOTE: It is a requirement that read-out instrumentation be capable of resolving
fractions of microvolt.
Remove one leg of the thermocouple from the ice bath and immerse it into the
reference well. Make sure that oil is used to fill the reference well to the top. Any
microvolt or fraction of microvolt off-set from the zeroed ice bath condition is a
measure of the bath's deviation from true ice-point conditions. NBS circular 561
can be used to determine the thermocouple sensitivity.
(3) PLATINUM RESISTANCE THERMOMETER. A calibrated Platinum
resistance thermometer fully inserted into the reference well can be used to
establish its temperature. If the accuracy of the calibration and read-out equipment
is such that there is any question about the overall reliability of the measurement,
a comparative technique using a freshly made ice bath should be used.
g. INSTRUMENT MAINTENANCE CHECKS AND ADJUSTMENTS.
(1) Should pilot light replacement be required on an instrument having a
miniature screw type bulb, a small piece of tape will prove helpful in grabbing the
slippery bulb. The operation of the unit is entirely independent of the operation of
the pilot light, hence the unit continues to operate when a bulb fails. The clicking
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