American Magnetics 185 & 186 Liquid Level Instruments (CE-Marked) - Manual (Page 33)

Rev. 3 23
Calibration
Closed dewar calibration
3.3.2.2 Completing the closed dewar calibration procedure
1. Install the sensor in the dewar and energize the instrument with
the sensor connected to the instrument via the oscillator and
extension cables (see the system diagram on page 7).
2. Set the LENGTH to the active length of the sensor. After setting
the LENGTH, set the units mode toggle switch to the % setting. For
details on setting the LENGTH and units mode, refer to the
Operation
section of this manual.
3. Connect a strip chart recorder to the recorder output terminals on
the rear panel of the instrument. If the recorder output is not
available, the 4-20 mA current loop output may be used if installed,
or an installed communications option can be used to query the
instrument for the liquid level at regular time intervals during the
calibration procedure. If no remote monitoring or communication
option is installed, the level display must be manually plotted vs.
time during the procedure.
4. Commence filling the dewar. While the sensor is cooling down,
there may be a slow drift in the displayed liquid level. However,
when the liquid actually touches the bottom of the sensor, contact
with the liquid surface may become apparent by virtue of more
random and frequent fluctuations in the displayed liquid level. The
liquid level trace will also start to show an increasing profile with
positive slope.
Once the indications of the contact between the sensor and liquid
become readily apparent, press the MIN push-button through the
small hole provided in the instrument front panel. When the
calibration point has been accepted, the display will show "bbb.b"
and the push-button can then be released. This point is the 0% level
of the sensor.
Note
If the sensor is installed in the dewar with some small amount of
liquid already in contact with the sensor, then the final MIN
calibration point can be set before filling begins but after any
thermally induced fluctuations in the observed output have
diminished. However, note that the measured span of the liquid
level is reduced by the initial level of liquid in contact with the
sensor.
5. Continue the transfer while observing the liquid level trace on the
strip chart recorder or computer display, whose slope is
proportional to the transfer rate. The slope of the liquid level trace