American Magnetics 286 Multi-Sensor Liquid Level Instrument (CE-Marked) - Manual (Page 38)

24
Calibration
Closed dewar calibration
available, use the 4-20 mA current loop output or a digital
communications option (if installed) to query the instrument for the
liquid level at regular time intervals during the fill. If no remote
monitoring or communication option is installed, the level display
must be manually plotted vs. time during the fill.
7. 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 button as prompted by the
display. When the calibration point has been accepted, the display
will change to
Entered and the Model 286 will beep once. 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.
8. Continue the transfer while observing the liquid level trace on the
strip chart recorder, computer display, or manual plot, whose slope
is proportional to the transfer rate. The slope of the liquid level
trace should decrease significantly when the liquid reaches the hole
in the top of the sensor.
When the break in the slope of the level trace occurs (i.e. the slope
of the level trace becomes 0 or horizontal), push the MAX button as
prompted by the display. When the calibration point has been
accepted, the display will change to
Entered and the Model 286 will
beep once. The level on the sensor when the MAX button is pressed
becomes the 100% level.
Note
If the instrument displayed a 100% reading before a break is
observed in the slope of the level trace, then the MAX calibration
point set prior to the current procedure has interfered. If this occurs,
the customer has two options: 1) stop the procedure, increase the