Calibration – American Magnetics 185 & 186 Liquid Level Instruments (CE-Marked) User Manual

Page 33

Rev. 3

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