American Magnetics 135 & 136 Liquid Helium Level Instruments (CE-Marked) - Manual (Page 32)

20 Rev. 3
Operation
Dirty Sensor Operational Mode
or 4-20 mA analog signal corresponding to 0-100% of liquid level. Refer to
the Appendix for the pinout diagram of connector J2.
3.3 Dirty Sensor Operational Mode
AMI expects the helium level sensor to be reasonably clean and free from
oil, water, ice, etc. for proper operation. However, it is recognized that
some experiments might result in some material being deposited on the
sensor wire. Ice formation at some point on the sensor is a typical
occurrence. Therefore, the Model 135/136 has the capability of increasing
the current for a short period of time at the beginning of the measurement
cycle (in the SAMPLE mode only) to try and drive the resistive zone of the
sensor wire past the dirty region. This operation may or may not be
successful depending on the degree of sensor contamination. This mode
should be viewed as a stopgap measure only. If correct readings cannot be
reestablished, the only choice is to warm the sensor or remove for cleaning
or replacement.
Note
Operation in the dirty sensor mode increases liquid helium losses.
Consequently, operation in this mode should not be used unless the
sensor is known or anticipated to become dirty or the helium level
measurement is in question due to unclean operation.
The procedure for configuring the Model 135/136 for the dirty sensor
operational mode is documented on page 52 in the Service section of this
manual.