16
4.3
"Err" Display
If no probe is plugged into a channel which has been configured to be in use, the display
will show a steady
"Err"
display. The same display will be produced to indicate a fault in
the probe, its wiring, or the ILM itself. In the case of a nitrogen probe, if the display
flashes between
"Err"
and a 100% reading every second, the inner and outer tubes of
the probe have become shorted together. This can be caused by the probe becoming
bent, by the inner tube touching the bottom of the cryostat, or by condensation within
the probe.
4.4
Sample Rate Selection
Helium level is measured by passing a current down a superconductive wire, such that the
wire is driven into its resistive state where it is in gas and therefore less well cooled. The
measuring process thus introduces a significant heat load into the cryogen. In order to
minimise this heat load, the probe is normally only energised for a brief pulse of around
2 to 3 seconds. Between pulses the level is assumed not to vary greatly and the last
measured value is displayed. When cryogen is being refilled, the level can change quickly
and is sensible to sample at a fast rate with the wire being pulsed every 10 - 30 seconds.
At other times the level will only change slowly and it is sufficient to sample it once or
twice per hour. At this rate, the heat load due to the pulse is negligible.
Pressing the RATE button switches between FAST and SLOW modes of operation.
Whenever the channel is switched to the FAST rate, a sample is taken straight away.
Thus if you are already in FAST but wish to take a sample immediately, pressing the RATE
button twice will switch to SLOW and back to FAST and so initiate a pulse.
The interval between pulses may be configured for both FAST and SLOW rates over a
wide range of values, from within test mode (see section 9.12). It is also possible to
configure ILM to switch back to SLOW automatically if left in FAST for more than 15
minutes (see section 9.7).
4.5
Calibration
To obtain accurate level readings of Helium or Nitrogen, ILM must be calibrated for a
specific probe. There are two levels of calibration. In normal use, it should only be
necessary to make small trim changes to the calibration. When an instrument is initially
set up for use with a new probe, it is necessary to establish a default starting-point
calibration. This will normally be done before the unit leaves the factory and should only
need changing if the probe is replaced by one of a different length.
4.5.1
Trimming the Calibration
For an accurate calibration it is necessary to set two points accurately near the ends of
the probe range. This is done by means of the recessed 0% and 100% buttons. These
may be pressed with a pointed object, such as a pencil. Whilst holding the button
pressed, RAISE and LOWER are used to adjust the display to the required value.
