17
With the cryogen at a known level near zero, the 0% button should be pressed and
RAISE and LOWER used so that the known level is displayed. The process is should then
be repeated at a known level near full, using the 100% button. It is often convenient to
set the 0% point to exactly 0 when liquid starts to collect during the cryostat cooldown
and to set the 100% point to exactly 100% when the reservoir is completely full. The
display has been designed to read slightly below 0% and slightly above 100% to simplify
this adjustment.
Although the above method may be convenient, it is not necessary to have the levels at
exactly 0% and 100% to perform an accurate calibration, provided the actual level is
known at the two calibration points. ILM automatically remembers the point at which
each setting was made and after any adjustment to one end of the range, it
automatically recalculates the complete calibration to ensure that the reading at the
other end remains as it was last calibrated. This ensures that there is no interaction
between the two calibration points. If this recalibration fails, the display will briefly
show "Err" after the calibration button is released. In this case the newly calibrated
point will remain correct but the other end of the range may have been moved. The
usual reason for this is that the default calibration established for the probe is in error.
This may also show as an inability to set the required display number by means of RAISE
and LOWER. In both cases check that the correct active length has been entered.
Note that it is not accurate to set the 0% point for either Helium or Nitrogen probes
when the cryostat is still warm, since the properties of warm gas are not the same as
those of the cold gas which will be present in the reservoir above the liquid surface
during normal operation.
4.5.2
Setting the Default Calibration
When the instrument is first configured for a probe, the nominal
active probe length
(in millimetres) must be supplied, using test mode t.07 as described in section 9.8. (The
active probe length is the working length of the probe. The physical length of the probe
will normally be longer than this since the probe will extend from the top of the cryogen
reservoir to the top of the cryostat.) For instruments with firmware version 1.08 or later,
it is also possible to configure any helium channels for the
probe wire resistivity
. This
is done by means of test mode t.14. If this is changed for any reason, it should be done
before setting the active probe length.
For a Nitrogen probe it is also necessary to establish the default 0% point. This is done
by pressing 0% and 100% buttons simultaneously whilst the probe is connected and is
immersed in air at room temperature, or preferably in cold nitrogen gas. This operation
sets the 0% and 100% trim adjustments to the midrange points, reads the probe and
defines this as the initial 0% point, then uses the entered length to compute a nominal
100% point.
For a Helium probe, it is not necessary to establish a 0% point in this way. The 100%
point is always defined for a Helium probe as zero resistance, so the entered length may
be used to calculate the 0% point. Pressing 0% and 100% simultaneously may used to
set the default midrange values to the 0% and 100% trim, but no probe reading is taken
during this process, so it does not matter whether the probe is in warm gas, cold gas or
liquid.
