38
When t.06 is selected the display for the first Helium channel will show
"ICAL"
and both
FAST and SLOW will light, indicating that current is flowing through the probe. For this
test a current meter should be connected in place of the probe. RAISE and LOWER may
be used to adjust the actual current flowing through the meter till this reads 100 mA.
Note that the calibration is done at exactly 100mA,
not
the actual wire current for a
probe. When the current has been set, press SILENCE again. The current will be turned
off and the process repeated for channel 2 if this also is to be used for Helium. Any
channels configured for Nitrogen will not be included in this step.
When all channels have been set, this test will automatically drop into t.08 and display its
first
"PCu"
prompt. The reason for this is that if the current source calibration is
changed, the value of the probe currents set in t.08 will change. Thus these must be re-
set to the required value after any change to the current source calibration. (Do not
forget to STORE before switching off.)
9.9
Test t.07 Setting Probe Active Length
An essential part of the calibration of ILM is to tell the meter the active length of the
probe. This is the length of the section of the probe between the 0% cryogen level and
the 100% cryogen level. ILM uses this to establish an approximate default calibration in
the absence of cryogen and to ensure that the accurate calibration is within the range of
the 0% and 100% adjustments. For a Helium probe, where 100% always corresponds to
the superconducting wire totally immersed, if an accurate figure is supplied for the active
length the default calibration will be within 10% of the final value. For a nitrogen probe
there will be an indeterminate length of probe between the 100% point and the physical
top of the probe. This makes predicting an accurate default calibration harder.
In some cases it may be necessary to set an "incorrect" figure for the active length in
order to ensure that the 0% and 100% points can both be calibrated correctly. If the
displayed reading is too low, try setting a longer active length. If it is too high, try
setting a shorter active length.
When test t.07 is selected the display will show
"LEn".
Pressing and holding SILENCE will
then show the nominal length in millimetres, which may be adjusted by RAISE and
LOWER.
9.10
Test t.08 Setting Helium Probe Currents
Setting the wire currents for a Helium level probe follows the same procedure as above.
Provided the current source(s) have been calibrated, the displayed figure is the actual
pulse current in mA. Note that any changes to the calibration of the current source, will
modify the value set in t.08. Hence test t.06 must be carried out
before
setting the
required probe current with t.08.
