39
ILM allows provision for setting two currents for each Helium probe. As well as the main
measuring pulse, there is provision for supplying a higher current
"Pre- pulse".
This can
have advantages with certain probes in unusual applications where for some reason it is
difficult to initiate the propagation of a resistive region in the probe at the normal
working current. Unless you have been advised otherwise by Oxford Instruments you
should assume that your probe will not require a pre-pulse. In which case, both the pre-
pulse current and the pre-pulse width should be set to zero.
When t.08 is selected the first prompt is
"PCu",
inviting entry of the pre-pulse current.
After this has been set (usually to 0) the prompt
"Cur"
will appear inviting entry of the
main pulse current. Typically a figure of 115mA will be used for the standard Oxford
Instruments probes (having a resistance of 178 ohms/metre). For operation of a probe in
a 1 Kelvin environment, a lower current will be required, typically 85mA.
9.11
Test t.09 Setting Helium Probe Pulse Widths
The same sequence is followed for setting the pulse width. The prompt
"PrE"
invites
entry of the pre-pulse width (usually zero) and the prompt
"PUL"
invites entry of the
main pulse width. Pulse widths are in seconds. Typical values are 2 seconds for a short
probe, 3 to 4 seconds for a long probe. The velocity with which the resistive region
propagates down the wire is approximately constant, so that a long probe will require a
longer pulse to ensure the pulse reaches the liquid surface when the reservoir is nearly
empty.
9.12
Test t.10 Setting Helium Probe Sample Rates
A similar procedure is followed to set the interval between sampling pulses for the fast
and slow rates. The prompts are
"FAS"
and
"SLO"
respectively.
BEWARE
, the figure for
the fast rate is entered in
SECONDS
, whilst that for the slow rate is entered in
MINUTES
.
This is done deliberately to reflect the normal usage of the meter and keep numbers
within the working range of the display. A typical interval of 20 seconds might be used
in the fast rate whilst filling a cryostat, whilst an interval of 60 minutes would be
adequate to follow the rate of cryogen fall between fills, without adding any significant
contribution to the cryogen consumption. Allowable settings are 1 to 1999 seconds for
the fast rate and 1 to 1999 minutes for the slow rate. (Note that the indicated times are
the intervals between the pulses. The pulse width and pre-pulse width should be added
to these to obtain the pulse repetition interval.)
9.13
Test t.11 Setting Full, Fill and Low Thresholds
A similar procedure is again used to set the thresholds for each channel in use. The
prompts for the thresholds are
"FUL", "FIL"
and
"LO"
respectively. Default settings are
90%, 20% and 10%.
