Helium Level Gauge probes - detailed description of options.


Collar option

The collar option adds a top section made from a larger diameter tube, to suit a convenient access port of the system. Top section tube diameters from 3.2 to 12.7mm are available.

The probe part number with the collar option specified is


where: rr gives the diameter of the tube at the seal point in tenths of mm (1/10mm); (WL) gives the length of the larger diameter tube section in mm, and is included within the total length of the probe.

Double hole spacing (D option)

This option specifies that the ventilation holes are every 25mm rather than the standard 50mm. This is preferred by some, so as to allow for greater circulation of helium, especially at and below 2.2K  This option is available for all types of probe.

Eureka (Constantan) wired probe (E option)

Probes can be supplied with the four wires running to the superconducting element in enamelled Eureka wire. For connector terminated probes, this option is useful in systems where there is an extreme flow of cold gas around the probe, for example during a very fast transfer in unfavourable installations. For wire ended probes, this option reduces the heat load on the helium bath because of the wires' reduced thermal conductivity.

PTFE (Teflon) insulated Eureka (Constantan) wired probe (T option)

Only available for wire ended probes The four lead-out wires are in Eureka wire, for the same reason as given for the E option. However, the PTFE insulation means that the probe wires have greater protection against damage to the insulation, and are more suitable for rough environments, either during installation or within the cryostat. If an Sx- series probe is required with the T option, a short collar is required to be fitted at the top of the probe to accommodate the wires. This collar is 5.0mm diameter and 18mm long.

Still Well (PW and SW options)

An Sx- series probe, supplied with an integral still well, for use in systems such as helium liquefiers. The still well prevents helium spray from a transfer syphon hitting the probe, and damps out fluctuations in helium level.; it provides a time constant of the level within the still well of about 3 seconds.

For the PW options, the outer still well probe runs the entre length of the probe.  PW range probes are therefore only avalilable up to 2000mm total length. The probe part number becomes Sx-TL-AL-7M-rrPW.  A larger diameter collar can be fitted if required, in which case the still well tube will run up to the point where the collar starts.

For the SW option, the probe is fitted with a collar of diameter r.r  typically 200mm long for a probe with total length of 2200mm or less, or longer if required.  The lower part of the probe is supplied with a detachable 4.0mm still well tube. The probe part number becomes Sx-TL-AL-7M-rrSW. A larger diameter collar can also be added.

The standard dimension for the still well probe outers is 4.0, but other diameters, such as 4.5 or 4.7mm are available.

Probes with two active elements

Certain circumstances mean that the probe requires two superconducting elements within the one tube. There are two different configurations of a dual element probe.

Elements in series

Probe part number

MT-TL-(AL1+ AL2)-CN-nn

In this probe, element 2 (Channel B) is closer to the connector or the leadout wires than element 1 (Channel A). This probe should be specified if the active length required is greater than 2000mm, the maximum possible on each channel of the HDI. For these probes, AL1 = AL2, each is half the required active length. The HDI can be configured to read both elements in sequence, and display the sum (Total) of the two readings.

Limitations on probes requiring this option

Sx- series: Probe diameter 2.4mm or greater.
Length: Probes with total length greater than 3000mm require special carriage requirements. Please contact Twickenham's sales office.

Elements in parallel

Probe part number

MT-TL-(AL1 & AL2)-CN-nn

In this probe, element 2 is alongside element 1. This probe should be specified if a fail-safe system is required, or if the probe, once installed, is so inaccessible that redundancy is required. The two elements share common I- and V- wires.

Limitations on probes requiring this option

Sx- series: Probe diameter 3.3mm or greater.

Twickenham Scientific Instruments Ltd.  Registered in England No. 2991094
Registered Office:  61d St. Margaret's Road, Twickenham, Middlesex. TW1 2LL, UK.
Telephone: +44 20 8892 7400
Email: sales@twicksci.co.uk