Helium level gauge probes.

A brochure for helium probes in Acrobat pdf format can now also be downloaded.

Each probe is made to customer order, and its part number describes all the required details.  There are six fields in the part number, which has the general form:

Each of these codes have a meaning as described here:  All dimensions are expressed in millimeters, except where stated.

• MR.  This now consists of two single letter codes, which specifies the probe series and range. The probe series (M) is based upon the physical construction of the probe, with the three most frequently specified are:
  • Stainless steel tube (Sx)
  • pultruded Glass Fibre (Gx)
  • PTFE/Teflon (Px)

  In addition, there is a variant of the Sx range, Rx, which is a radiation-hard version. A range using Polyetheretherketone (PEEK) tubing, to be labelled Kx, is being developed.

  The probe range (R) specifies the type of superconducting element that is used in the probe. The original probe range, xS, was supplimented by a new range, xG, in 2001. At that time, this change in nomenclature was introduced to the probe part number. In autumn 2002, the probe range is also specified in the product code number of the Helium Depth Indicator.

  A small number of product code changes have been necessitated by this; these are given in the Changes section.

• TL  This is the total length of the probe.
  The total length is defined by the way the probe is installed.
  • For probes which run from the helium and out of the cryostat, the total length is from the bottom to the pinch seal, or other point where the probe is sealed to prevent the atmosphere entering.  Details about the type of connector, or type of fitting do not affect the definition of the total length.
  • For probes which are installed within the cryostat, and have wires running from the probe up to a user supplied feedthrough, the total length is the entire length of the probe, including any special terminations for the wires into the probe.
• AL  The active length, that is the length of the superconducting element of the probe
• CN  The type of connector or wiring.  The standard is a 7 pin male (7M); which allows for connection to the Helium Depth Indicator.   Other types of connector can and have been used, with a different code to identify them.  Alternatively, a simple number indicates the wire length for a wire ended probe.
• NN  The diameter of the main section of the probe, expressed in tenths of millimeters (so 21 = 2.1mm)  The standard diameter for SS range probes is 2.1mm
• Option codes  The following is a short list of the more frequently specified options. either alone, or in some allowed combinations   Other designs and types can be accommodated
• T  For wire ended probes, this specifies that colour coded Teflon insulated copper-nickel alloy wires are to be used
• D The venting holes at double the standard frequency, i.e. 25mm instead of 50mm
• PW & SW  Two types of integral still well to be built around the probe as one complete item, for use in dewars attached to liquefier systems, for example.  The minimum outside diameter for a still well probe (the diameter NN) is 4.0mm
• /rr(WL) Defines a larger diameter collar (of diameter r.r mm, and length WL) at the top of the probe, to allow the probe to seal into an existing (larger) port on a cryostat with a separate adaptor

Due to the change in nomenclature necessitated by product improvements, there are a few cases where a product code has had to change. The changes are summarised here. The examples given here are where a code has been used in the past, but to specify the same unit with the change, a part code has to be changed. Other possible codes which were never required to be specified before are not listed here.