higher the pressure inside a vacuum system the cooler the filament or conversely, the
lower the gas pressure the hotter the filament becomes.The thermocouple junction
inside the gauge tube is positioned on the heated filament and is used to monitor its
temperature during system evacuation. A temperature rise or drop in the filament
produces a potential rise or drop in the thermocouple junction. This potential in
millivolts, is then calibrated to microns in a thermocouple gauge controller.
Because thermocouple gauge tubes and control electronics are available from a
multitude of manufacturers, it is important to note that individual thermocouple gauge
tubes are designed to operate at specific filament current ratings, which should be
carefully matched with a control instrument’s specifications. Always refer to the control
electronics manufacturer for specifications on filament current requirements before
purchasing or installing a thermocouple gauge tube.
MDC UHV gauge tubes feature all-welded construction and stainless steel casing tubes
that are bakeable to 150°C. Electrical connections are glass insulated and employ
a standard eight pin interface with a polarized center post. They are offered with
Del-Seal™ CF flanges and Swagelok’s VCR® fittings. HV gauge tubes are supplied with
male NPT pipe thread fittings and a nickel plated steel shell assembly for maximum
corrosion resistance and bakeable to 150°C.
Thermocouple gauge tubes are expendable vacuum components and must be replaced
periodically. Most gauges are typically calibrated for service in air (nitrogen) and
experience extreme variations in calibration when in the presence of other gases,
subsequently leading to erroneous vacuum measurement readings. An inexpensive solution
would be to include a non-gas-dependent gauge such as a Bourdon gauge to verify calibration.
Glass Enclosed Ionization Gauge Tube
MDC glass enclosed ionization gauge tubes are designed for high and ultrahigh vacuum
measurements between 1x10-3 to 2x10-10 Torr. Glass enclosed ionization tubes, commonly referred
to as a Bayard-Alpert gauge tubes, consist of several components including a resistive filament, a positively
charged grid and a negatively charged collector. The resistive filament is heated to
incandescence for the purpose of emitting electrons. The grid, a positively charged
wire, coiled in the shape of a spiral, attracts
and accelerates filament emitted electrons. The collector, a negatively charged wire, is
strategically placed in the path of oncoming electrons. As electrons collide with air
molecules inside the tube, the air molecules lose an electron and become positively
charged or ionized and thus attracted to the negatively charged collector. Upon colliding
with the collector air molecules regain their lost electron and return to their original
neutral charge. The flow of electrons from the collector to air molecules is measured
and calibrated for vacuum measurement. The number of air molecules is directly proportional
to their ionization and in direct proportion to the flow of electrons surrendered by the collector.
Nude Ionization Gauge Tube
MDC nude ionization gauge tubes are designed for high and ultrahigh vacuum measurements between
1x10-3 to 2x10-11 Torr. Like their glass enclosed counterparts, these are also hot cathode ionization
types. The main difference being their method of construction. Nude gauges allow for easy
replacement of perishable filaments and the added durability of ceramic-to-metal
electrical feedthrough insulation which makes them bakeable to 450°C. Two nude
gauge styles are offered, these are resistive heating or electron bombardment degas.