GT 340A & GT345B Thermistor Gauges
click to download GT-340A Product Manual


The 340 is an electrically powered gauge for use in harsh environments. The rugged electronics are designed for longevity and occasional accidental misuse. Two analog scales allow the user to take vacuum measurements from atmosphere to 1×10-3 torr. A separate meter scale for the 1 to 100 millitorr range provides increased readability in this critical area. Meter scales are calibrated in dry air pressure units; calibration curves for other gases are supplied.


  • Measures from atm to 1×10-3 torr
  • Rugged thermisitor sensor – excellent for high vapor applications
  • Interchangeable pre-calibrated tubes
  • Rugged circuitry for high reliability
  • Two scales for accurate pressure reading
  • External circuit control feature available
  • Leak detect capability on STA.1
  • Second station can be added on-site
  • Analog meter indicates direction of pressure change
  • 115/230V, 50/60 Hz operation

Principle of Operation

A single element is employed in a bridge-type detector driven from a constant regulated voltage source. The inverse resistive characteristics of the sensor unbalances the bridge as pressure changes occur, thereby causing a current change. A microammeter scaled in pressure units measures this current.

Leak Detection

Gauge has Leak Detect Mode. Front panel switch allows adjustment of the gauge sensitivity to aid vacuum leak detection.

Recorder Connection

Connections on the rear panel of the gauge cabinet provide a pressure-responsive output independent of the selector switch. A 10-, 50-, or 100-millivolt high-impedance recorder with suitable attenuating network can be connected here to cover the entire pressure range of the gauge at either sensing station.

Modular Construction

The Type GT-340A thermistor gauge and the GT-005A controller are each mounted in quarter-rack modular cabinets. The GT-345B controlling gauge combines these two modules in a single half-rack cabinet. Two GT-340A two-station gauges can be mounted side-by-side for applications requiring four-station operation, with the added advantages of two read-out meters and redundant circuitry.

Automatic Control of External Circuits (GT-345B)

Relay control unit available. This can either be built with the original gauge, or as an accessory (GT-005A). The trip points of the control are independent of the setting of the selector switch. Normally open or normally closed relay contacts rated at 3-amps, 115/230 volts are available for connecting external loads such as safety circuits for tuning off pressure-sensitive components or automatic circuitry for operating vacuum valves at preset pressures. By utilizing the separate adjustment provided, the pull-in and drop-out point of the relay contacts can be set so control is established at any two pressures within the operating range of the gauge. Indication lights on the front panel show position of relay contacts.

General Specifications

Width: GT-340A,  4″ ; GT345B, 8″
Depth: GT-340A, 6-1/2″ ; GT345B, 6-1/2″
Height: GT-340A, 6″ ; GT345B, 6″
Net Weight: GT-340A, 6 lbs. ; GT345B, 11 lbs.
Number of Stations: 1 or 2
Type of Mounting: Bench or panel
Normal Operating Temperature Range: 15 to 35ºC
AC Power Input: 115/230 volts, 50/60 Hertz
Meter Scale Ranges: 0 to 100 milliTorr; 0 to atmosphere
Recorder Output: 0 – 2.5 volts

External Control Circuit (GT-005A)

Operating Range: Entire range of gauge
Relay Differential: Adjustable
Relay Contact Capacity: SPDT, 3 amp, line voltage energized

GT-034 Tube Specifications

Envelope w/baffle housing length including base: 2-9/16″
Envelope O.D.: 1-1/4″
Tubulation Material: Nickel-plated steel
Tubulation Length:  1-3/8″
Tubulation O.D.:  0.405″ diameter sealing surface and 1/8″ pipe thread
Thermistor Element Operating Temperature:  90ºC
Maximum Bakeout Temperature of Sensor Envelope:  100ºC
Pressure Range: High Vacuum to 60 psig

Ordering information for the components mentioned on this page is listed below.

Gauge Type & Number of Stations

GT-340A-1:  Thermistor gauge, single-station, Part number 5506330-1
GT-340A-2: Thermistor gauge, two-station, Part number 5506330-2
GT-345B-1: Thermistor gauge, single-station w/ relay, Part number 5506789-1
GT-345B-2: Thermistor gauge, two-station w/ relay, Part number 5506789-2


GT-005A: Relay control unit, Part number 5506611
CGB port connector: Fittings for installing sensing tubes in a vacuum system (all have quick-disconnect seal for 3/8″ tubulation). To fit 1/2″ FPT hole, Part number 061083
Bushing for above P/N 061083: CGB Type 3/8″ nom. I.D., Part number 061081-2


Cable 10′ standard – Station 1, Part number 5505863-1
Cable 10′ standard – Station 2, Part number 5505889-1
Cable 15′ standard – Station 1, Part number 5505863-2
Cable 15′ standard – Station 2, Part number 5505889-2
Cable 25′ standard – Station 1, Part number 5505863-3
Cable 25′ standard – Station 2, Part number 5505889-3
Cable 50′ standard – Station 1, Part number 5505863-4
Cable 50′ standard – Station 2, Part number 5505889-4