SS Scientific Ltd
SSIG-17                                  IONISATION GAUGE                             SSIG-18
SSIG-17 Ionisation Gauge Head

SSIG-17 & SSIG-18 are new, improved Bayard Alpert ionisation gauges specifically designed for long life and reliability.

As well as using high specification materials, the surface area of the grid structure has been reduced, and the collector plus collector shield are of an improved design.  Compared to standard BA gauges, these improvements mean that the SSIG-17 / 18 has a lower x-ray limit (see technical note) that enables pressure measurement down to 3 x 10-11 mbar.

At higher pressure operation (up to 10-2 mbar), the rugged, robust characteristics of the gauge construction ensure a long-life of reliable, repeatable pressure measurement.

Whatever the application, the very competitive price of the SSIG-17 / 18 ion gauge make it a must for all vacuum systems. 

OVERVIEW

· Bayard Alpert Gauge

· Nude Head

· Conflat® Flange

· Low X-Ray Limit to 3 x 10-11 mbar

· High Reliability

· Long Life

· Robust, Efficient Design

· Twin Filaments:  Tungsten or
                                Thoria-coated iridium

· Bakeable to 400ºC 

SPECIFICATION    
GAUGE HEAD SSIG-17 SSIG-18
Flange 70mm OD 70mm OD
Pressure Range (mbar) 10-2 to 3 x 10-11 10-2 to 3 x 10-11
Degas Power (W) 40 to 100 40 to 100
X-ray limit (mbar) 3 x 10-11 3 x 10-11
Sensitivity (mbar-1) 19 19
Filament (replaceable) Twin tungsten Twin thoria coated iridium
SSIG-17 Assembly

PRINCIPLE OF OPERATION

A hot cathode ionisation gauge comprises three electrodes: a filament, grid and collector.  Electrons are produced from the filament by thermionic emission.  A positive potential on the grid attracts the electrons away from the filament, causing them to oscillate through the fine structure many times, until eventually they collide with the grid.  Alternatively, the electrons collide with gas molecules producing a positively charged ion, which are collected at a negative electrode (Collector) at the centre of the grid structure.  The current depends upon the number of ions which is directly proportional to the pressure in the gauge.

X-RAY LIMIT

The low pressure limit of the gauge is restricted by the ‘x-ray limit’. Electrons hitting the grid produce soft x-rays which in turn produce photoelectron emission (‘photoelectric noise’) from the collector. Since the measurement electronics cannot distinguish between collecting a positive ion or losing a photoelectron, a theoretical minimum detectable pressure is set. The effect is small and is kept to a minimum by careful design of the gauge.

GAUGE LEADS

To connect a SSIG-17 / 18 ion gauge head to a controller, a lead is required.  Bakeable leads are available for both gauges, which means that the gauge can be operated (and pressure monitored) whilst the system is being baked.  Also, the gauge heads can be degassed whilst the system remains hot.

The standard lead is 3 metres long (connections shown below), but any length can be supplied to order.

Gauge Leads

GAUGE CALIBRATION

Ionisation gauges have different sensitivities for different gases and are usually calibrated for nitrogen.  Many users accept the inaccurate pressure measurement for a gas mixture and rely on the long-term reproducibility of the gauge.

For accurate pressure measurements the gauge head should be calibrated for the gas species being measured.

The sensitivity of a gauge is defined as:

S  =          Collector current (A)

         Grid current (A)  x  Pressure (mbar)