Instruction Manual IGM402 Hornet
InstruTech
Page 77
9.5
R & D (Research) Diagnostic Display
In addition to the normal pressure measurement screen the
IGM402
provides a diagnostic display screen that
allows you to monitor the measurements of the critical electrical parameters used for measurement of very low
pressures, i.e., high vacuum, made by the ionization gauge.
Over time, the hot cathode (filament) of the ionization gauge may degrade. The IGM402 coated iridium
filaments are made using yttrium oxide to lower the work function of the heated wire material and enable
substantial emission of electrons from the hot cathode. These electrons are accelerated in an electric field
toward the grid structure of the ionization gauge device. The electron emission is commonly referred to as the
‘Emission Current’. As the emission current electrons orbit within the electric field formed by the bias voltages
applied to the grid, collector and filament electrodes, the molecules that the gas is comprised of will be ionized
by these energetic electrons.
The positive ions that are created within the grid structure are captured in another electric potential field
created by the grid structure (at +180 Vdc with respect to ground potential) and the collector wire (at zero volts,
i.e., ground potential). In simple terms, the ions collected by the collector electrode (a small diameter wire
within the grid electrode structure) are measured with an electrometer circuit connected to the collector. The
amount of ion current measured is directly proportional to the density of the gas within the ion gauge
transducer. This ion current is commonly referred to as the ‘Collector Current’.
From the collector current measured, which is directly proportional to the gas density inside the ion gauge
transducer enclosure, the pressure inside the ion gauge transducer can be calculated.
The R&D display screen is a very useful diagnostic tool to troubleshoot issues with the sensor or the electronics.
To access this screen, go to the SETUP DISP menu, select SHOW DATA and then select IG ONLY RND.
This mode displays the measured pressure, emission current, ion current, filament voltage and filament current.
In the following example, the measured pressure is 1.00E-9 Torr, (Pressure unit is based on selected units in
SETUP UNIT menu). Emission current is 4.00E-3 amperes, Ion current is 4.00E-11 amperes, filament voltage is
1.5 Vdc and filament current is 1.9 amperes.
Negative Exponent
Emission current value(amps)
Ion current value (amps)
PT= 1.00E-9
IE= 4.00E-3
IC= 4.00E-11
FVI 1.5 1.9
Measured Pressure
Filament Voltage (Vdc)
Filament current (amperes)
Example - R&D Display of Critical Hot-Cathode Ion Gauge Measurement Parameters
