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Gas Correction Curves for PG105
Gauges
It is important to understand that the pressure indicated by a PG105 convection-enhanced Pirani gauge
depends on the type of gas. All PG105 convection-enhanced Pirani gauges are factory-calibrated and
temperature-compensated for nitrogen (air). However the response of the gauge to other gases is very well
characterized and, with the proper calibration data, it is possible to obtain accurate pressure measurements
for other gases as well.
IGC100 controllers are factory-loaded with Nitrogen and Argon specific calibration curves compatible
with all PG105 gauges, and direct pressure measurements are possible for both gases.
If you must measure the pressure of gases other than Nitrogen or Argon, use Gas Correction Curves, like
figures I-1 and I-2 included in this application note, to convert "nitrogen equivalent pressure" readings
into "actual pressure" readings for those gases.
Gas Correction Factors (relative to nitrogen equivalent readings) can also be used for pressure
measurements below 1 Torr (See Table I-1)
PG105 users should generate their own conversion curves for gases, or mixtures of gases, not included in
this application note. A calibrated, gas-independent, capacitance manometer is recommended as a transfer
standard1.
In This Application Note
Gas Correction Curves and Factors 3
Nominal Gas Correction Factors for Figures I-1
and I-2. 4
Overpressure risks 5
References 6
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2 Gas Correction Curves for PG105 Gauges
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Gas Correction Curves for PG105 Gauges 3
Gas Correction Curves and Factors
Important
The conversion curves and factors listed in this application note only apply
1. when the pressure readings displayed by the controller are based on the nitrogen
calibration curve (i.e. PG Cal Curve = N2 Curve)
2. the gauge tube is mounted with its axis horizontal.
N2 Pressure Reading (Torr)
0.001 0.01 0.1
1
Actual Pressure (Torr)
0.1
Helium
Methane
Argon
Deuterium
Oxygen
0.01
0.001
Figure I-1. PG105 Gauge Indicated Pressure (N2 equivalent) vs. Actual Pressure Curve: 10-3 to 10-1 Torr.
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4 Gas Correction Curves for PG105 Gauges
N2 Pressure Reading (Torr)
0.1 1 10 100 1000
1000
100
10
1
Helium
Methane
Argon
Deuterium
Oxygen
0.1
0.01
Figure I-2. PG105 Gauge Indicated Pressure (N2 equivalent) vs. Actual Pressure Curve: 10-1 to 1000 Torr. Use
only when gauge axis is horizontal.
Nominal Gas Correction Factors for Figures I-1 and I-2.
Actual pressure = N2 equivalent reading x Kg
(Use for pressures below 1 Torr only!)
Gas Kg
Ar 1.59
He 1.10
Oxygen 1.03
Nitrogen 1.00
Deuterium 0.79
Methane 0.63
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Gas Correction Curves for PG105 Gauges 5
Overpressure risks
DANGER!
Using a PG105 convection gauge to backfill to atmospheric pressure should be avoided
unless the gas-specific calibration curve for the backfilled gas is used to calculate and
display pressures.
A serious danger can arise if the calibration data for one gas is applied without correction
to measure pressures for a different gas (or gases) at or above atmospheric pressure.
Argon provides an excellent example of how things can go very wrong. Applying the
nitrogen calibration data to measure argon pressures provides a "nitrogen equivalent"
reading of only 22 Torr when the gauge is exposed to an atmosphere of Argon gas (see
Figure I-2). The chamber could be seriously pressurized while the gauge controller
continues to display <100 Torr of nitrogen equivalent pressure. An oblivious operator,
looking for a 760 Torr pressure reading, might continue to increase the gas pressure
leading to the possibility of a dangerous explosion. Reports of accidents caused by this
effect have appeared in the vacuum literature2. Accidents such as these can occur only if
a thermal conductivity gauge is used to measure pressures at the upper end of the range
where the calibrations for different gases diverge widely. This is the one reason why
many vacuum practitioners reserve their convection gauges for measuring foreline and
roughing line pressures, or moderate backfill pressures only!
At pressures below a few Torr the danger of using the nitrogen (or argon) calibration to
measure the pressures of an uncalibrated gas (or gases) disappears. The only problem left
is the inaccuracy of the readings. However, it is generally possible to correct pressure
readings for uncalibrated gases using lookup tables, conversion curves and even simple
correction factors .
TIP
With systems that could be potentially backfilled to excessive pressures by failure of
gauges or regulator valves the inclusion of a pressure relief valve or burst disk is the
safest way to avoid over pressurization!
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6 Gas Correction Curves for PG105 Gauges
References
1
Consult R. E. Ellefson and A.P. Miller, "Recommended practice for calibrating vacuum gauges of the
thermal conductivity type", J. Vac. Sci. Technol. A 18(5) (2000) 2568, for information on thermal
gauge calibration and accuracy.
2
R. Chapman and J. P. Hobson, J. Vac. Sci. Technol. 16 (1979) 965, D. G. Bills, J. Vac. Sci. Technol. 16
(1979) 2109.
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