Chapter Four: Operation in Normal Mode
How To Span a Pirani Type Sensor to Atmosphere
113
How To Span a Pirani Type Sensor to Atmosphere
Spanning a Pirani type sensor should only be done at atmosphere. Spanning is done to adjust the
upper range of the gauge which may have drifted due to contamination or aging of the sensor.
The span adjustment assumes a pressure of 760 Torr (atmospheric pressure), as its reference.
Convection gauges display 760 Torr when spanned, and an HPS Pirani gauge displays Atm.
This spanning method is usually sufficient to bring the spanned gauge’s readings within its stated
measurement accuracy. If this is not the case, span the gauge with a reference (refer to How To
How To Span a Sensor with a Reference, page 115).
Actual Pressure
(or other measured parameter)
Spanning adjusts the slope of a gauge's
Pressure versus Actual Pressure (or other
measured parameter) line.
Original slope of Channel
to be Spanned
P
r
e
s
s
u
r
e
R
e
a
d
i
n
g
New slope
Figure 29: Spanning a Pirani-Type Sensor
Note
1.
Spanning a Pirani sensor preempts the factory set span. To return to
the factory set span, refer to How To Perform Sensor
Calibration/MFC Setup, page 190.
2.
Because of the high accuracy of capacitance manometers, they are
not spanned to atmosphere (which may not be exactly 760 Torr).
3.
Ion gauges cannot measure at atmosphere, so they are spanned to a
reference within their range.
4.
A Pirani sensor cannot be spanned if helium (He) or argon (Ar) is
the gas type chosen. The gas must be nitrogen (N
2
).
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