MKS HPS 945 Series, Operation And Maintenance Manual

Page: 26 / 38

20
Digital Pirani vacuum Sensor System
Before using the Series 945 Pirani System to measure
pressure of gases other than air or nitrogen, you should
read and understand this section. To answer further
questions, contact Applications Engineering at HPS
TM Products
at 1-303-449-9861 or 1-800-345-1967.
The Series 945 Pirani System is designed to give voltage output for air/
nitrogen according to the graph on page 16 or the table on page 15. If the
Pirani System is used to read pressure of gases with poorer heat transfer
properties than nitrogen, the true system pressure may be much higher than
indicated.
This reading error could lead to dangerous overpressure.
A system which is backfilled from a pressurized gas source
should contain a safety device, such as a burst disc.
Nitrogen Equivalent Pressure
The thermal loss from a heated sensor element is a function of the
transporting gas (see page A.1, Theory of a Thermal Conductivity Sensor.)
Since the Series 945 Pirani System's Sensor is such a gauge, the voltage
output depends upon the gas measured.
When used with gases other than nitrogen (N
2
), the System reads in nitrogen
equivalent pressure. For example, when the Sensor is set up to read pressure
for N
2
but is used with the gases helium (He) or
argon (Ar), the data for He and Ar in the graph
on page 16 are used to convert
nitrogen
equivalent pressure to true pressure.
1. Locate the indicated pressure on the
pressure axis of the graph on page 16 (as
shown in the example to the right). Locate the
point at which a vertical line from this point
intercepts the curve for N
2
.
2. Locate the point at which a horizontal line
from the point on the N
2
curve intercepts the He
or Ar curve.
3. Locate the point at which a vertical line from
the He or Ar curve intercepts the pressure axis.
This point corresponds to the true pressure for
He or Ar.
Using the Series 945 Pirani
System with Other Gases
4
3
2
1
0
1
10 -4
10
-3
10
-2
10
-1
10 +1 10 +2
He
N
2
Ar
#1
#2
#3
Output V
olta
g
e
(VDC)
Pressure
(Torr)