MKS HPS 947 Series, Operation And Maintenance Manual, Page: 28 / 40

22
HPS
®
Products Series 947 CEP System
Using the Series 947 CEP
System
with Other Gases
Before using the Series 947 CEP 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
® at
1-303-449-9861 or 1-800-345-1967.
The Series 947 CEP System is designed to give voltage output for air/
nitrogen according to the graph on page 17 or the table on page 16. If the
CEP 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 p. A.1, Theory of a Thermal Conductivity Sensor.)
Since the Series 947 CEP 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 17 are used to convert nitrogen equivalent
pressure to true pressure.
1. Locate the indicated
pressure on the pressure axis of the graph
on page 17 (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.