MKS HPS 945 Series, Operation And Maintenance Manual

Page: 24 / 38

18
Digital Pirani vacuum Sensor System
To avoid damage to the Sensor, vent the vacuum system to
atmosphere before removing it.
Buffered Analog Output
The Series 945 Pirani Controller's analog output voltage is buffered. The
temperature compensated signal from the Sensor is buffered by a unity gain
amplifier, then connected to the
Accessory port. This signal has an amplitude
of approximately 0.2 V in vacuum to 3.25 V at atmosphere and follows the
voltage-versus-pressure curve shown on the previous page. Since this signal
is not digitally processed, the Series 945 Pirani System is suitable for use in
applications in which measurement speed is important.
Use with Computers
The Series 945 Pirani System is designed to operate in highly automated
systems (especially those controlled by digital computers), as well as in
manual systems. It is compatible with a wide variety of computers,
interfaces, and software.
In order to take full advantage of the System’s capabilities, an analog-to-
digital (A/D) converter should be used with an input voltage span of 0 to 5 V.
The A/D conversion rate should be at least 6 Hz (150 msec conversion
time). A resolution of 12 bits is needed, corresponding to 1.22 mV per bit. The
A/D converter should have a differential input to prevent ground currents from
affecting the measurements.
Leak Detection
Its inherent sensitivity to gas type makes the Series 945 Pirani System
useful for detecting leaks, at rates greater than 10
-4
std cc/sec of helium, in
foreline and roughing systems. It is a useful complement to a mass
spectrometer leak detector, which locates smaller leaks.
1) Pump your vacuum system to a base pressure.
2) Slowly and methodically probe with a small amount of gas.
Flooding the leak or moving quickly past the leak can confuse
the search since system time lags may be significant.
3) Note the pressure reading.
The pressure will either rise or fall, depending upon the thermal conductivity
of the probe gas relative to the system gas. The largest change in value
indicates the probe gas is nearest the leak location.
4) Repeat the test to confirm.
As with any leak testing, many factors can influence the sensitivity of the
test. Explained in further detail below, these include chamber volume; system
pressure; probe gas; type of vacuum pump; location of the Sensor, leak, and
pump; and others such as pumping speed and size of tubing in the system.