MKS HPS 947 Series, Руководство по эксплуатации и обслуживанию, Страница: 25 / 40

19
HPS
®
Products Series 947 CEP System
open. When an LED is off, the measured pressure is above the set point
value, the normally open relay contact is open, and the normally closed
contact is closed.
Venting to Atmosphere
The CEP Sensor senses pressure by measuring heat loss from a heated fine
wire. Sudden venting at its port can greatly stress the Sensor physically.
To avoid damage to the Sensor, vent the vacuum system to
atmosphere
before removing it.
Buffered Analog Output
The Series 947 CEP 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 0.33 V in vacuum to 5.5 V at atmosphere and follows the
voltage-versus-pressure curve shown on page 17. Since this signal is not
digitally processed, the Series 947 CEP System is suitable for use in
applications in which measurement speed is important.
Use with Computers
The Series 947 CEP 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 6 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 1.46 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 947 CEP 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.