MKS 937B Operation Manual
65
7.3 Installing Pirani Sensors
7.3.1 Locating a Pirani Sensor
Locate a Pirani sensor appropriately for measuring a chamber or manifold pressure. Install the sensor
away from pumps and gas sources for the most representative data. Place the sensor in a location
with minimal vibration.
7.3.2 Preventing Contamination in a Pirani Sensor
Locate and orient the Pirani sensor so as to avoid contaminants that might affect the tube's element.
For example, if a sensor is installed directly above a roughing pump oil vapor could contaminate the
tube's filament wire and cause calibration shift.
Install a Pirani sensor with the vacuum port facing downward whenever possible. This helps to
prevent particulate and liquids from falling or flowing into the sensor. The use of a screen or porous
filter at the port can be helpful (i.e. a HPS
TM
Product seal and centering ring assembly with screen).
7.3.3 Orienting the Series 317 Pirani Sensor
When measuring pressures greater than 1 Torr, the Series 317 Sensor must be
mounted with its axis horizontal.
Measurements below 1 Torr are unaffected by position, but readings will be incorrect at higher
pressures. Incorrect readings could result in under- or over-pressure, damaging equipment or injuring
Mount the Sensor with the vacuum port facing downward to reduce particulates and liquids falling or
flowing into it. The sensors are calibrated in this position.
7.3.4 Orienting the Series 345 Pirani Sensor
Operating position has no effect on accuracy. The Pirani sensor was designed to minimize the effects
of convection. In a standard Pirani system, the output of the sensor changes very little in going from
the horizontal to vertical position.
The Series 345 Pirani sensor exhibits slight convection characteristics near atmospheric pressure.
Therefore, above 30 Torr the best accuracy can be achieved by calibrating the sensor in a vertical
position with the port facing down. The Pirani sensor can be calibrated at any pressure between 600
and 1000 Torr.
7.3.5 Connecting the Series 317/345 Sensors
Use an HPS
®
adaptive centering ring (HPS® PN 100315821) to fit a KF 16 port to a KF 10 port,.
To install the Sensor with a 1/8" NPT, do not apply torque to the case to tighten the connection. The
Sensor’s vacuum tubing has 9/16" hex flats for tightening. Wrap about two turns of Teflon
®
tape on
the threads of the Sensor in the direction of the threading to ensure a leak-free seal. Note: positive
pressures can blow the Sensor out of a compression fitting, damaging equipment and possibly
injuring personnel.
Do not use a compression mount (quick connect) to attach the Sensor to a
system in positive pressure applications.
A solid electrical connection between the Sensor and the grounded vacuum
system must be provided to shield the tube element from external radiation sources.
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