Vacuum Metrology

VACUUM METROLOGY  

In the above photo, the two large boxes contain the power supplies and analog meters for varian 531 thermocouple vacuum gauges. These are old units, have limited accuracy, and require frequent calibration, but they are still useful, particularly in the range from 10 to 1000 microns..

The two small LCD meters at the bottom are reading the high voltage power supply output voltage (in kilovolts) and current (in milliamperes)..

When this picture was taken the orange Meterman digital multimeter was being used as a temporary readout for a Barocel Capacitive Manometer vacuum gauge. The power supplies for the Barocel are not visible in the photo.

THERMOCOUPLE VACUUM GAUGE

Shown above is the Kurt J Lesker type 5311 Thermocouple Gauge installed in a QF-25 adaptive tee. The 5311 is a direct replacement for a Varian 531 and works well with the Varian 802-A and 804-A readouts shown in the top picture.

531 type thermocouple gauges are most useful in the 10 to 2000 micron range. Below  about 10 microns, the scale is compressed and difficult to read. They do require an initial zero calibration, which can be difficult to achieve without access to a diffusion or turbo pump.

They complement our 0 – 100 micron Capacitive Manometer gauges, which give amazingly good accuracy down to about one hundredth of a micron (10 e -5 Torr) but are useless above 100 microns.

CAPACITIVE MANOMETERS

Our first Capacitive Manometer type vacuum gauge, an MKS Baratron unit. We are currently using two Boc Edwards Barocel units. The Baratron and Barocel  are very simiular in appearance, and are directly interchangeable.

The Barocel units have a range of 0 – 100 microns, ideal for fusor work.

The output is a linear 0 – 10 volts for a pressure range of 0 to 100 microns, so a digital voltmeter works well as a readout. Both Baratron and Barocel units require regulated plus & minus 15 volts power supplies. A D-connector is used for all electrical connections. The base is a QF-25 flange.

 CALIBRATION

In the picture above, the newly completed diffusion pump was being used to calibrate a Barocel capacitive manometer vacuum gauge, using a temporary setup. The white cylinder visible to the right of the bell jar is the Barocel unit. To the right of the diffusion pump, and being used to cool it, is a 5 inch muffin fan, being temporarily supported by some text books.

Both thermocouple and capacitive manometer vacuum gauges need to have their zero adjusted periodically. This is normally the only type of calibration necessary. A diffusion pump is ideal for the zero adjustment.

The Leybold 2-stage vane-type pump (visible bottom left in the photo) is being used as a roughing pump for the diffusion pump.

Also visible in the picture is the safety screen around the glass bell jar. The inner surface of the bell has been darkened by the effects of the fusor operation. The large turquoise object below the meters is a Variac autotransformer, used to control the main high voltage power supply for the fusor.  See the  Description and picture of the power supply

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