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Cryostat Pressure Measurement |
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The SDSU utility card contains three un-dedicated op-amps that can be used to interface external analogue signals. Most cameras only use one of these for measuring the temperature of the detector. In the case of the INTWFC , a further channel is used to measure the cryostat pressure. The camera cryostat has a Balzers PKR251 full range gauge mounted permanently. The gauge power supply/display unit (TPG251) is mounted alongside the SDSU power supply in a 19" rack unit. The back of the display unit has a connector that gives an analogue output logarithmically proportional to the cryo pressure. A cable connects this signal to the SDSU front panel shutter connector (which is not used for any other purpose, the WFC shutter being controlled by the MCA)
The voltage output by the gauge PSU/display unit is = 6.8 + 0.6 x LOG10(pressure in mBar) This voltage appears on the circular control connector on the rear of the display unit, on pin 2 The gauge head produces a lot of interference ; the CCD readout noise doubles. This can be cured by partially unscrewing the grub-screw in the side of the gauge head so that the circuitry in the head becomes electrically isolated from the lower metal part of the gauge. This isolation can be checked with a DVM between the metal of the gauge head and pin5 of the 6 way connector on the back of the gauge. This should register open circuit, if it reads 10K Ohms then the grub screw has not been properly backed off. If the gauge measures <1e-7mBar it is assumed by uDAS that the head is disconnected. |
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