ITS40 GC/MS ( chromatomasspectrometer ) was produced since 1985 year of 20th sectury in cooperation of Finnigat MAT and Varian – very relieble longlasting device. Two of them are still running at my laboratory. But it has no gauge to check vacuum produced by forevacuum pump. This article is about schematic how to measure quality of vacuum by means of gauge ZJ-52T which can be bought on Aliexpress.
Method of measurement is very simple – special octal tube has filament wire ( pins 2 and 3 ), when we supply some voltage to it, becomes hot. Special thermoresistor connected to pins 1 and 4 and current through it depends of the quality of vacuum – as better is vacuum, as stronger is current.
PSU and measurement schemcatic is on the picture below
Secondary winding of the transformer T1 produces 20 V AC voltage, which after diode bridge and filtering gives 24 VDC volatage before the volage regulator T3 on LM317, which is connected as a current source – it’s value can be checked on mA meter. It’s value can be adjusted in 50 – 100 mA range by variale resistor R9. This current then applied to Wheatstone bridge on resistors R1 – R3, and as a fourth resistor of this bridge is used filament of the gauge ZJ-52T. Zero on the measurement microampermeter ( vacuum indicator in pct ) can be adjusted by means of resistor R14. Maximal range – 100 % can be adjusted by the resistor R5. I have used 100 uA microampermeter.
In order to extend life of the gauge longer, do not supply voltage to filament until good enough vacuum is maintained.
Hello, I would like to make a circuit for the ZJ-52T gauge. Do you know the maximum power to send to the filament? To avoid breaking it?
another question, what is the RVAR resistance when cold?
Hi, sorry for late reply. I have tried to contact AliExpress Seller with the similar question before purchase, but received to reply. Sensor’s filament wire is around 90 Ohms when cold, so if I apply to bridge 70 mA current, hence current through the filament is about 35 mA. If you wish to prolong life of the sensor, you can decrease this value, but sensitivity will be decreased also.