Safety system for compressed air producer - compressor is switched off when a resistance is not cooled due to low oil level

摘要

The two stage compressor has a probe screwed into the oil casing at the minimum desirable level of lubricating oil. The probe has a horizontal resistor usually immersed in the oil, the resistor having a high positive temperature coefficient of resistance between 160 deg.C and 150 Ohms and 200 deg.C and 1800 Ohms. The semi-conductor probe is heated by a continuous current but remains cool, due to the dissipation of the heat by the oil, and below 160 deg. Should the oil level fall below the minimum desirable level the probe temperature rises with the resistance to block a transistor circuit. This causes no signal to pass through a relay maintaining the compressor working and also operates an alarm.