Dew point

Drift free measurements in vacuum

The S8000 Integrale cooled mirror hygrometer, from Michell Instruments, provides highly reliable, drift-free dew-point measurements in vacuums as low as 0.05 mbar, making it unique in its category. It is ideal for applications, such as oil refilling of transformers, which involve vacuum pumping.

Many high voltage transformers, especially those switching many thousands of kVA (such as the types found in power plant sub-stations), are filled completely with a refined mineral oil. This serves several purposes; most importantly the oil must act as a main part of the transformer’s insulation, though it also serves to cool the transformer.

Moisture inside transformers adversely affects the dielectric and insulating properties of the oil. Vacuum pumping the transformer enclosure is a common practice in the industry, with the sole purpose of minimising the total moisture content prior to oil filling.

Optimal dryness is desirable at this critical stage and a reliable, accurate dew point measurement enables users to monitor the process and ensure that moisture is eliminated as much as possible. Together with its low-pressure capability, the S8000 can measure dew-points as low as -60°C with 0.1°C accuracy. Its fundamental chilled-mirror technology is drift-free so measurements are reliable over time. The design of the sensor head allows for excellent sealing both in high-pressure as well as in low pressure and vacuum applications, making it a highly versatile instrument.