IEEE C37.122.5-2013 IEEE Guide for Moisture Measurement and Control in SF 6 Gas-Insulated Equipment.
Chilled mirror hygrometers measure the dew point temperature of a gas using the following principles. Light shines onto a polished mirror surface, the temperature of which is controlled by a thermoelectric heat- pump known as a Peltier element. A light-sensitive receiver measures the intensity of the direct reflection. When the mirror is clean and dry, the intensity of the reflected light is at its maximum. Conversely, a cold mirror with water vapor condensed on its surface scatters the light, resulting in less directly reflected light and in reduced signal intensity. Using this received light signal as feedback in a closed loop control system, the mirror is cooled to the temperature at which the thickness of the condensed layer, detected through the intensity of the received light, remains constant. A condensate layer of constant thickness, with no further net increase or decrease in condensation, is in dynamic equilibrium with the gas surrounding the mirror. In this equilibrium condition, the dew point temperature of the gas is determined by measuring the temperature of the mirror. If the condensate is known to be in liquid form, even for temperatures below freezing, then the measured mirror temperature is taken as the dew point temperature. If the condensate is known to be in a solid form as ice or frost, then the measured mirror temperature is taken as the frost point. The typical flow rate is 40 l/h to 60 l/h.
These sensors use a hygroscopic polymer film which changes the capacitance of the sensor depending on the relative humidity. Hygrometers using capacitive polymer sensors are typically the most commonly used capacitive sensor type. These hygrometers usually measure the humidity content at atmospheric pressure, with the inlet connected to the GIS system being sampled. Depending on the instrument design, flow rates are maintained either automatically by the instrument, or by manual adjustment of a needle valve. Flow rates are approximately 20 l/h. The sensors measure relative humidity and recalculate to dew point temperature for indication. IEEE C37.122.5 pdf download.