This is part two of a seven-part series about measuring water vapor and hydrocarbon dew point in natural gas. Click here to read part one.
Chilled Mirror Hygrometry
A chilled mirror analyzer measures a physical property. The device accurately measures the temperature at which water condenses on a temperature-controlled surface that is in contact with the process gas. Chilled mirror instruments typically operate by continuously flowing sample gas across a temperature-controlled, polished surface (i.e., a mirror). As the temperature of the polished surface is slowly lowered, formation of condensate is identified visually, either by directly observing the surface of the mirror, or by means of a magnified viewer, which superimposes the mirror temperature on a magnified mirror surface, or electronically by monitoring light intensity or absorbance as reflected from the mirror surface. Once the condensate has formed and a “dew point” temperature has been established, the sample pressure and temperature may be recorded.
Chilled mirrors can be classified into two categories, manually operated (Figures 1A and 1B) or automated, online devices. Manual devices are typically available at a fraction of the cost of online water vapor measurement devices but require frequent maintenance and experienced technicians to perform the measurement.
Figures 1A and 1B. Manually operated dew point chilled mirror devices
Advantages of the manually operated chilled mirror device includes the ability to detect both moisture (water) and hydrocarbon dew points. The major disadvantage of using manually operated chilled mirrors is that manual devices require experienced operators to make reproducible observations of condensate formation. Figures 2a and 2b highlight the differences in the appearance of hydrocarbon and moisture (water) dew point condensate on the surface of a manually operated chilled mirror device.
Figure 2A. Water dew point with chilled mirror Figure 2B. Hydrocarbon iridescent ring (dew point)
Automated chilled mirror devices generally follow the same “direct” measurement criteria as do manual devices. Automated chilled mirrors provide a number of key features and benefits compared to their manual counterparts. Automated chilled mirrors provide a continuous update of dew point temperatures without requiring continuous supervision or intervention by experienced technicians. The major drawback of the automated chilled mirror devices is the initial cost of the system. Automated chilled mirrors generally require sample systems and some environmental control (installation in enclosures or shelters) to operate properly.
For more detailed information about this application, refer to our White Paper, “Analytical Devices for the Measurement of Water Vapor and Hydrocarbon Dew-Point in Natural Gas.”