Laser Hygrometer Provides Fast, Precise Moisture Measurement

The new Aurora Trace moisture analyser from GE Measurement & Control (USA), uses high definition laser absorption spectroscopy (HDLAS) to provide fast and accurate measurement of the moisture content of gases in the 0-10 parts per million (PPMV) range. It incorporates technology which ensures that its resolution is significantly higher than existing laser absorption equipment, while its sensitivity, or its ability to detect minute amounts of moisture, is increased by a factor of 30. In addition, the response time of the new instrument is the fastest of any currently available system. Typical applications for the new Aurora Trace include moisture measurement in the dry feed gas for the LNG liquefaction processes as well as a range of applications in the petrochemical sector where water-detection is process-critical.

Laser absorption spectroscopy is now an accepted moisture measurement technique in the gas sector but Aurora Trace brings new levels of accuracy and reliability. As Ken Soleyn, Product Manager at GE, explains, “As with all Laser Absorption moisture instruments, the new high definition technique scans a laser diode across a narrow frequency band and at certain frequencies the loss of laser intensity is directly proportional to the water concentration in the sample, as water absorbs the photonic energy. However, HDLAS incorporates two fundamental innovations to increase resolution and sensitivity. First of all, it incorporates a finesse cell to bounce the laser beam back and forth between two specially curved mirrors. This essentially increases the path length of the laser beam so that it encounters more water molecules in its passage through the gas. As a result, HDLAS offers thirty times the sensitivity of existing laser absorption technology. The second innovation is the application of vacuum in the measurement cell. This decreases the interactions between the water and carrier gas molecules, which can affect absorption, so that resolution is significantly increased by a factor of five and signal-noise ratio is improved.”

Aurora Trace is completely non-contact, as only the laser light and wetted parts come into contact with the process gas. It is designed to monitor continuously, without maintenance, for more than five years and has no need for field calibration or zero gas baselining. In addition to the measurement of moisture concentration, dew/frost point, temperature and pressure “instrument health” parameters are also continuously monitored to assure measurement integrity. An optional verification system is also available.

Aurora Trace is designed for use in hazardous environments and magnetic induction buttons enable it to be programmed remotely by a magnetic stylus. Consequently, a “hot permit” is not required for programming in a hazardous area, further reducing maintenance costs and simplifying operation. Aurora Trace data is transmitted as three programmable 4-20 MA signals or digitally via RS-232/485 with Modbus RTU. Ethernet and Foundation Fieldbus are also available.