There are four big reasons to use highly accurate condensation hygrometers in the industrial world:
The obvious role of the condensation hygrometer is for use as a reference instrument. Calibration standards are required for most measuring devices. For water vapor measurements, the condensation hygrometer is the gold standard of reference instruments. It is accurate, fundamental, and accepted internationally as a transfer standard.
Some applications require maximum measurement accuracy one can obtain only by using reference-grade instruments. The automotive industry does extensive calorimetry to understand the performance of cooling systems (engine, air-conditioning) over a wide range of environmental conditions.
Because humid air requires more energy to heat and cool, knowledge of water vapor content is critical to understanding cooling system performance. The SAW and other condensation hygrometers offer measurement accuracy that is one order of magnitude better than typical process instruments.
Mission-critical applications require not only a measurement but also the knowledge that the measurement is valid. Manufacturing polyethylene terephthalate (PET) soft-drink bottles requires that the PET resin be completely dry before melting for injection molding.
The hygrometer need only verify the dew point temperature of clean dry air, but undetected measurement errors can have serious consequences. Improperly dried resin may result in defective bottles, causing a shutdown of the line, loss of raw material, and loss of productive capacity.
SAW hygrometers have extensive internal diagnostics for alerting the user to system problems or any needs for maintenance. Their design is essentially drift free, so they do not develop insidious measurement errors. In short, either they work correctly or you know they don’t work correctly.
Finally, there are applications where a specific instrument design is uniquely well suited for the measurement. For example, corona discharge ozone generators (as used in water treatment plants) are water cooled to prevent overheating. Leakage of water into the generator can cause catastrophic destruction of a million-dollar device.
A dew point measurement of the generator’s output gas, which may contain as much as 10% ozone, is helpful for identifying very small leaks and can signal the need for shutdown and maintenance. However, ozone is extraordinarily corrosive and difficult to work with. The SAW hygrometer, with wetted parts of stainless steel, Teflon, and quartz, matches this severe application well.