On-line hygrometer is used to measure the dew point in air and process gases such as natural gas, compressed air and natural, with a wide range and multi-field display options. It provides reliable on-line measurement of moisture in virtually any gas, flammable or non-flammable, inert or corrosive across many applications. It can also be used for quality assurance in the supply of compressed air from heat regenerative adsorption dryers as well as for moisture control in refinery catalytic reformer recycle gas and many other petrochemical applications.