Dewpoint monitoring is an important requirement in many industrial applications. To insure safe and reliable dewpoint monitoring, Kahn Instruments offers the Cermet II hygrometer, which measures moisture over a range of -148°F to +68°F (-100°C to +20°C) dewpoint.

The instrument is designed to monitor dewpoint of non-flammable process gas such as compressed air, nitrogen, sulfur hexafluoride, carbon dioxide, helium or argon.  In addition to its wide dewpoint measurement range of          -148°F to +68°F, the Cermet II can measure 0.001 to 9999 PPMV moisture content in non-flammable gases, and an intrinsically safe model can measure 0 to 1000 #/MMSCF moisture content in natural gas.  Measurement accuracy is within +/-1°C from -59°C to +20°C, and +/2°C from -100°C to -60°C. Resolution is 0.1°C from -79°C to +20°C, and 1.0°C from -100°C to -80°C.

Dewpoint is defined as the temperature at which water vapor pressure of a gas equals the saturated water vapor pressure. It is a very convenient measure of actual water content of a gas, because it is not a function of temperature in the same way relative humidity is. Measuring dewpoint of non-flammable gases is important in ensuring the quality of gases used and products produced in many industrial applications, such as heat treating, pharmaceutical manufacturing, electric power generation, and semiconductor fabrication.

Inside the hygrometer

The Cermet II Hygrometer consists of a digital display with an integral signal conditioning board, ceramic dewpoint sensor and an interconnecting cable. The sensor can be installed directly in the gas line or in a separate sampling gas stream by utilizing the optional sensor block. The display can be easily mounted in a control panel or other appropriate location.

The hygrometer has two independent, adjustable set-point relays that can be used to provide an operator alarm, should the dewpoint exceed user programmable limits. A 4 to 20 mA scalable, linear output interfaces systems that use programmable logic controllers or other controllers.

The optional second input accepts a 4 to 20 mA signal from any two-wire transmitter. It can be configured as a separate display for pressure, temperature, flow or other process variable. When displaying in PPMV, #/MMSCF or g/m³, line pressure can be entered manually, or a pressure transmitter can be connected to the optional second input to provide automatic pressure compensation.

The hygrometer’s display, measuring 1.9” high x 3.8” wide x 5.6” deep, mounts in a 1/8 DIN cutout. It has programmable brightness and displays moisture measurements in units of °F or °C dewpoint, and pressure compensated PPMV, #/MMSCF and g/m³.

Ceramic sensor

One key to the hygrometer’s high performance is the ceramic sensor, made of metallized ceramic instead of traditional materials like aluminum, silicon and hygroscopic salts. The active surface of the ceramic sensor is vapor deposited to form a surface that is sensitive to small changes in water vapor pressure.

Kahn utilizes proprietary coating processes that enable the sensor to respond faster than other currently available impedance sensors. The sensor also features greater resistance to corrosive gases and other contaminants. All of Kahn’s ceramic sensors are fully interchangeable without display recalibration.

The digitally based sensor stores calibration data directly in its memory. It can be situated up to 4,000 feet from the digital display, without affecting calibration, and will operate at pressures from near vacuum to 5,000 psig and over a temperature range from -40°C to +60°C.  A built-in thermistor provides automatic temperature compensation.

An intrinsically safe variant of the sensor can be used with an isolation safety barrier for moisture measurement applications in environments containing flammable gases, such as natural gas, or in environments classified as hazardous (Class I, Divisions 1 or 2).  Options include a sensor block, second process input channel (e.g. for pressure), 10 micron sintered metal guard, sampling system, coalescing/particulate filter, NEMA 4 enclosure and an open sensor alarm relay.

The Cermet II Hygrometer operates on 85 to 265 VAC, or optionally, 9 to 60 VDC. The instrument’s calibration is traceable to the National Institute of Standards and Technology (NIST), and Kahn always provides a certificate of calibration with all new and recalibrated sensors.  All sensors are fully interchangeable without the need for display recalibration and all calibrations are guaranteed for one year.

For more information, please contact Robert Bailey at Kahn Instruments Inc., 885 Wells Road, Wethersfield, CT 06109, at 860-529-8643, or email: hygros@kahn.com.