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Head Office
®
Neptronic
400 Lebeau Blvd.
Montreal, Quebec, Canada H4N 1R6
Tel.: (514) 333-1433
Fax: (514) 333-3163
Toll Free: 1-800-361-2308
For Good Measure
Types of Hygrometers in the Market
There are multiple products available to measure humidity with different names, such as
hygrometers, humidistats, humidity sensors and dew point sensors. This article provides an
overview of these devices and their operation.
Wet bulb type:
A wet bulb hygrometer contains two thermometers, with the
bulb of one thermometer kept wet so that evaporation lowers
its temperature (wb) and the second thermometer is dry (db).
The relative humidity is determined by locating the
intersection of the wet and dry-bulb temperatures on a
psychrometric chart.
Metal-paper coil type:
The metal-paper coil hygrometer is very useful for giving a dial
indication of humidity changes. It appears most often in very
inexpensive devices, and its accuracy is limited, with variations
of 10% or more. In these devices, water vapor is absorbed by
a salt-impregnated paper strip attached to a metal coil, causing
the coil to change shape, which is reflected on the dial.
Hair tension type:
These devices use a human or animal hair under tension. Hair
tends to absorb moisture (hygroscopic), the length changes
with humidity and that length change is magnified by a
mechanism and indicated on a dial.
USA
NEP Inc.
P.O. Box 1151
Medford Oregon,
USA 97501
Tel.: (541) 531-5746
Middle East & Asia
NEP International FZE
P.O. Box 125687,
Dubai, UAE
Tel.: +97155 8825487
Fax: +9714 3426772
Singapore
Neptronic Pte Ltd
Office D6, #03-38,
Mountbatten Square
229, Mountbatten Road,
Singapore – 398 007
Mobile: +65 8118 4184
Tel: +65 6650 6212
Fax: +65 6491 6423
Chilled mirror dew point type:
Dew point is the temperature at which moist air at constant
pressure reaches water vapor saturation. At this saturation
temperature, further cooling results in condensation of water.
This device uses a chilled mirror and photo-detector to identify
condensation on the mirror's surface. The temperature of the
mirror is controlled by electronic feedback to maintain a
dynamic equilibrium between evaporation and condensation,
thus closely measuring the dew point temperature.
Capacitive humidity sensor:
Capacitive humidity sensors sense water by measuring the
change in capacitance between two plates. The sensor is a
made from two metal electrodes with a porous dielectric
substance between them. Water vapor is able to penetrate this
layer, changing the total capacitance. The incremental change
in the dielectric is nearly directly proportional to the relative
humidity.
Resistive humidity sensor:
Resistive type humidity sensors pick up changes in the resistance
value of the sensor element in response to the change in the
humidity. Resistive humidity sensors use membranes that change
conductivity in relation to water absorption. Typical materials are
salts and conductive polymers. As humidity changes, so does the
resistance of the electrodes.
Neptronic’s humidity sensors (wall mount model SHR10 and duct mount model SHC80) and
wall mount controllers (model HRO20 and HROB20 with BACnet) have the most advanced
humidity and temperature sensing solution for virtually any type of application. Each device has
a capacitive polymer sensor, which is individually calibrated and tested, to perform over a wide
humidity range.
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