Selecting the Right Humidity Sensor for Your Application
|There are a large variety of humidity sensors on the
market. The task of selecting the most suitable humidity sensor from a
cost and performance standpoint is not always an easy one. The range
of humidity sensors available extends from the simple hair hygrometer,
to a large variety of polymer sensors, to chilled mirror dew point
hygrometers, to the exotic Lyman-alpha or infrared hygrometers. All
have their place, but selecting the correct sensor for a particular
measurement is often confusing.
Yankee offers two of the most
popular and widely used types of humidity sensors; the chilled mirror
dew point hygrometer and the polymer-based relative humidity sensor.
The chilled mirror hygrometer offers one of the widest range and
highest precision humidity measurements. In addition, the sensor is
virtually indestructible. The polymer relative humidity sensor is much
less expensive, covers a narrower range and is less precise, but is
adequate for a large category of day-to-day environmental
|The chilled mirror hygrometer (CMH) dew point has
several distinct advantages over other water vapor sensing
- A CMH provides one of the few truly direct physical measurements
of humidity. It is recognized as the most precise method of
determining the water vapor content of a gas above 5% RH. Use
it when you need maximum precision of measurement.
- The CMH's optical sensor is a totally inert device. The sample
gas contacts glass and non-reactive metals. Thus, it can be easily
cleaned and can last indefinitely. Use it when the gas sample
contains contaminants and particulates, which would damage other
types of humidity sensors.
- Unlike polymer RH sensors, lithium chloride dew cells, and other
chemically-based sensors, a CM sensor does not lose its calibration. Use
it when you need long-term calibration stability.
The dew/frost point temperature defines the saturation point for
the water vapor in the gas. From this unique equilibrium temperature,
all other reporting formats of gas humidity can be derived. With
measurements of gas temperature and pressure, other reporting forms
for humidity can be derived.
|The measurement of the water vapor content of a gas by
the dew-point technique involves chilling a surface, usually a
metallic mirror, to the temperature at which water on the mirror
surface is in equilibrium with the water vapor pressure in the gas
sample above the surface. At this temperature, the mass of water on
the surface is neither increasing (too cold a surface) nor decreasing
(too warm a surface).
In the chilled-mirror technique, a mirror is constructed from a
material with good thermal conductivity such as silver or copper, and
properly plated with an inert metal such as iridium, nickel,
or gold to prevent tarnishing and oxidation. The mirror is chilled
using a thermoelectric cooler until dew just begins to form. A beam of
light, typically from a solid-state LED, is aimed at the mirror
surface and a photodetector monitors reflected light.
As the gas sample flows over the chilled mirror, dew droplets form
on the mirror surface, and the reflected light is scattered. As the
amount of reflected light decreases, the photodetector output also
decreases. This in turn controls the thermoelectric heat pump via an
analog or digital control system that maintains the mirror temperature
at the dew point. A precision miniature platinum resistance
thermometer (PRT) properly embedded in the mirror monitors the mirror
temperature at the established dew point.
If the mirror is controlled to an equilibrium condition above the
ice point, i.e., 0° C, the sensor is measuring the dew point.
Below 0° C, the deposit cannot long persist as liquid water, and it
is assumed that the deposit is frost, and that the sensor is measuring
the frost point. However, if the mirror is kept extremely
clean, it is possible for dew to exist below 0° C, and the only true
way to verify that the sensor is controlling on the frost point is to
visually inspect the mirror via a microscope. However, especially
outdoors, it is typically impractical to maintain a perfectly clean
mirror, as contaminants such as spores and other particulates serve as
motes on which frost deposits can nucleate. Consequently, errors due
to dew/frost point confusion at 0° C are seldom encountered.
|A chilled mirror hygrometer works by detecting the
scattered light from a condensation layer that forms on a cooled
mirror from a gas sample. To function properly, a chilled mirror
hygrometer must be able to cool its mirror to the dew or frost point
of the sample. The range of dew point temperatures that a chilled
mirror hygrometer can measure is dictated primarily by the heat
pumping capability of the solid state thermoelectric mirror cooler.
The graph below shows the typical useful working span of
hygrometers with single- and two-stage coolers. In a single-stage
hygrometer such as the YES 2010 series, if the sensor is
installed in an ambient temperature environment of +20° C, dew points
(frost point below 0° C) can be measured down to about -25° C. Thus,
the mirror can "depress" 45° C. Systems with two stage
coolers, such as the YES 2020 series, offer greater dew point
depression. A two-stage unit running at +20° C can measure frost
points down to -45° C. Thus, the mirror can "depress" 65°
C. Lower frost points can be measured by reducing the temperature of
the hot side of the cooler, or by using 3, 4, or 5 stage coolers or
via a customer-supplied mechanical chiller.
|Over the years a variety of sensing technologies have
evolved to measure the water vapor content of a gas including:
- Gravimetric train
- Aspirated (or sling) psychrometer
- Condensation type (chilled mirror)
- Capacitance-type electret
- Hair-type mechanical hygrometer
- Lyman Alpha hygrometer
- Surface Acoustic Wave type hygrometer
- LiCl saturated salt hygrometer
The absolute accuracy of the condensation-type (chilled mirror)
hygrometer is exceeded only by the gravimetric train, an elaborate
absorption system maintained as the primary standard for humidity
measurements. Over the mid-range of dew point temperatures (±
40° C), the uncertainties of measurement
for the chilled mirror hygrometer are in the order of one part in 100,
thus making it the most precise means of measuring water vapor
This graph was originally developed by the Mechanical/Humidity
Section at the National Bureau of Standards in the 1960's and shows
typical uncertainties for several popular humidity sensor
|You can view our full line of sensors here.