- The most important specifications to keep in mind when selecting a humidity sensor are:
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- Ability to recover from condensation
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- Resistance to chemical and physical contaminants
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Additional significant long-term factors are the costs associated
with sensor replacement, field and in-house calibrations, and the
complexity and reliability of the signal conditioning and data
acquisition (DA) circuitry. For all these considerations to make sense,
the prospective user needs an understanding of the most widely used
types of humidity sensors and the general trend of their expected
performance. Definitions of absolute humidity, dew point, and relative
humidity are provided in the sidebar, "Humidity Basics").
Capacitive Humidity Sensors Relative Humidity.
Capacitive relative humidity (RH) sensors (see Photo 1) are widely used
in industrial, commercial, and weather telemetry applications.
Photo 1. Capacitive RH sensors are produced in a wide range of
specifications, sizes, and shapes including integrated monolithic
electronics. The sensors shown here are from various manufacturers. |
They consist of a substrate on which a thin film of polymer or metal
oxide is deposited between two conductive electrodes. The sensing
surface is coated with a porous metal electrode to protect it from
contamination and exposure to condensation. The substrate is typically
glass, ceramic, or silicon. The incremental change in the dielectric
constant of a capacitive humidity sensor is nearly directly proportional
to the relative humidity of the surrounding environment. The change in
capacitance is typically 0.2–0.5 pF for a 1% RH change, while the bulk
capacitance is between 100 and 500 pF at 50% RH at 25°C. Capacitive
sensors are characterized by low temperature coefficient, ability to
function at high temperatures (up to 200°C), full recovery from
condensation, and reasonable resistance to chemical vapors. The response
time ranges from 30 to 60 s for a 63% RH step change.
State-of-the-art techniques for producing capacitive sensors take
advantage of many of the principles used in semiconductor manufacturing
to yield sensors with minimal long-term drift and hysteresis. Thin film
capacitive sensors may include monolithic signal conditioning circuitry
integrated onto the substrate. The most widely used signal conditioner
incorporates a CMOS timer to pulse the sensor and to produce a
near-linear voltage output (see Figure 1).
Figure 1. A near-linear response is seen in this plot of capacitance
changes vs. applied humidity at 25°C. The term "bulk capacitance" refers
to the base value at 0% RH. |
The typical uncertainty of capacitive sensors is ±2% RH from 5% to
95% RH with two-point calibration. Capacitive sensors are limited by the
distance the sensing element can be located from the signal
conditioning circuitry, due to the capacitive effect of the connecting
cable with respect to the relatively small capacitance changes of the
sensor. A practical limit is <10 ft.
Direct field interchangeability can be a problem unless the sensor is
laser trimmed to reduce variance to ±2% or a computer-based
recalibration method is provided. These calibration programs can
compensate sensor capacitance from 100 to 500 pF.
Dew Point. Thin film capacitance-based sensors provide
discrete signal changes at low RH, remain stable in long-term use, and
have minimal drift, but they are not linear below a few percent RH.
These characteristics led to the development of a dew point measuring
system incorporating a capacitive sensor and microprocessor-based
circuitry that stores calibration data in nonvolatile memory. This
approach has significantly reduced the cost of the dew point hygrometers
and transmitters used in industrial HVAC and weather telemetry
applications.
The sensor is bonded to a monolithic circuit that provides a voltage
output as a function of RH. A computer-based system records the voltage
output at 20 dew point values over a range of –40°C to 27°C. The
reference dew points are confirmed with a NIST-traceable chilled mirror
hygrometer. The voltage vs. dew/frost point values acquired for the
sensor are then stored in the EPROM of the instrument. The
microprocessor uses these values in a linear regression algorithm along
with simultaneous dry-bulb temperature measurement to compute the water
vapor pressure.
Once the water vapor pressure is determined, the dew point
temperature is calculated from thermodynamic equations stored in EPROM.
Correlation to the chilled mirrors is better than ±2°C dew point from
–40°C to –7°C and ±1°C from –7°C to 27°C. The sensor provides long-term
stability of better than 1.5°C dew point drift/yr. Dew point meters
using this methodology have been field tested extensively and are used
for a wide range of applications at a fraction of the cost of chilled
mirror dew point meters.
Resistive Humidity Sensors
Resistive humidity sensors (see Photo 2) measure the change in
electrical impedance of a hygroscopic medium such as a conductive
polymer, salt, or treated substrate.
Photo 2. Resistive sensors are based on an interdigitated or bifilar
winding. After deposition of a hydroscopic polymer coating, their
resistance changes inversely with humidity. The Dunmore sensor (far
right) is shown 1/3 size. |
The impedance change is typically an inverse exponential relationship to humidity (see Figure 2).
Figure 2. The exponential response of the resistive sensor, plotted
here at 25°C, is linearized by a signal conditioner for direct meter
reading or process control. |
Resistive sensors usually consist of noble metal electrodes either
deposited on a substrate by photoresist techniques or wire-wound
electrodes on a plastic or glass cylinder. The substrate is coated with a
salt or conductive polymer. When it is dissolved or suspended in a
liquid binder it functions as a vehicle to evenly coat the sensor.
Alternatively, the substrate may be treated with activating chemicals
such as acid. The sensor absorbs the water vapor and ionic functional
groups are dissociated, resulting in an increase in electrical
conductivity. The response time for most resistive sensors ranges from
10 to 30 s for a 63% step change. The impedance range of typical
resistive elements varies from 1 k
to 100 M
.
Most resistive sensors use symmetrical AC excitation voltage with no
DC bias to prevent polarization of the sensor. The resulting current
flow is converted and rectified to a DC voltage signal for additional
scaling, amplification, linearization, or A/DRconversion (see Figure
3).
Figure 3. Resistive sensors exhibit a nonlinear response to changes in
humidity. This response may be linearized by analog or digital methods.
Typical variable resistance extends from a few kilohms to 100 MV. |
Nominal excitation frequency is from 30 Hz to 10 kHz.
The "resistive" sensor is not purely resistive in that capacitive effects >10–100 M
makes the response an impedance measurement. A distinct advantage of
resistive RH sensors is their interchangeability, usually within ±2% RH,
which allows the electronic signal conditioning circuitry to be
calibrated by a resistor at a fixed RH point. This eliminates the need
for humidity calibration standards, so resistive humidity sensors are
generally field replaceable. The accuracy of individual resistive
humidity sensors may be confirmed by testing in an RH calibration
chamber or by a computer-based DA system referenced to standardized
humidity-controlled environment. Nominal operating temperature of
resistive sensors ranges from –40°C to 100°C.
In residential and commercial environments, the life expectancy of
these sensors is >>5 yr., but exposure to chemical vapors and
other contaminants such as oil mist may lead to premature failure.
Another drawback of some resistive sensors is their tendency to shift
values when exposed to condensation if a water-soluble coating is used.
Resistive humidity sensors have significant temperature dependencies
when installed in an environment with large (>10°F) temperature
fluctuations. Simultaneous temperature compensation is incorporated for
accuracy. The small size, low cost, interchangeability, and long-term
stability make these resistive sensors suitable for use in control and
display products for industrial, commercial, and residential
applications.
One of the first mass-produced humidity sensors was the Dunmore type,
developed by NIST in the 1940s and still in use today. It consists of a
dual winding of palladium wire on a plastic cylinder that is then
coated with a mixture of polyvinyl alcohol (binder) and either lithium
bromide or lithium chloride. Varying the concentration of LiBr or LiCl
results in very high resolution sensors that cover humidity spans of
20%–40% RH. For very low RH control function in the 1%–2% RH range,
accuracies of 0.1% can be achieved. Dunmore sensors are widely used in
precision air conditioning controls to maintain the environment of
computer rooms and as monitors for pressurized transmission lines,
antennas, and waveguides used in telecommunications.
The latest development in resistive humidity sensors uses a ceramic
coating to overcome limitations in environments where condensation
occurs. The sensors consist of a ceramic substrate with noble metal
electrodes deposited by a photoresist process. The substrate surface is
coated with a conductive polymer/ceramic binder mixture, and the sensor
is installed in a protective plastic housing with a dust filter.
The binding material is a ceramic powder suspended in liquid form.
After the surface is coated and air dried, the sensors are heat treated.
The process results in a clear non-water-soluble thick film coating
that fully recovers from exposure to condensation (see Figure 4).
Figure 4. After water immersion, the typical recovery time of a
ceramic-coated resistive sensor to its pre-immersion, 30% value is 5-15
min., depending on air velocity. |
The manufacturing process yields sensors with an interchangeability
of better than 3% RH over the 15%–95% RH range. The precision of these
sensors is confirmed to ±2% RH by a computer-based DA system. The
recovery time from full condensation to 30% is a few minutes. When used
with a signal conditioner, the sensor voltage output is directly
proportional to the ambient relative humidity.
Thermal Conductivity Humidity Sensors
These sensors (see Photo 3) measure the absolute humidity by
quantifying the difference between the thermal conductivity of dry air
and that of air containing water vapor.
Photo 3. For measuring absolute humidity at high temperatures, thermal
conductivity sensors are often used. They differ in operating principle
from resistive and capacitive sensors. Avbsolute humidity sensors are
left and center; thermistor chambers are on the right. |
When air or gas is dry, it has a greater capacity to "sink" heat, as
in the example of a desert climate. A desert can be extremely hot in the
day but at night the temperature rapidly drops due to the dry
atmospheric conditions. By comparison, humid climates do not cool down
so rapidly at night because heat is retained by water vapor in the
atmosphere.
Thermal conductivity humidity sensors (or absolute humidity sensors)
consist of two matched negative temperature coefficient (NTC)
thermistor elements in a bridge circuit; one is hermetically
encapsulated in dry nitrogen and the other is exposed to the environment
(see Figure 5).
Figure 5. In thermal conductivity sensors, two matched thermistors are
used in a DC bridge circuit. One sensor is sealed in dry nitrogen and
the other is exposed to ambient. The bridge output voltage is directly
proportional to absolute humidity. |
When current is passed through the thermistors, resistive heating
increases their temperature to >200°C. The heat dissipated from the
sealed thermistor is greater than the exposed thermistor due to the
difference in the thermal conductively of the water vapor as compared to
dry nitrogen. Since the heat dissipated yields different operating
temperatures, the difference in resistance of the thermistors is
proportional to the absolute humidity (see Figure 6).
Figure 6. The output signal of the thermal conductivity sensor is
affected by the operating temperature. Maximum output is at 600°C;
output at 200°C drops by 70%. |
A simple resistor network provides a voltage output equal to the range of 0–130 g/m
3
at 60°C. Calibration is performed by placing the sensor in
moisture-free air or nitrogen and adjusting the output to zero. Absolute
humidity sensors are very durable, operate at temperatures up to 575°F
(300°C) and are resistant to chemical vapors by virtue of the inert
materials used for their construction, i.e., glass, semiconductor
material for the thermistors, high-temperature plastics, or aluminum.
An interesting feature of thermal conductivity sensors is that they
respond to any gas that has thermal properties different from those of
dry nitrogen; this will affect the measurements. Absolute humidity
sensors are commonly used in appliances such as clothes dryers and both
microwave and steam-injected ovens. Industrial applications include
kilns for drying wood; machinery for drying textiles, paper, and
chemical solids; pharmaceutical production; cooking; and food
dehydration. Since one of the by-products of combustion and fuel cell
operation is water vapor, particular interest has been shown in using
absolute humidity sensors to monitor the efficiency of those reactions.
In general, absolute humidity sensors provide greater resolution at
temperatures >200°F than do capacitive and resistive sensors, and may
be used in applications where these sensors would not survive. The
typical accuracy of an absolute humidity sensor is +3 g/m
3; this converts to about ±5% RH at 40°C and ±0.5% RH at 100°C.
Summary
Rapid advancements in semiconductor technology, such as thin film
deposition, ion sputtering, and ceramic/silicon coatings, have made
possible highly accurate humidity sensors with resistance to chemicals
and physical contaminants?at economical prices. No single sensor,
however, can satisfy every application. Resistive, capacitive, and
thermal conductivity sensing technologies each offer distinct
advantages. Resistive sensors are interchangeable, usable for remote
locations, and cost effective. Capacitive sensors provide wide RH range
and condensation tolerance, and, if laser trimmed, are also
interchangeable. Thermal conductivity sensors perform well in corrosive
environments and at high temperatures. For most applications, therefore,
the environmental conditions dictate the sensor choice.
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