air. The amount of water vapor in air
can affect human comfort as well as
many manufacturing processes in
industries. The presence of water
vapor also influences various physical,
chemical, and biological processes
Humidity measurement in industries
is critical because it may affect the
business cost of the product and the
health and safety of the personnel.
Hence, humidity sensing is very
important, especially in the control
systems for industrial processes and
human comfort.
Controlling or monitoring humidity is
of paramount importance in many
industrial & domestic applications. In
semiconductor industry, humidity or
moisture levels needs to be properly
controlled & monitored during wafer
processing. In medical applications,
humidity control is required for
respiratory equipments, sterilizers,
incubators, pharmaceutical
processing, and biological products.
Humidity control is also necessary in
chemical gas purification, dryers,
ovens, film desiccation, paper and
textile production, and food
processing. In agriculture,
measurement of humidity is
important for plantation protection
(dew prevention), soil moisture
monitoring, etc. For domestic
applications, humidity control is
required for living environment in
buildings, cooking control for
microwave ovens, etc. In all such
applications and many others,
humidity sensors are employed to
provide an indication of the moisture
levels in the environment.
RELEVANT MOISTURE TERMS
To mention moisture levels, variety of
terminologies are used. The study of
water vapour concentration in air as a
function of temperature and pressure
falls under the area of psychometrics.
Psychometrics deals with the
thermodynamic properties of moist
gases while the term “humidity’ simply
refers to the presence of water vapour
in air or other carrier gas.
Humidity measurement determines
the amount of water vapor present in
a gas that can be a mixture, such as
air, or a pure gas, such as nitrogen or
argon.
CHARACTERISTICS
Sensor characterisation is done based
on the n-point(usually 9)
characterisation of the sensor.
Characterisation is performed at a
specific temperature (25°C) and
excitation.
In 9 point characterisation method,
humidity levels are swept the through
the RH values and measuring the
corresponding dc output voltage for
the individual sensor: Values are taken
at humidity levels of 0%, 25%, 53.2%,
75.3%, 93.8%, 75.3%, 53.2%, 25% and
0%. Based on the characterisation
results, Best Fit Straight Line (BFSL) is
plotted and sensor characteristics are
specified in the datasheets.
- Acuracy
Accuracy is specified based on the
specific calibration curves for any
individual sensor. It is specified using
the linear Best Fit Straight Line (BFSL)
and the non-linear 2 nd order curve.
As an example let us consider a
sensor with an accuracy of ±2% RH
(BFSL). If the sensor has an output
voltage of 0.689 V at 0%RH, an
average slope(BFSL) of 0.036 V/%RH
and offset of 0.662, then its BFSL
accuracy error is given by (0.689 -
0.662)/0.036 = 0.75% RH. As sensors
accuracy is ±2% RH (BFSL), i.e. 0.072V,
the sensor should always output
0.662 ±0.072 V or a value in the range
of 0.59 V to 0.734 V. - Hysteresis
Hysteresis is the difference between
the two voltages to %RH conversions
(using average BFSL slope) at each of
the four duplicated points in the nine
point characterization. Hysteresis is
recorded in absolute %RH terms.
The value taken is the largest %RH
figure for an individual sensor over
each of the four characterization
points. - Interchangeability
Interchangeability defines the range of
voltages for any population of sensors
at this RH point.
As an example let us consider a
sensor from a particular company
with an interchangeabilty of ±5% at
0% RH. With an average slope (BFSL)
of 0.036 V/%RH and offset of 0.662 V,
±5% RH is equal to ±0.18 V. This
means that the output voltage for this
device is 0.662 V ±0.18 V, or a range of
0.482V to 0.842 V. When exposed to
an RH of 0%, the output of the entire
population of sensors will fall within
this range. - Linearity
Linearity indicates the voltage
deviation from the BFSL value and the
measured output voltage value,
converted to RH. - Reliability
Sensors are subjected to accelerated
stress tests. If the tests causes the
sensor to drift and report RH outside
prescribed specifications, the sensor
is considered a failed sensor. Based
on such tests, reliability figures like
MTTF(Mean-time-to-failure) and FIT
(Number of Failures per billion
operating hours) are specified. - Repeatability
Repeatability is the maximum
variation between sensor outputs for
repeated sweeps of humidity levels
across the sensors’ measurement
range under identical conditions.
For example, if the point value is
0.013 V using the 31 mV/RH slope this
is 0.42% RH. - Response Time
Response Time is measured in “slow
moving air” (less than 5 m/s).
Typically, maximum time required for
the output voltage of the sensor to
rise to 63% of its final value or to fall
to 37% of its final value when exposed
to a step rise or fall in humidity is
specified as response time - Temperature Compensation
Voltage output for an individual
sensor at a given excitation and RH is
affected by temperature. In many
sensors, the temperature is measured
and the effect of temperature of
humidity measurement is reduced
and this is referred to as temperature
compensation. - StabilityOutput voltage stability is the output
voltage drift in time at the specified RH
level converted to a %RH value.
This figure is also generated through
accelerated stress tests and is typically
taken as the change in mean output
voltage from a large batch of sensors
in specific environmental conditions.
SELECTING A HUMIDITY SENSOR
As there no real physical standard for
relative humidity calibration, humidity
instruments are not specified
properly. And it makes it really difficult
for a user to compare the sensors
from different manufacturers. This
makes it mandatory for a user to go
deeper into the specifications and
attempt to verify the claims of the
instrument manufacturer. Various
sensor characteristics, viz., accuracy,
linearity, hysteresis, calibration errors,
long term stability of sensor and
electronics, needs to be examined
using the support documentation
from the OEM.
Source: Engineersgarage
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