Humidity Sensor: Types of Humidity Sensors & Working Principle

Humidity is the presence of water in
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

Operational princle of SCR

SCR working principle
The SCR is a four-layer, three-junction
and a three-terminal device and is
shown in fig.a. The end P-region is the
anode, the end N-region is the
cathode and the inner P-region is the
gate. The anode to cathode is
connected in series with the load
circuit. Essentially the device is a
switch. Ideally it remains off (voltage
blocking state), or appears to have an
infinite impedance until both the
anode and gate terminals have
suitable positive voltages with respect
to the cathode terminal. The thyristor
then switches on and current flows
and continues to conduct without
further gate signals. Ideally the
thyristor has zero impedance in
conduction state. For switching off or
reverting to the blocking state, there
must be no gate signal and the anode
current must be reduced to zero.
Current can flow only in one direction.
In absence of external bias voltages,
the majority carrier in each layer
diffuses until there is a built-in voltage
that retards further diffusion. Some
majority carriers have enough energy
to cross the barrier caused by the
retarding electric field at each
junction. These carriers then become
minority carriers and can recombine
with majority carriers. Minority carriers
in each layer can be accelerated
across each junction by the fixed field,
but because of absence of external
circuit in this case the sum of majority
and minority carrier currents must be
zero.
A voltage bias, as shown in figure, and
an external circuit to carry current
allow internal currents which include
the following terms:

The current I_x is due to

• Majority carriers (holes) crossing
  junction J₁
  
• Minority carriers crossing junction
  J₁
  
• Holes injected at junction J₂
  diffusing through the N-region and
  crossing junction J₁ and
  

Minority carriers from junction J₂
diffusing through the N-region and
crossing junction J₁ .
Similarly I₂ is due to six terms and I₃
is due to four terms.
The two simple analogues to explain
the basic action for the thyristor are
those of the diode and the two
transistor models.
1. Diode Model. The thyristor is
similar to three diodes in series as
there are three P-N junctions.
Without gate bias, there is always
at least one reverse biased junction
to prevent conduction irrespective
of the polarity of an applied voltage
between anode and cathode. If the
anode is made positive and the
gate is also biased positively with
respect to cathode, the P-layer at
the gate is flooded by the electrons
from the cathode and loses its
identity as a P-layer. Accordingly
the thyristor becomes equivalent
to a conducting diode.

Scr working
2. Two Transistor Model.
Imagine the SCR cut along the
dotted line, as shown in fig. a.
Then we can have two devices, as
shown in fig.b. These two devices
can be recognized as two
transistors. The upper left one is P-
N-P transistor and the lower right
N-P-N type. Further it can be
recognized that the base of the P-
N-P transistor is joined to the
collector of the N-P-N transistor
while the collector of P-N-P is
joined to the base of N-P-N
transistor, as illustrated in fig. c.
The gate terminal is brought out
from the base of the N-P-N
material. This construction has
been conceived merely to explain
the working of SCR, otherwise in
physical shape the SCR has four
solid layers of P-N-P-N type only.
Now we can see that the two
transistors are connected in such a
manner that the collector of Q₁ is
connected to the base of Q₂ i.e. the
output collector current of Q_t
becomes the base current for Q₂ . In
the similar way the collector of Q₂ is
joined to the base of Q₁ which shows
that the output collector current of Q₂
is fed to Q₁ as input base current.
These are back to back connections of
transistors in such a way that the
output of one goes into as input of
other transistor and vice-versa. This
gives net gain of loop circuit as β₁x
β₂ where β₁ and β₂ are current gains
of two transistors respectively.
When the gate current is zero or the
gate terminal is open, the only current
in circulation is the leakage current,
which is very small in case of silicon
device specially and the total current
is a little higher than sum of individual
leakage currents. Under these
conditions P-N-P-N device is said to
be in its forward blocking or high
impedance ‘off state. As soon as a
small amount of gate current is given
to the base of transistor Q₂ by
applying forward bias to its base-
emitter junction, it generates the
collector current as β₂ times the base
current. This collector current of Q₂ is
fed as input base current to Q : which
is further multiplied by β₁ times as I_Cl
which forms input base current of Q₂
and undergoes further amplification.
In this way both transistors feedback
each other and the collector current
of each goes on multiplying. This
process is very quick and soon both
the transistors drive each other to
saturation. Now the device is said to
be in.on-state. The current through
the on-state SCR is controlled by
external impedance only.
Souce: circuitstoday