Application of Physical variable Transducer

1.Resistive sensors:

The input measured is transformed into a change in resistance.

Eg-Potentiometer,strain gauge,resistance thermometer,photo conductive cells etc. The basic block diagram of Potentiometer is given below:

Some of the Resistive sensors are explained in detail below:

a.Potentiometer:

A potentiometer is one of the most commonly used resistive sensors for measuring the displacement.The displacement may be either linear or rotatory. Thus,there are 2 types of Potentiometer as:

1.Linear potentiometer

2.rotatory potentiometer

Let ,

Input excitation voltage=e_i

Total resistance of Potentiometer=R_p

Total length of Potentiometer=x_t

.^.. Resistance per unit length = R_p / x_t

From equation (3) , we can conclude that there exist a linear relationship between the output and input.

The sensitivity of the device is given by,

Sensitivity,S=output/input = e₀ /x_i =e_i /x_t =constant

Thus,the sensitivity of the device is constant. The above relations are applicable for rotatory potentiometer if we replace x_i by ɵ_i and x_t by ɵ_t .

Loading effect:

If the resistance across the output terminal is infinite , we get a linear relation between the output and input voltage given by e₀ =ke_i =(x_i / x_t )*e_{i .} However, the output terminal of the POT, in practice ,is connected to the device whose internal resistance(R_m) is finite. Thus, an electrical instrument is connected to the output terminal. So, the indicated voltage is less than given by above equation . This effect is loading effect,due to which there exists a non-linear relationship between output and input. The loading effect is mainly caused by the input resistance is the output device.

Where R_L = internal resistance of the load and the output is taken along resistance R_L

The output voltage measured by the voltmeter is given by,

Let us assume Rm/Rp = then

Thus from above equation , we can conclude that there exist a non-linear relationship between output and input due to the loading effect.

Loading error

There are two types of types of a loading error. They are:

1.Relative error

2.Absolute error

It is given by,

Thus , the error will be maximum when wiper is at the center of the potentiometer.In actual practice , error is maximum at k=0.6T.

Lets plot the variation of error with a variation of ‘K’ for different value of α .

From the above graph , it is clear that as α increases Relative error(E_r) decreases.

Method to reduce Loading error effect

To decrease loading error, we should increase the value of i.e- R_m > R_p.

1.Use digital meter instead of analog meter because digital meter has higher input impedance.

2.Connect a buffer amplifier in combination with the voltmeter.

3.Modify the construction of potentiometer.

By connecting a compensating resistor R_comp = R_m across the remaining portion of the POT as shown in the figure above.

The power rating of the potentiometer.

It is the maximum value of heat that can be dissipated by the potentiometer without any damage. The heat dissipation is given by P=e_i²/R_p . Hence , the power rating is given by,

P_max =e_imax ²/R_p

So, e_imax=(p_max .R_p)^1/2

Linearity and sensitivity of the potentiometer:

As we know that, output voltage during loading is given by,

This equation shows that non-linear relation between e_o and e_i .

For linearity,α tends to infinity

i.e- R_m/R_p =∞

or, R_m >> R_p

i.e- R_p should be low as possible . however , for sensitivity,

S=e_o/x_i =e_i/x_t

The above relation shows that , For higher sensitivity , e_i must be high. But we know that

P_max =e_imax ²/R_p

i.e- The value of Pmax should be kept within certain limit. It is possible if e_i is low as possible and R_p is high as possible.

Thus, from above two relations, we can conclude that Linearity and sensitivity are the conflicting requirements of the Potentiometer.

b.The Strain Gauge

The strain is the amount of deformation of a body due to an applied force. More specifically, strain (ε) is defined as the fractional change in length, as shown in Figure below.

Strain can be positive (tensile) or negative (compressive). Although dimensionless, the strain is sometimes expressed in units such as in./in. or mm/mm. In practice, the magnitude of measured strain is very small. Therefore, the strain is often expressed as microstrain (µε), which is ε × 10^–6.While there are several methods of measuring strain, the most common is a strain gauge, a device whose electrical resistance varies in proportion to the amount of strain in the device. For example, the piezoresistive strain gauge is a semiconductor device whose resistance varies nonlinearly with strain. The most widely used gauge, however, is the bonded metallic strain gauge.

The metallic strain gauge consists of a very fine wire or, more commonly, metallic foil arranged in a grid pattern. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction (Figure below). The cross-sectional area of the grid is minimized to reduce the effect of shear strain and Poisson Strain. The grid is bonded to a thin backing, called the carrier, which is attached directly to the test specimen. Therefore, the strain experienced by the test specimen is transferred directly to the strain gauge, which responds with a linear change in electrical resistance. Strain gauges are available commercially with nominal resistance values from 30 to 3000 Ω, with 120, 350, and 1000 Ω being the most common values.

Let us consider a strain gauge made of circular wire Assuming ‘L’ be the length and ‘D’ be the diameter of wire before being strained. Suppose the material of the wire has resistivity ρL/A Then,

Resistance of unstrained gauge,R= ρL/A---------------(1)

This is the required equation for the gauge factor of the strain gauge.

Note: Tensile strength is taken positive and Compressive strength is taken negatively.

C.Resistance thermometer

It is one of the examples of a resistive sensor and is used to measure temperature. It is also termed as resistance temperature detectors(RTDs) and it is used to measure temperature by correlating the resistance of the RTD element with temperature .The basic block diagram of resistance thermometer is given below:

There are basically two types of resistance thermometer. They are :

i.)Metal Resistance thermometer

ii.)Semiconductor Resistance thermometer

i).Metal Resistance thermometer

This thermometer is an instrument used to measure the temperature variation in the control room.

Working principle

In each metallic conductor, their resistance changes when its temperature is changed. By calculating the variation in resistance, the temperature variations may be calculated. The thermometer which utilizes this phenomenon is called “resistance thermometer”.

Construction

The construction of resistance thermometer detector is shown in the figure. RTD uses platinum, nickel or copper as a resistance element. Generally, platinum wire is wound on either ceramic bobbin to form a resistance element. This resistance element is placed inside the hollow structure called protection tube. It is made up of stainless steel or carbon steel.

Internally lead wire is used to connect resistance element with external lead terminals. Lead wire covered by insulated tube for short circuit prevention. Fiber glass is used for low and medium temperature and a ceramic insulation for high temperature.

Protection tube is used to protect the resistance element and internal lead wires from ambient conditions. Protection tube is fitted with the mounting attachment to install the resistance temperature detector to the measuring point.

Operation

Initial resistance is measured by using Wheatstone bridge. The probe tip of the RTD is placed near the heat source. Outer cover uniformly distributes heat to sensing resistance element. As the temperature varies, the resistance of the material also varies. Now, final resistance is again measured. From the above measurement, variation in temperature can be calculated as follows,
Rt = R0 ( 1+Dt)
Dt = ( (Rt/Ro)-1)/x

Where,
Rt = resistance at C.
R0 = Resistance at room temperature.
Dt = Difference in temperature.
X = Temperature coefficient of RTD material.

Thus from the above formula by knowing Rt, R0 and x, the difference in temperature can be calculated.

ii).Semiconductor Resistance thermometer(thermistor)

It is also known as a thermally sensitive resistor(thermistor). A thermistor is a type of resistorwhoseresistanceis dependent on temperature, more so than in standard resistors. Thermistors are widely used as inrush current limiter, temperature sensors, self-regulating heating elements etc. They are made of oxides of manganese,chromium, cobalt. These oxides are semiconductor in nature . So as temperature increases, resistance decreases. Radiation between temperature and temperature is given by,

R_t = R₀ e^B(1/T-1/t₀⁾

Where B is constant depending upon the type of material used.

Thus, as temperature increases the resistance of the thermistor decreases exponentially.

Thermistors are of two opposite fundamental types:

• With NTC, resistance decreases with temperature to protect against inrush overcurrentconditions. Installed series in a circuit.
• With PTC, resistance increases with temperature to protect against overvoltage Installed parallel in a circuit.

D.Photoconductive cell

Photoconductive cells are light-sensitive resistors in which resistance decreases with an increase in light intensity when illuminated. These devices consist of a thin single-crystal or polycrystalline film of compound semiconductor substances. Most commercially available photoconductive cells are manufactured from cadmium sulfide (CdS), which is sensitive to light in the visible spectrum. Other materials that are less commonly used in photoconductive cells include lead sulfide (PbS), lead selenide (PbSe), and lead telluride (PbTe), although they react to infrared light, not the visible spectrum.CdS photoconductive cells (CdS cells) are often referred to as light dependant resistors (LDR). They function within the same general spectral range as the human eye and are therefore widely used in applications where this type of spectral response is required.

Photoconductive cells function by receiving light energy, which in turn free electrons from their valence bonds in the semiconductor material. At room temperature, the number of free charges in a semiconductor is relatively limited, but the addition of light-released electrons raises conductivity (and thereby reduces resistance). This change is resistance may be as large are several hundred thousand ohms from a darkened state to only a few hundred ohms in sunlight. Embedding the conductive path within the semiconductor substrate, in a zig-zag pattern, will enhance the level of resistance. In addition to an increase in dark resistance, this same pattern will reduce the current as well, changing the output current to about one milliampere (mA) per lumen.

Photoconductive cells are generally inexpensive, and their small size and ease of use make them popular in many applications. Some of the many uses include making street lights turn on and off automatically according to the level of daylight, in point-of-sale and inventory bar code reading devices, in security devices such as motion sensing lights and cameras, and in alarm systems. They also are used as light meters in photographic applications.

2.Inductive sensor

These are analog passive transducers used for the measurement of displacement. These transducers work generally upon one of the following three principles:

1. Variation of self-inductance of the coil
2. Variation of mutual inductance of the coil
3. production of eddy currents

The general block diagram of the inductive sensor are:

Generally , there are two types of inductive transducers to be used to translate the linear motion and rotational motion into an electrical signal. They are:

a.Linear Variable Differential Transformer(LVDT)

b.Rotatory Variable Differential Transformer(RVDT)

a.Linear Variable Differential Transformer(LVDT)

The linear variable differential transformer (LVDT) is an accurate and reliable method for measuring linear distance. It is a type of electricaltransformerused for measuring linear displacement (position). LVDTs find uses in modern machine-tool, robotics, avionics, and computerized manufacturing.

Principle of LVDT:

LVDT works under the principle of mutual induction, and the displacement which is a non-electrical energy is converted into an electrical energy. And the way how the energy is getting converted is described in working of LVDT in a detailed manner.

LVDT consists of a cylindrical former where it is surrounded by one primary winding in the center of the former and the two secondary windings at the sides. The number of turns in both the secondary windings are equal, but they are opposite to each other, i.e., if the left secondary windings are in the clockwise direction, the right secondary windings will be in the anti-clockwise direction, hence the net output voltages will be the difference in voltages between the two secondary coil. The two secondary coil is represented as S1 and S2. Esteem iron core is placed in the center of the cylindrical former which can move in to and fro motion as shown in the figure. The AC excitation voltage is 5 to 12V and the operating frequency is given by 50 to 400 HZ.

Working of LVDT:

LVDT consists of a cylindrical former where it is surrounded by one primary winding in the center of the former and the two secondary windings at the sides. The number of turns in both the secondary windings are equal, but they are opposite to each other, i.e., if the left secondary windings are in the clockwise direction, the right secondary windings will be in the anti-clockwise direction, hence the net output voltages will be the difference in voltages between the two secondary coil. The two secondary coil is represented as S1 and S2. Esteem iron core is placed in the center of the cylindrical former which can move in to and fro motion as shown in the figure. The AC excitation voltage is 5 to 12V and the operating frequency is given by 50 to 400 HZ.

Working of LVDT:

Let’s study the working of LVDT by splitting the cases into 3 based on the iron core position inside the insulated former.

Case 1:On applying an external force which is the displacement, if the core reminds in the null position itself without providing any movement then the voltage induced in both the secondary windings are equal which results in net output is equal to zero

i.e., Esec1-Esec2=0

Case 2:When an external force is applied and if the steel iron core tends to move in the left-hand side direction then the emf voltage induced in the secondary coil is greater when compared to the emf induced in the secondary coil 2.

Therefore, the net output will be Esec1-Esec2

Case 3:When an external force is applied and if the steel iron core moves in the right-hand side direction then the emf induced in the secondary coil 2 is greater when compared to the emf voltage induced in the secondary coil 1. therefore the net output voltage will be Esec2-Esec1

Advantages of LVDT:

* Infinite resolution is present in LVDT

* High output* LVDT gives High sensitivity

* Very good linearity

* Ruggedness

* LVDT Provides Less friction

* Low hysteresis

* LVDT gives Low power consumption.

Advantages of LVDT

• High Range - The LVDTs have a very high range for measurement of displacement.they can use for measurement of displacements ranging from 1.25mm to 250mm
• No Frictional Losses - As the core moves inside a hollow former so there is no loss of displacement input as a frictional loss so it makes LVDT as a very accurate device.
• High Input and High Sensitivity - The output of LVDT is so high that it doesn’t need any amplification.the transducer possesses a high sensitivity which is typically about 40V/mm.
• Low Hysteresis - LVDTs show a low hysteresis and hence repeatability is excellent under all conditions
• Low Power Consumption - The power is about 1W which is very as compared to other transducers.
• Direct Conversion to Electrical Signals - They convert the linear displacement to electrical voltage which is easy to process

Disadvantages of LVDT

• LVDT is sensitive to stray magnetic fields so they always require a setup to protect them from stray magnetic fields.
• They are affected by vibrations and temperature.

It is concluded that they are advantageous as compared than any other inductive transducers.

Applications of LVDT

1. They are used in applications where displacements ranging from a fraction of mm to few cm are to be measured. The LVDT acting as a primary Transducer converts the displacement to electrical signal directly.
2. They can also act as the secondary transducers .E.g. the Bourbon tube which acts as a primary transducer and covert pressure into linear displacement.then LVDT coverts this displacement into an electrical signal which after calibration gives the ideas of the pressure of fluid.

b.Rotatory Variable Differential Transformer(RVDT)

A Rotatory Variable Differential Transformer(RVDT) is a type of electrical transformer used for measuring angular displacement. More precisely, a Rotary Variable Differential Transformer (RVDT) is an electromechanical transducer that provides a variable alternating current (AC) output voltage that is linearly proportional to the angular displacement of its input shaft. When energized with a fixed AC source, the output signal is linear within a specified range of the angular displacement.

It is used to measure rotational angles and operates under the same principles as the LVDT sensor. Whereas the LVDT uses a cylindrical iron core, the RVDT uses a rotary ferromagnetic core. A schematic is shown below.

3.Capacitive sensors

Capacitive sensors consist of two parallel metal plates in which the dielectric between the plates is either air or some other medium. The capacitance C is given by C=εoεrA/d,

Where

εo = the absolute permittivity,

εr =relative permittivity of the dielectric medium between the plates,

A = area of the plates and

d =distance between them.

The basic block diagram of capacitive sensors is given below:

Capacitive devices are often used as displacement sensors, in which motion of a moveable capacitive plate relative to a ï¬Âxed one changes the capacitance. Often, the measured displacement is part of instruments measuring pressure, sound or acceleration. Alternatively, plate capacitors can also be used as sensors, in which the capacitance value is changed by causing the measured variable to change the dielectric constant of the material between the plates in some way.

Capacitive sensors can generally be divided into three categories, based on their mode of operation:Load mode,transmit mode and shut mode . Another distinction can be made between capacitive sensors that are designed for contact and ones that are not.

Load Mode (or "Unloaded") Sensing

An unloaded capacitive sensor is one in which the circuit anticipates a certain capacitive load and an external capacitance is applied, resulting in a change of total capacitance .

The Thereminis a well known electronic instrument that operates on the principle of unloaded capacitive sensing. Before a performer plays the instrument, he or she must calibrate the device to establish a reference capacitance between the performer and the instrument. As the performer plays the instrument, she varies a capacitance between her body and one of the antennae which in turn modulates an internal oscillator. This internally modulated signal is then translated into frequency and amplitude.

Transmit Mode (or "Loaded") Sensing

A loaded capacitive sensor is one in which a signal is capacitively coupled through an object or performer and the amplitude of the signal received varies with the distance between the two “plates” of the capacitor. The body/object is very close to the electrodes in this mode and becomes effectively an extension of them.

A familiar loaded capacitive sensor is used inMax Matthew's Radio Baton. In this instrument, two transmitting batons are directly connected to oscillators. Four receive electrodes embedded in a large plate acquire and demodulate the signal. Each of the receive electrodes is patterned such that it varies in the surface area across either the x or y dimension of the plate, allowing the 2D position to be sensed. By comparing the signals from all four electrodes, it is also possible to sense the height of the batons over the plate.

Shunt Mode Sensing

Shunt mode capacitive sensing is very similar to Transmit mode, in that an expected capacitive load is present between a transmit electrode and a receive electrode. In this case, however, the body of the performer is not connected to the transmit electrode, and effectively screens/absorbs the electrical field. The current measured at the second electrode then correlates to the electrical field over the cross-section of the body. The difference between Shunt and Transmit mode is that the distance to the second electrode is known in Shunt mode, and thus allows more meaningful measurements.

Capacitive liquid level sensor

It consists of two concentric cylinders with an inner radius and outer radius.It is placed inside the liquid whose height is to be measured. It results in the formation of two dielectric one having liquid as dielectric and other having air as dielectric .

Working principle

A capacitor has 2 conducting plates (Electrodes). When a charge is applied to these plates, the space in between the plates will also get a charge. The charge that can be between these plates depends on the material in between the plates (the Dielectric). The ability of a material to be charged is called relative permittivity.The capacitive level sensor has the 2 conducting plates in the form of 2 electrically isolated aluminum tubes, a smaller tube in a larger tube. The space between the tubes is the dielectric. When the tube is empty, space is occupied by air. when the tube starts to fill, more and more of the space will be occupied by water. Water holds more charge than air and thus the capacitance will rise (mostly) linearly with the water level.

4.Thermoelectric sensor

It is that type of sensor which works on the principle of thermoelectric effect i.e. direct conversion of temperature differences to electric voltage. The basic block diagram of thermoelectric sensor is given below:

eg- thermocouple etc

Thermocouple

A thermocouple is a device used extensively for measuring temperature.A thermocouple is comprised of at least two metals joined together to form two junctions. One is connected to the body whose temperature is to be measured; this is the hot or measuring junction. The other junction is connected to a body of known temperature; this is the cold or reference junction. Therefore, the thermocouple measures the unknown temperature of the body with reference to the known temperature of the other body.

Working Principle

The working principle of the thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. They are as follows:

1) Seebeckeffect:The Seebeckeffectstates that when two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals.

2) Peltier effect:As per the Peltier effect, when two dissimilar metals are joined together to form two junctions, emf is generated within the circuit due to the different temperatures of the twojunctionsofthe circuit.

3) Thomson effect:As per the Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to the temperature gradient along the entire length of the conductors within the circuit.

In most of the cases, the emf suggested by the Thomson effect is very small and it can be neglected by making a proper selection of the metals. The Peltier effect plays a prominent role in the working principle of the thermocouple.

Diagrams:

How it Works

The general circuit for the working of the thermocouple is shown in the figure above. It comprises of two dissimilar metals, A and B. These are joined together to form two junctions, p, and q, which are maintained at the temperatures T₁and T₂respectively. Remember that the thermocouple cannot be formed if there are not to junctions. Since the two junctions are maintained at different temperatures the Peltier emf is generated within the circuit and it is the function of the temperatures of two junctions.

If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net current flowing through the junction is zero. If the junctions are maintained at different temperatures, the emf’s will not become zero and there will be a net current flowing through the circuit. The total emf flowing through this circuit depends on the metals used within the circuit as well as the temperature of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the suitable device.

5.Hall effect sensors

Basically, a Hall-effect sensor is a device that is used to measure the magnitude of a magnetic field. It consists of a conductor carrying a current that is aligned orthogonally with the magnetic field,as shown in Figure 13.4. This produces a transverse voltage difference across the device that is directly proportional to the magnetic field strength. For an excitation current I and magnetic field strength B, the output voltage is given by V=KIB, where ‘K’ is known as the Hall constant.

The conductor in Hall-effect sensors is usually made from a semiconductor material as opposed to a metal because a larger voltage output is produced for a magnetic field of a given size. In one common use of the device as a proximity sensor, the magnetic field is provided by a permanent magnet that is built into the device. The magnitude of this field changes when the device becomes close to any ferrous metal object or boundary.

Working principle

When a beam of charged particles passes through a magnetic field, forces act on the particles and the beam is deflected from a straight path. The flow of electrons through a conductor is known as a beam of charged carriers. When a conductor is placed in a magnetic field perpendicular to the direction of the electrons, they will be deflected from a straight path. As a consequence, one plane of the conductor will become negatively charged and the opposite side will become positively charged. The voltage between these planes is called Hall voltage.When the force on the charged particles from the electric field balances the force produced by a magnetic field, the separation of them will stop. If the current is not changing, then the Hall voltage is a measure of the magnetic flux density. Basically, there are two kinds of Hall effect sensors. One is linear which means the output of voltage linearly depends on magnetic flux density; the other is called threshold which means there will be a sharp decrease of the output voltage at each magnetic flux density.

6.Piezoelectric sensors

A Piezoelectric sensor is a device that uses the piezoelectric effect, to measure changes in pressure,acceleration,temperature,strain or force by converting them to an electrical charge. The word piezoelectric is derived from Greek word piezo, which means to squeeze or press. The piezoelectric effect states that when mechanical stress or forces are applied to a quartz crystal, produce electrical charges on quartz crystal surface. One of the unique characteristics of piezoelectric effect is that it is reversible means when a voltage is applied to them ,they tends to change dimension along certain plane i.e quartz crystal structure is placed into an electric field, it will deform quartz crystal by an amount proportional to the strength of electric field. If same structure is placed into an electric field with the direction of field reversed, the deformation will be opposite.

Eg of piezoelectric materials- Barium Titanate, Lead zirconate titanate (PZT), Rochelle salt

Working principle

There are mainly three main operational modes of Piezoelectric sensors:

• Transverse
• Longitudinal
• Shrear

Transverse effect

A force applied along a neutral axis (y) generates charges along the (x) direction, perpendicular to the line of force. The amount of charge (Cx ) depends on the geometrical dimensions of the respective piezoelectric element. When dimensions a,b,c apply,

where a is the dimension in line with the neutral axis, b is in line with the charge generating axis and d is the corresponding piezoelectric coefficient.

Longitudinal effect

The amount of charge produced is strictly proportional to the applied force and independent of the piezoelectric element size and shape. Putting several elements mechanically in series and electrically in parallel is the only way to increase the charge output. The resulting charge is

,

where dxx is the piezoelectric coefficient for a charge in x-direction released by forces applied along x-direction (in pC/N). Fx is the applied Force in x-direction [N] and n corresponds to the number of stacked elements.

Shear effect

The charges produced are strictly proportional to the applied forces and independent of the element size and shape. For n elements mechanically in series and electrically in parallel, the charge is

..

In contrast to the longitudinal and shear effects, the transverse effect makes it possible to fine-tune sensitivity on the applied force and element dimension.

Advantages of Piezoelectric Transducer

1. No need of external force. 2. Easy to handle and use as it has small dimensions . 3. The high-frequency response it means the parameters change very rapidly.

Disadvantages of Piezoelectric Transducer

1. It is not suitable for measurement in a static condition.
2. It is affected by temperatures .
3. The output is low so some external circuit is attached to it.
4. It is very difficult to give the desired shape to this material and also desired strength.

Measurement of humidity

Humidity is the amount of water vapor in the air. Water vapor is the gaseous state of water and is invisible.Humidity indicates the likelihood of precipitation,dew, or fog. Higher humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin.

There are three main measurements of humidity: absolute, relative and specific.Absolute humidity is the water content of air at a given temperature expressed in gram per cubic meter. Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum (highest point) for that temperature.Specific humidity is a ratio of the water vapor content of the mixture to the total air content on a mass basis.

To measure the humidity , a class of transducer or sensor device is used , which is named as Hygrometer. A hygrometer is an instrument used for measuring the moisture content in the atmosphere. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity.Generally ,the output of the hygrometer is used to indicate its relative humidity. Basically, there are following types of sensor devices to measure the relative humidity:

1. Resistive hygrometer
2. Capacitive hygrometer
3. Aluminum hygrometer
4. Crystal hygrometer etc

1.Resistive/Capacitive Hygrometer

Resistive and capacitive hygrometers have sensing elements that absorb or give up moisture in a hygroscopic layer until equilibrium is reached with the ambient water vapor pressure. Two electrodes are in contact with the layers, whose resistance and/or capacitance (often leaky capacitance) varies with the humidity. A variety of materials are used to form the layer; among them are polymer films, lithium chloride, an aqueous solution of hygroscopic salt, a carbon-powder suspension in a gelatinous cellulose, aluminum oxide, some experimental materials like lead iodide, polyelectrolyte combinations with ion exchange resins, and other materials.
One of the modifications of the capacitive hygrometer is a capacitor formed by two concentric electrodes. The measured gas flows between them. The dielectric constant of the gas depends on the quantity of water vapor in the gas.

2.Crystal hygrometer

The crystals are used as frequency determining elements in an electronic oscillator. When humidity changes, the crystals coated with hygroscopic polymers absorb water content hence change their mass. Due to such changes ,frequency also changes which is calibrated in terms of humidity. These hygrometers are most widely used in telemetry systems as the frequency range is adjustable according to the standard frequency of a telemetry.

Reference:

1.S.morris.alan(2001). Measurement and Instrument principle(3 ed.).A division of Reed Educational and Professional Publishing Ltd

2.Webster.G.John(1999). The measurement, Instrumentation, and sensors. CRC Press

3.Anderson, Norman A. (1998). Instrumentation for Process Measurement and Control (3 ed.). CRC Press

4.Doebelin, Ernest O. (1966). Measurement Systems: Application and Design.

5.Fraden J. (2016). Handbook of Modern Sensors: Physics, Designs, and Applications(5th ed)

6.https://en.wikipedia.org/wiki/Transducer