Sub. Mechanical Engineering Measurement. Ch. no. 3 pressure and temperature measurement Babasaheb Phadatre Polytechnic, Kalamb. Department of Mechanical Engineering. Prepared By- Prof. Kokare Amol Yashwant Sir

1. UNIT NO. 3 PRESSURE AND
TEMPERATURE MEASUREMENT
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.
Subject- Mechanical Engineering
Measurement

2. Unit No. 3:
Pressure and Temperature
Measurement (12 Marks)
Course Outcomes (CO: c):
Use relevant pressure and
temperature measuring
instruments.
Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.

3. Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.
Pressure Measurement
Pressure is defined as the force acting per unit area | exerted by
a fluid on a surface. Its unit is N/m2 or Pa.
1 mm of Hg (Mercury) = 133 Pa = 133 N/m2 = 1 torr
1 bar = 105N/m2.
Atmospheric Pressure (Patm)
Atmospheric pressure is the pressure on the surface of earth due
to the atmospheric gases. It is normally expressed at sea level.
1atm = 101.36 kPa = 760 mm of Hg
Gauge Pressure (P gauge)
Gauge pressure is the pressure measured with respect to
atmospheric pressure. It is the difference between absolute
pressure and atmospheric pressure.
Pgauge = Pabs - Patm

4. Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.
Absolute Pressure (P abs)
It is the pressure measured with reference to vacuum. For a
complete vacuum, pressure measured is absolute zero pressure.
Pabs = Pgauge + Patm
Static Pressure
It is the pressure where no
motion is occurring of the
liquid. Its value increases as
the liquid head in the tank
increases.
Dynamic Pressure
It is the pressure that it
exerts on its surroundings
while the fluid is in motion. It
increases as the liquid velocity
increases.

5. Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.
Pressure Measurement Gauges
(A) Low Pressure Gauges.
(a) McLeod Gauge.
(b) Thermal conductivity gauge.
(i) Thermocouple vacuum gauge.
(ii) Pirani gauge.
(C) lonization gauge.
(B) High Pressure Gauges.
(a) Elastic pressure gauges.
(i) Diaphragm.
(ii) Bellows.
(iii) Bourdon tube.
(b) Electrical pressure gauges
(i) Resistance type
(ii) Photo electric type
(iii) Piezoelectric type
(iv) Variable capacitor type

6. Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.
McLeod Gauge
McLeod gauge comprises a system of glass tubing made of
tough glass and mercury is used to trap the known volume of
gas. The gauge is connected to the unknown gas whose
pressure is to be measured.
The plunger moves up, lowers the mercury level to the cut off
positions, entering the gas at unknown pressure through the
tube.

7. Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.
Advantages of McLeod Gauge
1. It is very simple in use.
2. Measurement is independent of gas composition and it is
related to physical dimensions of gauge
3. It is a very accurate pressure measuring device.
4. It can be used as a standard to calibrate other low pressure
gauges.
Limitation of McLeod Gauge
1. If the gas contains the vapor, it may not give correct result.
2. It is applicable to those systems where mercury is tolerable.
3. It does not give continuous output.

8. Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.
Pirani gauge (Thermal Conductivity Gauge)
When a current flow in the wire filament, the filament gets heated
depends upon the gas pressure.
The resistance of the wire changes with temperature and bridge
becomes unbalanced. The change in the value of the resistance
of wire filament gives the value of unknown pressure.
Advantages
1. It is simple in design and easy to use.
2. They are more accurate than
thermocouple gauges.
3. They can measure between 20 micron to 1
mm of Hg.
Limitation
1. The gauge must be calibrated.
2. It is passive type of gauge & need
electrical power.

9. Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.
Thermocouple Vacuum gauge
The heater element is supplied with constant electric current and
its temperature which is the function of pressure is measured by
thermocouple. The output voltage of the thermocouple is
measured with the help of voltmeter whose scale is directly
calibrated in terms of pressure being measured.
Advantages
1. They have rugged construction and
simple to use.
2. They are inexpensive.
3. Remote reading is possible.
Limitation
1. Components can be damaged by organic
vapours.
2. Need calibration for different gases.
3. Need electrical power supply to operate.

10. Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.
lonization Gauge
Extremely low pressures can be measured with ionization gauge
(10 torr and below) Ionization means process of producing free
electron and a positively charged ions by knocking off an
electron from an atom. The heated cathode emits the electrons
which move past the grid. The positive grid accelerates these
electrons where they collide with gas molecules causing its
ionization.
Advantages
1. Very low pressure measurement up to
10-11 torr is possible.
2. It can give continuous pressure reading.
3. They have good linearity.
Limitation
1. Few gases like oxygen carbon ecomposed
by the hot filament.

11. Prof. Kokare A.Y.
High Pressure gauges:
Diaphragm
If unknown pressure is applied on one
side of diaphragm, It gets deflected.
The deflection of diaphragm is
proportional to applied pressure. The
measure of the deflection is the
calibrated in terms of pressure.
Advantage
1. They have moderate cost and simple in
construction. 2. They have good linearity .
3. Easy calibration with dead weight tester.
4. They can measure gauge, absolute and
differential pressures.
Limitation 1. It cannot avoid overloading.
2. It cannot tolerate vibrations and shocks

12. Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.
Bellows
The pressure to be measured is applied from the bottom as
shown in since bottom end is fixed, expansion of bellows takes
place to upper side to which rod is connected. The
displacement of this rod is directly proportional to the pressure
inside the bellows. This displacement is transferred to pointer
moving over a calibrated scale.
Advantage
1. They have simple and rugged construction.
2. They are good for low to moderate pressure
measurements.
3. They are relatively less expensive.
Limitation
1. They are not suitable for high pressures.
2. Friction and dirt may affect sensitivity.

13. Prof. Kokare A.Y.
Bourdon Tube
It consists of a metal tube oval-cross section, bent in the form of
circular shape having 200 to 250°.
The tube has two ends, out of which one end is sealed and closed.
This end is connected to pointer and scale through deflection
arrangement. Types of Bourdon Tube
1. C-Tube, 2. Helical, 3. Spiral, 4. Twisted
Advantage
1. Bourdon gauges are more robust than
manometers.
2. Bourdon tubes are also used in liquid, gas
filled thermal system for measurement of temp.
3. They are relatively less expensive.
Limitation
1. Accuracy in precision measurements is limited
2. These are influenced by shock and vibrations.

14. Prof. Kokare A.Y.
Electrical Resistance Pressure Gauges
Potentiometer may be made curved so that the wiper can pivot
in a circular motion. where wiper of a potentiometer is attached
to a bellows element which is a primary sensing element.
Unknown pressure is applied to bellows element such that it
expands moving the wiper downward increasing the resistance
of the potentiometer.
Advantage
1. Easy conversion of resistance into
voltage or current.
2. Easy design flexibility to suit
specific applications.
3. They are simple in design and
have rugged construction.
Limitation
1. They have poor resolution.
2. The problem of electrical "noise" may occur.

15. Prof. Kokare A.Y.
Strain gauge pressure transducer
Strain gauge is a resistance type gauge whose electrical
resistance changes when it is stretched or compressed.
Two gauges measure strain in circumference direction while other
two measures strain in radial direction. The four strain gauges are
then attached to Wheatstone bridge circuit to find it.
Advantage
1. They are compact in nature
and easy to install.
2. They give good accuracy over wide
pressure range.
3. It contains no moving parts.
4. They have good frequency response.
Limitation
1. Requires signal conditioning.
2. Relatively complex to fabricate.

16. Prof. Kokare A.Y.
Photoelectric Pressure Transducers
When the pressure to be measured is applied through port to
the pressure sensing member, it changes the position of
window. As the light source and phototube are separated
by a window it changes the amount of light falling on phototube,
causing change in current.
Advantage
1. It can measure both static and dynamic
pressures.
2. It is highly efficient.
3. Easy portability and compact size.
Limitation
1. Less stable for long term measurement.
2. Considerable displacement in force
membrane is necessary.

17. Prof. Kokare A.Y.
Piezoelectric Pressure Gauge
Piezoelectric crystal placed between a solid base and the
pressure sensing element like diaphragm. Apart from quartz,
some other crystalline materials used are barium titanate,
tourmaline, Rochelle Salts etc. under the action of pressure, the
diaphragm deflects producing deformation in crystal, there by
produces an emf which is the function of applied pressure.
Advantage
1. They are self generating type transducer
i.e. Active one.
2. Their high frequency response is very good.
3. Range is 0 to 20 MPa.
Limitation
1. These transducers cannot measure
2. static pressures.
2. They are sensitive to temperature changes.

18. Prof. Kokare A.Y.
Variable Capacitor Type Pressure Gauge
We know that Capacitance of a parallel plate capacitor is
inversely proportional to the spacing between the plates.
The diaphragm is free to move as the pressure applied
changes. According to change in pressure, the diaphragm
deforms and distance d becomes less and capacitance
between the plate increases and vice versa.
Advantage
1. Response time to change in pressure is less.
2. It can withstand shock and vibrations.
3. It can measure both static and
dynamic pressures.
Limitation
1. They have poor thermal stability.
2. Care must be taken for dirt and
other contaminants.

19. Prof. Kokare A.Y.
Temperature measurement Classification
(A) Glass thermometers with mercury
(B) Pressure gauge thermometer- Liquid, Vapour, Gas
(C) Bimetallic thermometer
(D) Electrical resistance thermometer
(E) ) Thermometer pyrometer
(F) Optical pyrometers
(G) Radiation pyrometers
(H) Fusion pyrometer
(a) Calorimetric pyrometers
(b) Color temperature
Temperature measurement
1. – 35° C to 510°C: Pressurized mercury glass thermometer
2. - 65° C to 430°C : Bimetallic thermometers
3. - 100° C to 315°C : Thermistor.
4. - 15° C to 3870°C : Radiation pyrometers.

20. Prof. Kokare A.Y.
Liquid - in - Glass Thermometers
Changes in the temperature will cause the fluid to expand and
contract in the stem since the area of the stem is much less
than the bulb, the relatively small changes of fluid volume will
result is significant fluid rise or fall in the stem
Advantage
1. Simplicity of use and relatively low cost. 2. Easily portable.
3. Checking for physical damage is easy. 4. Good accuracy
Disadvantages
1. Can not be adopted for automatic recording. 2. Time lag in
measurement. 3. Range is limited to about 600°C.

21. Prof. Kokare A.Y.
Bimetallic Thermometer
Different two materials having different coefficient of thermal
expansion rigidly joint together, one on other form a bimetallic
strip. When bimetallic strip is fixed at one end heated from free
end then it bends in the direction of material having low thermal
coefficient of expansion.
Advantage
1. Compact and robust construction.
2. No maintenance is required.
Disadvantages
1. Material is subjected to creep at high
temperature. 2. Speed of response is low.
Application
1. Household devices like oven, air
conditioner clocks etc.
2. Industrial devices like refineries, hot
wires, heater. tempering tanks etc.

22. Prof. Kokare A.Y.
Pressure Thermometer –a) Liquid pressure thermometers
The bulb is connected by means of a capillary tube to a pressure
measuring device, like a bourdon tube pressure gauge. An increase
in temperature causes the liquid to expand, thereby increasing the
pressure on the gauge.
a) Vapour pressure thermometers
The bulb is partially filled so that when the bulb senses the temp.
vapour will be formed in the remaining space in the bulb, and passed
through a capillary tube to the indicator with a bourdon tube.

23. Prof. Kokare A.Y.
Platinum Resistance Thermometer (RTD)
Resistance Thermometers or Resistance Temperature Detector
(RTD) works on the principle of positive temperature coefficient
of resistance. i.e. as temperature increases, resistance offered
by thermometer also increases. Rt= Ro (1+ α0 t)
Advantage
1. The response time compared to thermocouples is very fast. 2. An RTD
will not experience drift problems 3. Within its range it is more accurate and
has higher sensitive. 4. RTD does not require special extension cable.
Disadvantages
1. they are much more expensive than thermocouples. 2. RTD is not
capable of measuring as wide.
a temperature range

24. Prof. Kokare A.Y.
Thermistor
Thermistors are a temperature measurement device that has a
negative temperature coefficient.
As the temperature increases the resistance goes down and as
the temperature decreases the resistance goes up.
Thermistors are made from semiconducting materials mixtures of
metallic oxides such as chromium, nickel, cobalt, copper, iron.
Advantage
1. Thermistors have a relatively large but
negative resistance change.
2. Metals have an approximately linear temp.
resistance relationship.
3. The range for resistance thermometers is
much greater, being from – 160 to 600°C.
Disadvantages
1. It requires external energy source.
2. Unsuitable for a wide range.

25. Prof. Kokare A.Y.
(Thermoelectric Methods) Thermocouple:
The basic principle of a thermocouple is that when two dissimilar
metal wires are joined to form a complete electric circuit, and the
two junctions are maintained at different temperatures, an
electromotive force is setup due to the algebraic sum of an emf
developed between two dissimilar metals placed in contact which
is known as Seebeck effect or an emf developed between the
two ends of a homogeneous wire
when one end is heated, which is
called the Thomson effect.
Material:
Used mtl are copper, iron, platinum,
rhodium or iridium all-alone, & alloys
like constantan (60% Cu + 40% Ni),
chromel (10% Cr + 90% Ni), Alumel
(2A1, + SONi + balance Si & Mn).

26. Prof. Kokare A.Y.
Radiation Pyrometer
The pyrometer is designed to collect the radiations from the
radiating object (furnace) and focus it by means of mirrors or lens
on to a detector (say hot junction of thermocouple).
The emf developed by the thermocouple circuit is measured by a
suitable mill voltmeter or potentiometer which after suitable
calibration becomes a measure of the temperature of the
radiating object.
Advantages
1. A pyrometer can measure temperature without contact.
2. It can be used for measurement of high temp.
i.e. above 550°C.
Disadvantages
1. Installation cost is also very high.
2. It requires observer's skill for
accurate reading.

27. Prof. Kokare A.Y.
Optical Pyrometer
An image of radiating source is produced by lens and made
coincide with filament of lamp. In optical pyrometer wavelength
of radiation accepted is restricted by colour filter and its
brightness is compared with standard lamp
Advantages: 1. Optical pyrometer is portable. 2. It can be used to
detect the temperature of moving object.
Disadvantages: 1. More chances of human error while adjusting
the image. 2. It measures the temperature of only hot surfaces.

28. Babasaheb Phadtare Polytechnic ( DME) Prof. Kokare A.Y.