Electrical Engineering Course

1. Lecture No. 20
Pressure Measurement
and Sensors
1
Muhammad Aseer Khan
Electrical Measurements and Instrumentation
EE383 (Fall 2022)
Department of Electrical Engineering
Air University Islamabad, Kamra Pakistan

2. Bellows Pressure Sensors 2
 A bellows looks like a small can with flexible sides and a rigid top and bottom.
 These devices are usually made of brass or stainless steel.
 Process pressure makes the unit expand or contract.
 The resilience of the flexible sides, and a possibly a spring on one end, resists the effects
of the process pressure.
 Thus, the bellows expands or contracts in response to pressure challenges.

3. Bellows Pressure Sensors 3
Construction:
 As the pressure increases inside, the bellows expands.
 The top rises and compresses the spring.
 As the top of the bellows continues to rise, it compresses the spring even more.
 If the upward force from inside the bellows (pressure multiplied by the area of the top)
equals the downward force of the spring, the bellows stop expanding.
 The spring pushes down on the bellows with a force proportional to the amount of
compression. That is, if you double the distance the spring compresses (measured from
its relaxed length), the spring pushes down with double the force.
 For example, if the spring is compressed 1 in. from its relaxed length, it might push down
with a force of about 10 lb. If you now compress the spring another inch, making the
total compression 2 in., the spring exerts a force of 20 lb.

4. Bellows Pressure Sensors 4
Sealed-case Bellows:
 In the figure shown below, the process pressure is applied in the sealed case surrounding the bellows.
 As the pressure increases, the bellows contracts, compressing the spring inside.
 The bellows works according to the same basic principle as presented earlier.
 The process pressure usually is applied inside the bellows for measurements below atmospheric pressure.
 If the process pressure is higher than the atmospheric pressure, it is applied in a sealed case around the
bellows.

5. Bellows Pressure Sensors 5
Sensor Application Comparisons:
 What sensor is right for a given process measurement application?
 The answer to this question depends largely on the range of pressures to be measured.
 Table given below offers a guide to appropriate kinds and materials for different applications.
Animation
 https://instrulearning.com/pressure/bourdon-tube-pressure-gauge/

6. Bellows Pressure Sensors 6
Sensor Application Comparisons:

7. Pressure Measurement 7
Maintaining Accuracy
 Pressure gauges must be accurate in order to provide precise control on
industrial processes.
 You can calibrate a gauge (adjust it for accuracy) if you are sure what
pressure is being applied to it.
 Manometers are very accurate and used in parallel connection with
instruments under test to determine the “true” pressure being applied to
the instruments.
 If test pressure is unavailable, or if test pressures exceed manometer limits,
you can use a deadweight tester for producing and determining the exact
pressure applied to a gauge.

8. Pressure Measurement 8
Deadweight Tester
 Deadweight tester is simple in design and operation, but it must be used properly
and carefully because of the extremely high pressures that it can produce-up to
100,000 psi (with an error of less than ±0.1%).
 Another common design uses a hydraulic pump and a manifold with two
connections. One connection is for a cylinder with free-floating piston and the
other connection is for the gauge to be calibrated.
 While deadweight testers may vary in design but they all operate on the same
basic principle.
“Pressure equals weight divided by area”
 The primary or free-floating piston is of a very precise area. Also, the weights
(Selected to equal the range of the device you are testing) are calibrated with
great precision.
 Therefore, the pressure divided by the deadweight tester is very accurate.

9. Pressure Measurement 9
Deadweight Tester

10. Pressure Measurement 10
Deadweight Tester
 To use the deadweight tester, you first attach the gauge to be calibrated so it senses the
pressure of the liquid in the cylinder.
 Then you place the desired weight on the primary piston.
 Turning the screw depresses the secondary piston and forces the liquid inside to lift the
primary piston and its weights, applying the desired pressure to the gauge through the
liquid.
 The deadweight tester would be a very accurate pressure gauge, but it responds too
slowly to pressure changes and requires a full-time operator to make measurements.
 The tester is therefore impractical for measuring process pressures.

11. Pressure Switches 11
 A pressure switch turns on an electric circuit on or off at a preset pressure. This pressure is
referred to as the setpoint of the switch.
 The switch usually is a sensitive reed switch, Microswitch®, or a mercury tilt-switch.
 A bourdon tube, a diaphragm, or a bellows can actuate a switch.
 The contacts in a pressure switch may be normally open or normally closed if the pressure is
below the setpoint.
 The contacts in a N.O. (normally open) switch remain open until the pressure increases above
the setpoint. Then the sensing element makes the contacts snap to the closed position. The
contacts again open when the pressure decreases below the setpoint.
 Deadband is the difference between the value at which a control action occurs (setpoint) and
the value at which the control action is cancelled (reset point).
 All pressure instruments have some inherent deadband due to moving parts and free play.
 From mechanical point of view, the less deadband, the better it is. However from control point
of view some deadband is desirable. Pressure control instrument usually have some provision
for deadband adjustment.

12. Pressure Switches 12
 One important use of pressure switches is in limiting pressure.
 For example, in steam power plants the pressure of steam entering a turbine must not
exceed an upper limit. Suppose this limit is 1250 psia in a particular plant. A switch then
can be used to operate a safety valve.
 The safety valve vents steam if the pressure reaches 1200 psia, keeping the pressure
below 1250 psia limit.

13. Pressure Switches 13