1. Temperature Measurement

2. How to measure temperature
Temperature can be measured by detecting changes in various temperature-dependent properties
Volume  liquid-in-glass thermometer
Pressure  gas thermometer
Displacement  bimetallic strip
Voltage  Thermocouple
Resistance  RTD & thermistor
Radiation spectrum  Infra Red detectors 2

3. Source: Mechanical Engineering magazine, March 2010

4. Thermocouple
Thermoelectric effect: when any two different metals are connected together, an emf that is a function of the temperature is generated at the junction between the metals:
For certain pairs of materials,
Hot junction
Reference junction
(a) Thermocouple; (b) equivalent circuit 4

5. Characteristics of thermocouples
Thermocouple tables
Sensitivity 5

6. Resistance Temperature Detectors (RTDs)
Varying resistance devices
Rely on the fact that the resistance of a metal varies with temperature
Also known as resistance thermometers or thermistors depending on material used (metal or semiconductor)
Variation can be non-linear, resulting in inconvenient measurement
Platinum exhibits most linear behavior
Platinum is also chemically inert

7. Resistance thermometers or Resistance Temperature Devices (RTDs)
Two common designs:
Coil wound on mandrel
Film deposited on substrate
Wheatstone bridge: used to measure resistance change for an RTD
Excitation voltage has to be chosen carefully: while a high voltage is desirable for high sensitivity, this causes self-heating

8. Mechanical temperature sensing devices
Liquid-in-glass thermometer
Bimetallic thermometer
Commonly used as a thermostat (on-off switch in control applications)
When displacement is measured, it acts as a thermometer
Tip displacement: against a calibrated scale, or electrical output such as LVDT
Pressure thermometer
Liquid-in-glass thermometer
Pressure thermometer
Bimetallic thermometer

9. Pressure measurement

10. Diaphragm & bellows
Pressure causes displacement of diaphragm (thin sheet), which can be measured by a displacement transducer
Can be used with an LVDT or strain gauge
Diaphragm
Bellows

11. Application: sound measurement
Sound is measured as sound pressure level:
Microphone: diaphragm-type pressure sensor
Converts sound pressure into displacement
Displacement is commonly measured using a piezoelectric-type transducer

12. Bourdon tube Pressure causes Bourdon tube to unwind
displacement transducer
Can also be used with an LVDT or strain gauge

13. Manometer Can be used to measure gauge pressure:
Can also measure differential pressure:
Type of liquid
Water is cheap & convenient
Water evaporates & is difficult to see through
Not to be used if reacts with fluid
Well-type: need only to measure liquid level in one tube
Inclined-type: better sensitivity

14. Force measurement

15. Force sensing Elastic Sensing: Strain Sensing: Pressure Sensing:
Beams, rings
Strain Sensing:
Strain gauges
Pressure Sensing:
Piezoelectric elements
Acceleration Sensing:

16. Load cell Force produces measurable displacement Design objectives:
Achieve linear input/output relation
Make the instrument less sensitive to forces not applied along sensing axis
Based on strain gauge measurement
Hydraulic load cell
Use of strain gauges in a torque cell

17. Flow measurement

18. Conveyor-based methods
To measure flow of solids or particles
Mass is measured with a load cell
M = mass of material
L = length of conveyor
v = velocity
Q = mass flow rate

19. Pipe flow Flow measurement :
Local properties (velocity, pressure, temperature, density, viscosity)
Integrated properties (mass flow rate, volumetric flow rate)
Global properties (visualization of entire flow)
If the velocity profile is known, it is enough to measure one velocity (centerline) to determine the total flow rate
Otherwise, cross-section must be mapped by a grid of velocity data
Laminar Flow
Turbulent Flow

20. Principles of flow measurement
Bernoulli’s equation:
Conservation of mass: z2
For a horizontal pipe: z1
Q = volume flow rate
Pressure difference is a measure of flow rate

21. Orifice Plate D d

22. Types of flowmeters

23. Differential pressure meters
Rely on the insertion of some device info a fluid-carrying pipe to obstruct the flow, thus creating a pressure difference
Obstruction-type meters or flow-restriction meters
Common devices: orifice plate, Venturi tube, flow nozzle
Pressure difference usually measured with a differential pressure transducer
Advantage: no moving parts; robust, reliable & easy to maintain
Disadvantage: permanent loss of pressure

24. Pitot static tube Negligible obstruction of flow
Measures flow at a single point
Measures average flow velocity

25. Turbine flowmeter
Speed of rotation of turbine is proportional to flow rate

26. Electromagnetic flowmeter
Used for electrically conductive fluids
Non-invasive device (no obstruction to fluid flow)
No pressure loss

27. Hot wire anemometer
Consists of an electrically heated fine wire which is immersed in the flow.
As the fluid velocity increases, the rate of heat flow from the heated wire to the flow stream increases.
Thus a cooling effect on the wire occurs, causing its electrical resistance to change.
In a constant current anemometer, the fluid velocity is determined from measurement of the change in resistance.