1. Mechanical Measurement and Instrumentation MECN 4600
Professor: Dr. Omar E. Meza Castillo
Department of Mechanical Engineering
Inter American University of Puerto Rico
Bayamon Campus

2. Tentative Lecture Schedule
Topic
Lecture
Basic Principles of Measurements
Response of Measuring Systems, System Dynamics
Error & Uncertainty Analysis
1, 2 and 3
Sensor & Transducers
Basic Electronics, Signal Processing 4
Measurement of Pressure 5
Measurement of Temperature 6
Measurement of Fluid Flow 7
Measurement of Level 8
Measurement of Stress-Strain 9
Measurement of Time Constant 10

3. Topic 6: Measurement of Temperature
Thermocouple

4. Course Objectives
Understand the main operating characteristics of temperature measurement devices such as resistor temperature detectors, thermistors, and thermocouples
Be able to experimentally determine temperature curves (Equations) for temperature measurement devices.

5. Temperature
Temperature is the most common used and measured variable. Actually, the definition of temperature and scale is not well understood.
Temperature is described as the property of an object that describes its hotness and coldness.
The zeroth law of thermodynamics states that two systems in thermal equilibrium with a third system are in thermal equilibrium with each other.
The conversion equations for the four modern temperature scales are:
oC = 5/9 (oF - 32) oF = 9/5 oC + 32
K = oC oR = oF

7. Temperature
Temperature can be measured via a diverse array of sensors. All of them infer temperature by sensing some change in a physical characteristic. Six types with which the engineer is likely to come into contact are: THERMOCOUPLES, RESISTIVE TEMPERATURE DEVICES (RTD and THERMISTORS), INFRARED RADIATORS, BIMETALLIC DEVICES, LIQUID EXPANSION DEVICES, AND CHANGE-OF-STATE DEVICES.
The most common method of measuring and controlling temperature uses an electrical circuit called a THERMOCOUPLE.

8. Devices to Measure Temperature
Thermocouple
RTD
Thermistor

9. Thermocouple
Thermocouples utilize the so-called SEEBECK effect (1821) in order to transform a temperature difference to a voltage difference (electromotive force emf). A THERMOCOUPLE consists of two electrical conductors that are made of dissimilar metallic materials and have at least one electrical connection. This electrical connection is referred to as a junction. A thermocouple junction may be created by welding, or by any method that provides good electrical contact between the two conductors, such as twisting the wires around one another.
The output of a thermocouple circuit is a voltage, and there is a definite relationship between this voltage, and temperatures of the junctions that make up the thermocouple circuit.

10. Thermocouple Thermocouple Circuit
To demonstrate the errors introduced in this procedure, introduced the junction of a type T thermocouple in boiling water (known to be at 100ºC) and read the voltage across the leads. The reading was mV, which corresponds to 86.1ºC.
emf
Copper
Constantan T
Probe junction
Thermocouple Type T

12. Thermocouple
This temperature error arises because the connection of the thermocouple leads to the voltimeter constitutes two additional thermoelectric junctions that substract voltage from the signal being measured.
This problem can be remedied using the arrangement show in this figure:
emf
Copper
Constantan T
Ice bath
Reference junction
Probe junction T To

13. Thermocouple
One thermocouple junction is held in an ice bath at 0ºC. This called THE REFERENCE JUNCTION. The other thermocouple junction is THE TEMPERATURE PROBE. If the probe is at 0ºC, then there is no thermoelectric voltage across the leads because the thermoelectric voltage created by each junction cancel each other out.
The general form of the electromotive force emf as a function of temperature T for all thermocouples is:

14. Type of Thermocouple

15. Type of Thermocouple

16. Type of Thermocouple

17. Experiment Setup

18. Experiment using a Thermocouple Type K
Temperature (oC)
emf (mV) Experimental
emf (mV) Standard 1
0.039 21
0.886
0.839 30
1.252
1.204 40
1.680
1.612 50
2.105
2.023 65
2.770
2.644 75
3.217
3.059 85
3.621
3.474 92
3.918
3.765

19. Laboratory 3  WebPage
Omar E. Meza Castillo Ph.D.