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Lecture

7Mechanical Measurement and

Instrumentation MECN 4600

Professor: Dr. Omar E. Meza Castilloomeza@bayamon.inter.edu

http://www.bc.inter.edu/facultad/omezaDepartment of Mechanical EngineeringInter American University of Puerto Rico

Bayamon Campus

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Tentative Lecture Schedule

Topic Lecture

Basic Principles of MeasurementsResponse of Measuring Systems, System DynamicsError & Uncertainty Analysis

1, 2 and 3

Sensor & TransducersBasic Electronics, Signal Processing

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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

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Thermocouple

Topic 6: Measurement of Temperature

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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.

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Course Objectives

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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 + 273.15 oR = oF + 459.67

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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.

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Devices to Measure Temperature

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Thermocouple

RTD

Thermistor

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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.

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Thermocouple

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emf

Copper

Constantan

T

Thermocouple Type T

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 3.634 mV, which corresponds to 86.1ºC.

Thermocouple Circuit

Probe junction

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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:

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emf

Copper

Constantan

T

Copper

Ice bathReference junction

Probe junction

T

To

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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:

13

nnTaTaTaTaaemf ...3

32

210

CinT

mVinemf

Where

o

:

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Type of Thermocouple

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Type of Thermocouple

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Type of Thermocouple

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Experiment Setup

https://www.youtube.com/watch?v=ETsyJR_p49M

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Experiment using a Thermocouple Type K

Temperature (oC)

emf (mV) Experimental

emf (mV) Standard

1 0.039 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

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Omar E. Meza Castillo Ph.D.

Laboratory 3 WebPage

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