2142-391 Engineering Mechanical Laboratory

Semester 2013-1

Lab 4 (b): Thermocouple

Lab 4 (a): Viscosity

Lecturer: Nopdanai Ajavakom (NAV)

Lab 4 (a): Viscosity MeasurementLubricant = a substance (usually a liquid) introduced between two moving surfaces to reduce the friction and wear between them.

Viscosity = one of the most important characteristics of the lubricant

What is viscosity? Viscosity• the resistance of a fluid to deform under shear stress. • commonly perceived as "thickness” or resistance to

flow. • describes a fluid's internal resistance to flow

How do we measure it?

You will learn how to measure the “Viscosity” and compute “Viscosity Index” (the number indicating viscosity-temperature characteristics)

Measuring Viscosity

Saybolt Viscometer

n = f (t) ?

“Thick” (high viscosity) “Light” (low viscosity)

Liquid which has high viscosity,takes more time to pass the tube.

(Strock, Centistroke)B

Att

Viscosity vs TemperatureViscosity is one of most important property of lubricant, while temperature is the most important factor affecting viscosity.

Thus the viscosity of a lubricant can be matched to the mechanism’s requirement only if the lubricant’s viscosity temperature characteristics are known over the considerable temperature range.

Dean and Davis’s VI (“Viscosity Index”) method

The fluid usually becomes less viscous, when temperature arises.

One way to evaluatingviscosity-temperaturecharacteristics is

Viscosity Index• Indicator for: “how less n changes, when T changes”

• One single number

Temperature

viscosity

A

C

B VI of B = 51

VI of C = 73

VI of A = 32

n changes much : V.I. is low:

n changes little : V.I. is high: betterExample:

Lab Objectives

To understanding the concept of Viscosity Index and the basic process of determining it.

To understand the theory underlying the viscosity measurement, and develop some expertise in the measurement of viscosity using Saybolt Viscometer.

Measure Viscosity at

Find V.I of a lubricant

100oT F 210oT F

Lab 4 (b): Calibration of ThermocoupleTemperature • is a thermodynamic property of a body, which is

important for analyzing the internal state of the object.• one of the most frequently measured quantities in

science and industry. Since it is not possible to measure a temperature change directly, “changes of other properties” (that can be observed easily) are used as indicators of temperatures.

RTD:resistance thermometer

thermocouple

pyrometerliquid-in-glass thermometer

Thermocouple• is a widely used type of temperature transducer. • relatively cheap, interchangeable, having standard

connectors and can measure a wide range of temperature.

Thermocouple

Two wires of dissimilar metals, connected at both ends to form a circuit.

thermocouple

wire A

wire B

reference junction

measuring junction

meas refT T T

electromotive force (emf) (change in voltage)

In this experiment, we will study the relation of DT and the emf

Thermocouple Type, Range, Sensitivity

E Chromel & Constantan

-270 to 1000ºC

~ 60.9 µV/ºC

J Iron & Constantan

-210 to 1200ºC

~ 51.7 µV/ºC

K Chromel & Alumel

-270 to 1350ºC

~ 40.6 µV/ºC

T Copper & Constantan

-270 to 400ºC

~ 40.6 µV/ºC

R Platinum & 87%Platinum

-50 to 1750ºC

~ 6 µV/ºC

S 90% Platinum & 10% Rhodium

-50 to 1750ºC

~ 6 µV/ºC

Constantan is a 55% copper and 45% nickel alloy. Chromel is a 90 % nickel and 10 % chromium alloy. Alumel is a 95% nickel, 2% manganese, 2% aluminium and 1% silicon alloy.

Calibration• is a comparison between measurements – one of known

magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device.

• The device with the known or assigned correctness is called the standard. The second device is the unit under test, test instrument, etc.

Lab Objectives

To understand the concept of calibration and basic process of calibrating an instrument. Specifically, to realize the purposes of calibration, why an instrument should be calibrated before use.

To understand the theory of the thermoelectric temperature measurement, especially thermocouples and develop some expertise in the measurement of temperature with thermocouples.

Calibration Graph

Temperature(from a reliable source)

EMF (mV)from voltmeter

Type J

∆V

∆T

∆V / ∆T = sensitivity (V/ºC)

Thermocouple

• low cost• less likely to be broken• wide temperature range• short response time• Repeatability and acuracy

• Sensitivity is low• Known temperature

reference• nonlinearity

Saybolt Viscometer

• To measure viscosity at specific temperature.

• To find V.I. 100oT F 210oT F

32

5 9

C F

Saybolt Viscometer