Transcript Viscosity

Semester 2013-1
2142-391 Engineering Mechanical
Laboratory
Lab 4 (a): Viscosity
Lab 4 (b): Thermocouple
Lecturer:
Nopdanai Ajavakom (NAV)
Lab 4 (a): Viscosity Measurement
Lubricant = 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
You will learn how to measure the
“Viscosity” and compute “Viscosity
Index” (the number indicating
viscosity-temperature characteristics)
How do we measure it?
Measuring Viscosity
Liquid which has high viscosity,
takes more time to pass the tube.
“Thick” (high viscosity)
“Light” (low viscosity)
Saybolt Viscometer
B= f (t) ?
n
  At 
(Strock, Centistroke)
t
Viscosity vs Temperature
Viscosity is one of most important property of lubricant,
while temperature is the most important factor affecting
viscosity.
The fluid usually becomes less viscous, when
temperature arises.
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.
One way to evaluating
viscosity-temperature
characteristics is
Dean and Davis’s
VI (“Viscosity Index”)
method
Viscosity Index
• Indicator for: “how less n changes, when T changes”
• One single number
n changes much : V.I. is low:
n changes little : V.I. is high: better
Example:
viscosity
C
VI of A = 32
VI of B = 51
VI of C = 73
B
A
Temperature
Lab Objectives
 To understand the theory underlying the viscosity
measurement, and develop some expertise in the
measurement of viscosity using Saybolt Viscometer.
Measure Viscosity at
T  100o F
T  210o F
 To understanding the concept of Viscosity Index and
the basic process of determining it.
Find V.I of a lubricant
Lab 4 (b): Calibration of Thermocouple
Temperature
• 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.
liquid-in-glass
thermometer
RTD:
resistance
thermometer
pyrometer
thermocouple
Thermocouple
• is a widely used type of temperature transducer.
• relatively cheap, interchangeable, having standard
connectors and can measure a wide range of temperature.
Thermocouple
measuring junction
Two wires of dissimilar metals,
connected at both ends to form a circuit.
T  Tmeas  Tref
electromotive force (emf)
reference junction
thermocouple
(change in voltage)
In this experiment, we will study the
relation of T 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 &
-50 to 1750ºC
~ 6 µV/ºC
-50 to 1750ºC
~ 6 µV/ºC
87%Platinum
S 90% Platinum & 10%
Rhodium
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 theory of the thermoelectric
temperature measurement, especially thermocouples
and develop some expertise in the measurement of
temperature with thermocouples.
 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.
Calibration Graph
EMF (mV)
from voltmeter
Type J
∆V
∆T
∆V / ∆T = sensitivity (V/ºC)
Temperature
(from a reliable source)
Thermocouple
•
•
•
•
•
• Sensitivity is low
low cost
• Known temperature
less likely to be broken
reference
wide temperature range
• nonlinearity
short response time
Repeatability and acuracy
C F  32

5
9
Saybolt Viscometer
• To measure viscosity at specific temperature.
• To find V.I.
Saybolt Viscometer
T  100 F
o
T  210o F