Transcript Thermocouple and Viscosity

Term 2009-1
2142-391 Engineering Mechanical
Laboratory
Lab 9(a): Viscosity
Lab 9 (b): Thermocouple
Lab 9 (a): Viscosity Measurement
Lubricant is a substance (usually a liquid)
introduced between two moving surfaces to
reduce the friction and wear between them.
“Viscosity” is one of the most important
characteristics of the lubricant and it should be
match with the mechanism requirement.
You will learn how to measure the “Viscosity”
and “Viscosity Index” (the number indicating
viscosity-temperature characteristics)
What is viscosity?
the resistance of a fluid to deform
under shear stress. It is commonly
perceived as "thickness", or resistance to
flow. Viscosity describes a fluid's
internal resistance to flow and may be
thought of as a measure of fluid friction.
How do we measure it?
Liquid with high viscosity,
takes more time to pass the tube.
Principle
concept of
Saybolt Viscometer
Measuring Viscosity
Liquid which has high viscosity,
takes more time to pass the tube.
“Thick” (high viscosity)
“Light” (low viscosity)
Saybolt Viscometer
  At n
B=
t
f (Strock,
(t) ? Centistroke)
Objective
To determine a viscosity and viscosity index (VI) of lubricating oil
by Saybolt viscometer
Viscosity and 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
Dean and Davis’s V.I. (Viscosity Index)
• Indicator for: “how much n changes, when T changes”
• One single number
viscosity
B
In the sense that
n changes much : V.I. is low
n changes little : V.I. is high
C
VI of A = 32
A
VI of B = 51
VI of C = 73
Temperature
Lab Objectives
 To gain some understanding on the theory underlying
the viscosity measurement, and develop some
expertise in the measurement of viscosity using
Saybolt Viscometer.
Measure Viscosity
T  100o F
T  210o F
 To gain an understanding on the concept of Viscosity
Index and basic process of determining it.
Find V.I of a lubricant
T  100o F
T  210o F
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
Lab 9(b) : Calibration of Thermocouple
Temperature is a thermodynamic property of a body.
It is one of the most frequently measured quantities in science and industry.
Its measurement is vital for analyzing the internal state of the object.
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.
Sensor
liquid-in-glass
thermometer
pyrometer
Transducer
RTD:
resistance
thermometer
thermocouple
Thermocouple
Thermocouple is a widely used type of temperature transducer.
They are cheap, interchangeable, having standard connectors and
can measure a wide range of temperature.
measuring junction
Two wires of dissimilar metals,
connected at both ends to form a circuit.
T  Tmeas  Tref
(change in voltage)
electromotive force (emf)
reference junction
thermocouple
In this experiment, we will study the
relation of T and the emf
same temp.
Thermocouple
Usage
Law of intermediate metals
Thermocouple Type
E
Chromel - Constantan -270 to 1000’C
60.9 uV/’C
J
Iron & Constantan
-210 to 1200’C
51.7 uV/’C
K
Chromel & Alumel
-270 to 1350’C
40.6 uV/’C
T
Copper & Constantan -270 to 400’C
40.6 uV/’C
R
Platinum &
-50 to 1750’C
87%Platinum
90% Platinum & 10% -50 to 1750’C
Rhodium
94% Platinum & 6% -50 to 1750’C
Rhodium
6 uV/’C
S
B
6 uV/’C
6 uV/’C
Pro and Cons
•
•
•
•
•
low cost
Less likely to be broken
wide temperature range
short response time
Repeatability and acurracy
• Sensitivity is low
• Accuracy
• Known temperature
reference
• nonlinearity
Lab Objectives
 To gain some understanding on the theory underlying
the thermoelectric temperature measurement,
especially thermocouples and develop some expertise
in the measurement of temperature with
thermocouples.
 To gain an understanding on 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.