Measurements and Instrumentation
ECCE3036
ECCE3036
Resistance Temperature Detectors
Thermistors
Thermocouples
Other Sensors
Bimetal Strips
Gas Thermometer
Liquid Expansion Thermometers
A Linear approximation means that we may develop an
equation for a straight line that approximates the
resistance versus temperature (R-T) curve over some
specified span.
The equation of the straight line approximation to the
curve over the span T1 to T2 can be written as :
R (T ) = R(T0 )(1 + 0 T )
T1 < T < T2
Where:
R (T) = approximation of resistance at temperature T
R(T0) = resistance at temperature To
T = T - T0
0 =
fractional change in resistance per degree of temperature
at T0
RTD is a temperature sensor that is based
on the principle that metal resistance
increasing with temperature
Metals used in these devices such as
platinum, which is very repeatable, quite
sensitive, and very expensive.
Nickel is also used which is not quite
repeatable, more sensitive, and less
expensive.
An estimate of RTD sensitivity can be noted
from typical values of 0. For platinum, this
number is typically on the order of 0.004/oC,
and for nickel a typical value is 0.005/oC.
With platinum, for example, a change of only
0.4 would be expected for a 100 RTD if
the temperature changed by 1oC.
In general, RTD has a response time of 0.5 to
5 s or more
The slowness of response is due to the
slowness of thermal conductivity in bringing
the device into thermal equilibrium with its
environment.
Generally, time constants are specified either
for a free air condition or an oil bath
condition.
An RTD is simply a length of wire whose
resistance is to be monitored as a function
of temperature.
The construction is typically such that the
wire is wound on a form (in a coil) to
achieve small size and improve thermal
conductivity to decrease response time.
In the view of the very small fractional change
of resistance with temperature (0.4%), the
RTD is generally used in a bridge circuit.
10
Because RTD is a resistor, there is an I2R
power dissipated by the device that causes
a slight heating effect, a self-heating. This
may cause an erroneous reading.
Thus, the current through the RTD must be
kept sufficiently low and constant to avoid
self-heating.
Typically, a dissipation constant is provided
in RTD specifications. Thus, a 25 mW/oC
dissipation constant shows that if I2R power
losses in the RTD equal 25mW
25mW. The RTD will
be heated by 1oC.
11
The dissipation constant equation is:
P
T =
PD
Where:
T = temperature rise because of self-heating in
oC
P = power dissipated in the RTD from the circuit
in W
PD = dissipation constant of the RTD in W/oC
12
An RTD has 0=0.005
0.005/
/oC, R= 500 , and a
dissipation constant of PD=30 mW/
mW/oC at 20
oC. The RTD is used in a bridge circuit with R
1
500
= R2 = 500
and R3 a variable resistor used
to null the bridge. If the supply is 10 V and
the RTD is placed in a bath at 0oC, find the
value of R3 to null the bridge
13
A thermistor is a temperature sensor that has
been developed from the principles of
semiconductor resistance change with
temperature.
The particular semiconductor materials used
varies widely to accommodate temperature
ranges, sensitivity, resistance ranges, and
other factors.
14
The sensitivity of the thermistors is a
significant factor in their application.
Changes in resistance of 10
10%
% per oC are
common. Thus, a thermistor with a nominal
resistance of 10 k at some temperature may
change by 1 k per 1 oC.
15
The temperature range of thermistors
depends on the materials used to construct
the sensor. In general, there are three range
limitation effects:
Melting or deterioration of the semiconductor
Deterioration of encapsulation material
Insensitivity of higher temperature
16
The response time of the thermistors
depends principally on the quantity of
material present and the environment.
Thus, for the smallest bead thermistors in an
oil bath, a response of s is typical. The
same thermistor in still air will response
typically in 10 s.
17
In many cases, a bridge circuit is used.
Since these devices are resistances, care
must be taken to ensure that power
dissipation in the thermistor does not
exceed the limits specified or even interfere
with the environment.
Dissipation constants are quoted for
thermistors as the power in milliwatts
required to raise a thermistors temperature
by 1oC above its environment. Typical
values vary from 1 mW/oC in free air to
10mW/
10
mW/oC or more in an oil bath
18
A thermistor is to monitor room temperature.
It has a resistance of 3.5 K at 20oC with a
slop of -10
10%/
%/oC. The dissipation constant is
PD=5 mW/oC. It is proposed to use the
thermistor in a voltage divider with V=
V=10
10V
10
V
and R=
R=3.5
3.5 k in order to provide a voltage of
5 V at 20oC. Evaluate the effects of self
heating.
19
When a temperature differential maintained
across a given metal, the vibration of atoms
and motion of electrons is affected so that a
difference of potential exists across the
material.
This potential difference is related to the
fact that electrons in the hotter end of the
material have more thermal energy than
those in the cooler end, and thus tend to
drift towered the cooler end.
20
Using solid state theory,
= (T2 T1 )
Where = emf produced in volts
= constant in V/K
T1, T2 = junction temperatures in K
This equation, which describes the Seebeck effect,
shows that the emf produced is proportional to the
difference in temperature and, further, to the
difference in the metallic thermal transport
constants.
21
To measure an unknown temperature, we
see first that one temperature must be
known since the Seebeck voltage is
proportional to the difference between
junctions temperature.
22
23
The thermocouple tables simply give the voltage
that results for a particular type of thermocouple
when the reference junctions are at a particular
reference temperature.
In most cases, the measured voltage does not fall
exactly on a table value. When this happens, it is
necessary to interpolate between table values that
bracket the desired value. Use the following
formula:
TH TL
TM = TL +
(VM VL )
VH VL
24
A voltage of 23.76
23.76mV
mV is measured with a type
K thermocouple at a 0 oC reference. Find the
temperature of the measurement junction.
25
Find the voltage of a type J thermocouple with
a 0 oC reference if the junction temperature is
-172.4 oC.
26
V(400 oC) = 21.85 mV (Type J, 0 oC ref)
and
V(30 oC) = 1.54 mV
(Type J, 0 oC ref)
then
V(400 oC) = 20.31 mV (Type J, 30 oC ref)
or
VJ30 (400 oC) = 20.31
27
A type J thermocouple with a 25 oC reference
is used to measure oven temperature from
300 oC to 400 oC. What output voltages
correspond to these temperature?
28
A type K thermocouple with a 23.9 oC reference
produces a voltage of 35.56 mV. What is the
temperature?
29
A review of tables shows that the range of
thermocouple output is typically less than
100 mV. The actual sensitivity strongly
depends on the conditioning circuit and on
the TC itself.
Examples of the worst and best sensitivity:
Type J: 0.05 mV/oC (typical)
Type S: 0.006 mV/oC (typical)
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The TC has the greatest range of all the
types considered.
Examples:
Type J TC is usable from -150o to 745oC.
Type S TC is usable up to 1765oC.
TC time response is simply related to the
size of the wire and any protective
material used with the sensor.
Large industrial TCs using thick wire may
have time constants as high as 10 to 20 s.
Whereas, a TC may from very small gage
wire can have a time constant as small as
10 to 20 ms
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Designed to amplify input signals to a right
level to be noticeable for further uses.
Typical input signals are:
thermocouple, RTD, pressure, strain, flow, etc.
Typical outputs include:
high level dc voltages (0 to 5 or 0 to 10 volts),
process current (0 to 20 mA or 4 to 20 mA)
There are commercial signal conditioners
with computer interface ready.
ready.
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V+ = VIN.
By virtual ground, V- = V+
Thus Vout = V- = V+ = VIN !!!!
Due to the infinite input impedance of an op
amp, no current at all can be drawn from the
circuit before VIN. Thus this part is effectively
isolated.
Very useful for interfacing to high
impedance sensors such as microelectrode,
microphone
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Point B is grounded, so
does point A (very small).
RF
V1
V2
R1
A
R2
B
R3
V out =
RF
(V 2 V 1 )
R1
Vout
Voltage across R1 is V1,
and across R2 is V2.
Normally: R1 = R2, and
R F = R 3.
Commonly used as a
single op-amp
instrumentation
amplifier.
34
Design a single op-amp
instrumentation amplifier.
R1 = R2, RF = R3
Determine the instrumentation gain.
RF
R1
-
V1
V2
A
R2
R3
Vout
V1 V A
V V OUT
= A
iA
R1
RF
V2 VB
V
+ iB = B
R2
R3
i A = iB 0
VA VB
V out =
RF
(V 2 V1 )
R1
V V OUT
V1 V A
V
V VB
= A
B + 2
R1
RF
R3
R2
V OUT (V A V B
RF
) = (V 2
V 1 ) (V A V B
R1
35
In order to avoid high current driving to the
circuit, V1 and V2 input lines are connected
straight to the inputs of two voltage-follower
op-amps, giving very high impedance. The two
op-amps on the left now handle the driving of
current through the resistors instead of letting
the input voltage sources (whatever they may be)
do it.
36
A Sensor outputs a voltage range of 20 to
250 mV. The sensor output has to feed
computer based controller that work at
voltage range of 0 to 5 V. Design the
required conditioning circuit in order to get
the demand voltage range.
37
J-type thermocouple was used to measure
temperature for plating operations.
Temperature must be measured for control
within a range of 260 oC to 320 oC. Develop a
measuring system that scales this
temperature into 0 to 5 V.
38
RTDs are commonly used in applications where
repeatability and accuracy are important
considerations.
If a process will be run at a specific temperature,
the specific resistance of the RTD at that
temperature can be determined in the laboratory
and it will not vary significantly over time.
RTDs also can be used with standard
instrumentation cable for connection to display or
control equipment
39
RTDs are more expensive than thermocouples
because there is more construction required to
make the RTD including manufacture of the
sensing element, the hooking up of extension
wires and assembly of the sensor.
RTDs do not do as well as thermocouples in high
vibration and mechanical shock environments due
to the construction of the sensing element.
RTDs are also limited in temperature to
approximately 1200F where thermocouples can be
used as high as 3100F
40
Thermocouples can be used to temperatures as
high as 3100F.
Generally cost less than RTDs and they can be
made smaller in size (down to approximately .020
dia) to allow for faster response to temperature.
Thermocouples are also more robust than RTDs
and can therefore be used in high vibration and
shock applications.
41
Thermocouples are less stable than RTDs when
exposed to moderate or high temperature
conditions.
In critical applications, thermocouples should be
removed and tested under controlled conditions in
order to verify performance.
Thermocouple extension wire must be used in
hooking up thermocouple sensors to thermocouple
instrument or control equipment.
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