Lab 2: Measuring Temperature

1
Lab 2 Measuring Temperature

• Temperature
• Common measurements
• Weather
• Bank signs
• Heater/AC thermostat
• Body temperature
• We will talk about
• Measurement methods
• Sensors and their applicability
• Sources of error

2
Common Temperature Sensors

• Change in volume of gas or liquid
• Alcohol or mercury
• Increase in T ? Increase in volume
• Calibrated according to temperature
• Example Mercury Thermometer (why mercury?)

• Bimetallic Strip
• Two different metals with different coefficients
  of thermal expansion (CTE)
• Bonded together
• Bend proportional to temperature
• Calibrate to get temperature
• Example Thermostat

3
Infrared Transducers

• Measure intensity of IR radiation
• Intensity proportional to T4
• Useful for measuring temp of surfaces (remote)
• Chapter 10

4
Electronic Methods

• Good for data logging
• Store temperature values without anyone present
• Preserve T maximum, minimum, hourly, etc.
• Come in many shapes and sizes
• Important to know where and when each is
  appropriate

• Basic Types
• Thermocouples
• Thermistors
• Platinum Resistance Thermometers (PRT)
• Integrated circuit (chip) sensors

5
Basic Electronics Review

• Units
• Voltage (V) - Volts potential difference that
  drives electron flow
• Current (I) - Amps flow of electrons through a
  circuit
• Resistance (R) - Ohms resistance to current
  flow
• Power (P) - Watts energy dissipated by current
  flow through a circuit
• Ohms Law V IR
• P VI

6
Thermocouples

• Two wires of dissimilar metal joined together
• Sebec effect two junctions at different T will
  produce an electrical current

current
7
Thermocouples

• Voltage can be measured across open circuit
• Voltage proportional to ?T

T1
T2
Voltmeter
8
Thermocouples
Reference Temperature

• Voltage response to temperature can be logged
• MUST KNOW TEMPERATURE AT ONE JUNCTION (Tref)

Constantan
Cu
To logging Device
9
Calculating Thermocouple Temperature from Voltage

• Ref Temp 22.5 C
• Measured TC voltage 0.98 mV
• Cu-Constantan TC calibration 40 µV/C

TC Temp ?
10
Calculating Thermocouple Temperature from Voltage

• Ref Temp 22.5 C
• Measured TC voltage -0.13 mV
• Cu-Constantan TC calibration 40 µV/C

TC Temp ?
11
Thermocouple Cautions, Advantages and
Disadvantages

• Reference T must be measured at the TC reference
  junction
• Advantages
• Inexpensive
• Many sizes
• No individual output
• (somewhat) Linear calibration

• Disadvantages
• Changes in µV are difficult to measure
• Requirement for accurate reference temperature
  measurement

12
Thermistors

• Change resistance with temperature
• R vs. T relationship follows form

• where a and ß are calibration constants for
  individual thermistors

13
Thermistor response
14
Thermistor response
15
Reading thermistors
Or, use a resistance meter
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Thermistor

• Disadvantages
• More expensive
• Require individual calibration
• Limited size and shapes
• Generate heat internally if not set up properly

• Advantages
• Output easily measurable when set up properly
• No reference temperature requirement

17
Other Temperature Devices

• Integrated circuit (chip)
• Often use a diode
• Look at voltage drop across PN junction
  (semiconductor) at a constant current
• Advantages
• Linear with temperature
• Disadvantages
• Complex circuitry

• Platinum Resistance Thermometer
• Electrical resistance increases 0.4 per oC
• Similar circuit to thermistor (see lab chapter)
• Advantages
• Excellent calibration stability
• Excellent accuracy
• Disadvantages
• Lower voltage increase per oC than thermistor
• More expensive than thermistor

18
Temperature Measurement Errors

• Assuming calibration is good
• Errors are due to difference between thermometer
  and medium
• Time constant of device
• Radiant heating
• Heat flow in thermocouple wires
• Requires high thermal resistance in wires

19
Time Constant

• Body temperature example
• If t 0.5 min, Ti 20 oC and Tf 37 oC, what
  will the temperature be after 1 minute?
• 2.3 oC below actual
• Waiting 5t will result in a reading within 1 of
  actual

• Time constant (t) time to change to 63 of final
  reading
• Time constant depends on
• Heat capacity of sensor (cp)
• Rate of heat transfer
• What happens to the time constant if we increase
  cp?

20
Radiant heating of thermometer measuring air
temperature

• 100 degrees in the shade

as short wave absorptivity Si Solar
radiation d diameter of thermometer Cp
specific heat of air u windspeed