The Measurement of Temperature

1
Chapter 21

• The Measurement of Temperature

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Introduction to Temperature Measurement

• The importance and Definition of Temperature
• Biological system in the lab sensitive to
  temperature
• Enzymes
• Specific temp for optimal activity
• PCR
• Reaction mixture cycle between certain specific
  temperatures

3
The Principles and Methods of Temperature
Measurement

• Overview
• Various transducers-sensors-to measure
  temperature
• All respond to effects of temp on a physical
  system
• Mercury
• Expansion/contraction of mercury in a glass tube
• Types
• Liquid expansion device
• Bimetallic expansion devices
• Change-of-state indicators
• Metallic resistance devices
• Thermistors
• thermocouples

4
The Principles and Methods of Temperature
Measurement

• Overview, cont..
• Types
• Liquid expansion and bimetallic are similar
• Most materials expand and temp increases/contract
  when temp decreases
• Mechanical measuring
• Do not require electricity
• Use you eyes-no direct computer interface
• Mercury, bimetallic and change-of-state
• Automated
• Generate electrical signal
• Interface well with other equipment
• Accurate, precise, versatile, compact
• Use in wide range of temperatures and remote
  sensing
• Resistance thermometers, thermistors, and
  thermocouples

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The Principles and Methods of Temperature
Measurement

• Liquid expansion thermometers
• Works by expansion/contraction of fluid in
  response to heat/cold
• Fluid moves up/down through a stem-usually used
  in range
• -38OC to 250OC
• Usually mercury or alcohol
• Mercury
• Does not adhere to glass
• Silvery appearance makes it easy to read
• Has better linear response than alcohol
• Alcohol or spirit
• Dyed red to make easier to read
• Safer-less environmental threat
• Can be used below -38OC (mercury freezes)

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The Principles and Methods of Temperature
Measurement

• Liquid expansion thermometers
• Components
• Liquid
• Stem
• Capillary tube which fluid moves through in
  response to temp change
• Scale
• Graduations
• To indicate degrees or fractions of degrees
• Contraction chamber
• Enlargement of capillary tube
• Takes up some volume of the liquid
• Reduces length of thermometer

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The Principles and Methods of Temperature
Measurement

• Liquid expansion thermometers
• Components
• Expansion chamber
• Enlargement of capillary tube at top
• Prevents buildup of excessive pressure
• Stem enlargement
• Thickening of the stem
• Assists proper placement of thermometer in a
  device (oven)
• Immersion line
• Line etched onto stem to how far the thermometer
  should be immersed
• Upper and lower auxiliary scales
• Extra markings at zero and 100 degree areas
• Assists in calibration and verification
  performance

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The Principles and Methods of Temperature
Measurement

• Liquid expansion thermometers-Limitations
• Fairly wide tolerances
• 5OC
• Capillary tube must be smooth-no impedance of
  liquid
• Very difficult to achieve
• Scale must be carefully etched onto the stem for
  accuracy
• Working range is limited
• Fragile
• Mercury

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The Principles and Methods of Temperature
Measurement

• Bimetallic Expansion Thermometers
• Two different metals fused together
• Expand and contract with change in the
  temperature
• Thermal expansion coefficient
• Measures how much the material expands for a
  specific increase in temperature
• Strips of metal distort and bend based on the
  temperature

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The Principles and Methods of Temperature
Measurement

• Bimetallic Expansion Thermometers
• Bimetallic dial thermometer
• Fusing of two metal strips
• Brass and iron
• Different coefficients of expansion
• Two metals respond unequally-results in bending
• Simple and convenient
• Not intended to be accurate
• Good for
• Photography darkrooms
• Ovens in the home

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The Principles and Methods of Temperature
Measurement

• Change-of-State Indicators
• Varied products that change color or form when
  exposed to hear
• May be liquid crystal
• Colors
• Have narrow temperature range
• Useful for biological systems

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The Principles and Methods of Temperature
Measurement

• Resistance Thermometry Metallic Resistance
  Thermometers and Thermistors
• Based on electrical resistance of different
  materials change as temperature changes
• Metallic resistance thermometers
• Use metallic wires in which resistance increases
  as the temperature increases
• The best use platinum wire
• Used as references against other thermometers are
  calibrated
• Important with thermocyclers

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The Principles and Methods of Temperature
Measurement

• Resistance Thermometry Metallic Resistance
  Thermometers and Thermistors
• Thermistors
• Hard, ceramic-like material
• Compressed mix of metal oxides.
• Molded into many shapes
• Rapid response to temperature change
• Can be miniaturized
• Useful in wide range of temperatures
• Used in production setting for temperature
  control devices

15
Verifying the Performance of Laboratory
Thermometers

• Many uses in laboratories
• Checking temperature in solutions, water baths,
  incubator, ovens, refrigerators, and freezers
• GLP to verify thermometers performing acceptably
• Check that readings correct within specified
  tolerance range
• 95 of errors occur at the ice point end of the
  scale
• 0OC or 32OF
• Perform Ice Point check (pg. 378)

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Verifying the Performance of Laboratory
Thermometers

• Ice Point check
• Only verifies performance at one temperature
• Check other temps with NIST-traceable thermometer
  at different readings in the range
• Calibrated against standard thermometer
• In turn calibrated against NIST standard