14. Temperature Measurement Flashcards
What is heat?
Heat is a form of energy
associated with the kinetic motion of molecules
within a substance.
Heat energy gets transferred from a
hotter to a colder substance.
What is temperature?
Temperature refers to the
thermal state of a substance.
It is the degree of ‘hotness’ of a substance
and reflects its potential for heat transfer.
What is the SI unit of temperature?
What is it based on
The standard international (SI) unit of
temperature measurement is the kelvin (K).
It is based on the triple point of water,
which is the temperature
(at a specific pressure)
at which water exists in
all three phases
273.16 K or 0.01 °C
at a pressure of 611.73 Pa or 0.006 atm).
1 unit kelvin = 1/273.16 of the
thermodynamic triple point of water.
A change in temperature of 1 K is
equivalent to a change in temperature of 1 °C.
What methods can be used to
measure temperature?
non-electrical,
- Liquid Expansion
- Gas expansion thermometers
- Bimetallic strip dial thermometer
- Chemical thermometer
electrical
infrared-based methods.
Non-electrical:
- Liq Expansion
- > Liquid expansion thermometers
(e. g. mercury and alcohol thermometers):
• Principle:
Based on the volumetric expansion
of a liquid with increasing temperature.
Bulb containing the liquid is in
communication with a
narrow, linear, calibrated capillary tube.
As the temperature increases the liquid expands
and its volume increases causing it to
rise up the capillary tube.
Alternatively, a small bobbin sitting above the liquid gets left at the maximum reading point until the device is shaken.
• Uses:
Mercury thermometers were previously used to measure body temperature
(mercury freezes at about −39 °C and boils at about
250 °C)
while alcohol thermometers are used to measure
very low temperatures
(alcohol freezes only at −114 °C and boils at 78 °C).
- Advantages: Cheap and easy to use.
- Disadvantages:
Slow (2–3 min),
glass thermometers can break
causing injury and
mercury is now classified as a hazardous substance.
- Gas expansion thermometers
(e. g. Bourdon gauge dial thermometer):
Gas expansion thermometers
(e.g. Bourdon gauge dial thermometer):
• Principle:
Based on the volumetric expansion of a gas
with temperature and
the associated pressure changes
that ensue due to this volume expansion.
A bulb containing volatile liquid
or saturated vapour is in
communication with a hollow, elliptical, spiral tube.
As the temperature rises,
the volume in the bulb increases
, and as the hollow tube tries to accommodate the expanded gas it changes shape from elliptical to circular in order to give it the largest possible crosssectional area.
This shape change causes the tube to uncoil, moving a
pointer across a temperature scale.
- Uses: Used outdoors in harsh environments.
- Advantages: Cheap, robust and gives continuous measurements.
- Disadvantages: Poor accuracy and requires recalibration.
Non electric
3. > Bimetallic strip dial thermometer:
- > Bimetallic strip dial thermometer:
• Principle:
Coil consisting of two different metals
with different expansion coefficients.
As the temperature increases these metals
expand by different amounts
causing the coil to tighten and moving a
pointer over the temperature scale.
- Uses: Used outdoors in harsh environments.
- Advantages:
Cheap, robust and gives continuous measurements.
• Disadvantages:
Poor accuracy and requires recalibration.
- > Chemical thermometer:
> Chemical thermometer:
• Principle:
Strip of small cells containing a
chemical mixture that melts
over a range of temperatures
to produce temperature-dependent colour changes.
Newer reusable models use liquid crystal technology, where tiny colourless, solid crystals melt with increases in temperature and then realign themselves, causing a colour change.
- Uses: Clinical body temperature measurement.
- Advantages: Fast response time (<30 s), disposable and no risk of glass breakage.
- Disadvantages: Not very accurate at temperature differences less than 0.5 °C.>
Electrical
1.> Thermocouple:
- Thermocouple:
• Principle:
Consists of two different metals
(e.g. copper and constantan, an alloy of copper and nickel)
joined to form two separate junctions.
One junction is kept at constant temperature
and is known as the reference junction
while the other junction acts as the
temperature-measuring probe.
When there is a temperature difference
across these two junctions,
a small voltage is produced.
This voltage is proportional to
the temperature difference
across the junctions and
is measured using a galvanometer.
This phenomenon is known as the Seebeck effect.
• Advantages: Rapid response time, accurate to within ± 0.1 °C and small.
• Disadvantages: Voltage produced is very small and needs signal amplification and the reference junction needs to be at a constant temperature (or requires compensation).
graph page 191
junctional voltage y temperature x
Electrical
- > Resistance thermometers
> Resistance thermometers
(e.g. platinum wire resistance thermometer):
• Principle:
Linear relationship between
temperature and electrical resistance of a wire
(e.g. platinum, copper or nickel)
such that as temperature increases,
the resistance within a platinum wire increases
in a predictable manner.
• Advantages:
Extremely accurate to within ±0.0001 °C,
with linear
relationship between 0 and 100 °C.
• Disadvantages:
Slow response time, bulky and fragile.
graph resistance on y and temp on X
- Thermistor:
• Principle:
Semiconductor composed of heavy metal oxide
(e.g. nickel, iron or manganese)
that displays a negative exponential relationship
between electrical resistance and temperature.
• Uses:
Used clinically in PA catheters to
measure core temperature.
• Advantages: Rapid response time (<0.2 s), very small, accurate and cheap.
• Disadvantages: Hysteresis, ageing, variability within a batch and non-linear relationship requires recalibration
graph 192 negative exponential
resitance y
tempp x
Infrared:
> Infrared tympanic membrane thermometers:
• Principle:
All objects emit electromagnetic radiation,
the wavelength of which is
dependent on the temperature of that object.
At body temperature, infrared radiation
is the primary electromagnetic radiation
given off by objects.
Tympanic membrane thermometers receive infrared radiation from the tympanic membrane, which is close to the brain and therefore represents core body temperature.
There are two main types of sensors that are used in these devices –
the pyroelectric sensor
and the thermopile sensor.
The pyroelectric sensor contains crystals
that alter their polarisation
depending on the temperature.
The thermopile sensor is made up of
numerous thermocouples connected
in series and allows
continuous measurements to be made.
• Uses:
Clinical measurement of core body temperature.
• Advantages:
Non-invasive,
accurate with a rapid response time (<5 s).
Mock viva qs
temperature- electric non electric - ways and where- how it works - risk of ear , issue with thermistor-( cleaning can damage)- where’s the most accurate to take, thermistors vs thermocouples - graphs
