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Measuring temperature
PHYSICS
Measuring temperature
Learning objectives
E Byron Howells After reading this article, you should be able to: C define heat and temperature C describe fixed points and temperature scales in common use C explain the principles of action of thermometers in common use, with the particular advantages and limitations of each type
Abstract In the perioperative period patients tend to lose heat and become hypothermic. An understanding of the causes and prevention of heat loss is therefore important to the anaesthetist. Heat and temperature are measures of energy. Heat is a measure of the total kinetic energy (joules, J) of a body, and depends on the size of the body and its specific heat capacity. Temperature is a measure of the average kinetic energy, and describes the potential for heat transfer from a body at high temperature to one at lower temperature until both bodies reach equilibrium at the same temperature. Temperature is measured on a scale (e.g. Fahrenheit, Celsius, or Kelvin) which is defined by fixed points related to predictable physical events (e.g. the freezing point, steam point, and triple point of water).
Heat and temperature Heat is a measure of energy and therefore has the SI unit joule (J) (and non-SI unit calorie: 1 calorie ¼ 4.186 KJ). It is the quantity of thermal energy in a substance and represents the total kinetic energy of the molecules. The total amount of heat energy in a body depends on the mass of the body and its specific heat capacity. The heat capacity of a body is the heat required to produce a unit temperature rise, and has the SI unit joule per kelvin (JK1). Specific heat capacity is the heat capacity per unit mass of a substance, and has the SI unit joule per kelvin per kilogram (JK1Kg1). Temperature is the degree of hotness (or coldness) of a body. It is a measure of the average kinetic energy of the molecules within a substance, although individual molecules will have a wide range of kinetic energy. In a solid the molecules vibrate faster and with greater amplitude when heated. In liquids and gases where the molecules are free to move, they move with higher velocity when heated, resulting in more frequent collisions with each other and with the vessel walls. Temperature also represents the tendency of a substance to lose or gain heat relative to its surroundings. Heat tends to flow down a temperature gradient from a hot body towards a cold one, and given time this process will continue until they are both at the same temperature.
Keywords Fixed points; heat; temperature; thermometer; thermometric properties Royal College of Anaesthetists CPD matrix: 1A03
Introduction Humans are homeothermic, that is, they control core body temperature within a narrow range (36.8 0.4 C). When a patient undergoes surgery under anaesthesia they will inevitably lose heat due to vasodilatation, exposure of tissues, cold intravenous fluids and ventilation with dry cold gases. The mechanisms of heat loss are: convection, conduction, radiation and evaporation. It is important for the anaesthetist to make every effort to minimize heat loss as hypothermia has many adverse effects, including impairment of cardiac output, oxygen delivery, coagulation and drug metabolism. Further, shivering in the postoperative period increases pain and oxygen consumption. It is therefore very important to monitor patients’ body temperature to diagnose hypothermia or hyperthermia (due to, for example, fever or malignant hyperpyrexia). Core body temperature can be measured in the pulmonary artery, oesophagus, nasopharynx and tympanic membranes, while peripheral body temperature can be measured sublingually, in the axilla, rectum, bladder and on the forehead, and is usually within 0.5 C of core temperature. Heat loss can be minimized by: maintaining theatre ambient temperature >22 C use of blood/fluid warmers pre-warming intravenous fluids in warming cabinets forced air convection heaters/blankets warming mattresses hats for small children and babies.
Temperature scales In order to establish a temperature scale it is necessary to make use of fixed points. A fixed point is the single temperature at which a particular physical event always occurs, for example: The ice point: The temperature at which ice exists in equilibrium with water at standard pressure (101.3 kPa). This is designated 0 C or 32 F. The steam point: The temperature at which pure water exists in equilibrium with its vapour at standard pressure, that is, the temperature at which the saturated vapour pressure of pure water equals atmospheric pressure. This is designated 100 C or 212 F. The triple point of water: The temperature at which pure water, water vapour, and ice can exist in equilibrium. This is designated 0.01 C and can only occur at 611 Pa (0.006 atm.). The Fahrenheit scale was devised by German physicist Daniel Gabriel Fahrenheit in 1724. There is no particular logic to the scale and its evolution, and it has been revised several times. Fahrenheit defined the temperature of a mixture of ice, salt and sal ammoniac (ammonium chloride) as 0 F, the ice point as 32 F and the boiling point of water as 212 F; and divided the
E Byron Howells MB Bch (Wales) FRCA is a Consultant Anaesthetist at the Princess of Wales Hospital, Bridgend, UK. Conflict of interest: none.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 16:7
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Ó 2015 Elsevier Ltd. All rights reserved.
PHYSICS
ice and boiling points into 180 divisions or ‘degrees’ ( ). His temperature scale was widely adopted because it was used in his accurate and commercially successful mercury thermometers. The Celsius scale was devised by Swedish astronomer Anders Celsius shortly after Fahrenheit’s death in 1736. Celsius divided the fixed points of the boiling and freezing points of water into 100 (hence until 1948 this was sometimes also called the ‘centigrade scale’). Celsius originally represented the boiling point as 0 C and the freezing point as 100 C; but this convention was reversed after his death by one of his students. It became known at the ‘Celsius scale’ from the early part of the 19th century, but was only officially recognized after the 9th Conference on Weights and Measurements in 1948. The Kelvin scale was proposed by Irish physicist William Thomson, First Lord Kelvin. It is also called the absolute or thermodynamic scale. His scale uses the fixed points of absolute zero and the triple point of water. Absolute zero is defined as the theoretical temperature at which the molecules in a substance have no kinetic energy. Absolute zero has been approached experimentally, but is impossible to achieve and therefore cannot be measured directly. The scale has the same intervals as the Celsius scale (1 C ¼ 1 K) with absolute zero representing 0 K and the triple point of water representing 273.16 K; therefore 1 K (kelvin) is defined as the fraction 1/273.16 of the triple point of water.* The International Temperature Scale of 1990 (ITS-90) is the current standard used to calibrate modern thermometers. ITS-90 ranges from 0.65 K to >1300 K and uses several fixed points including the triple points of water (273.16K), neon (24.56K), oxygen (54.36K), argon (83.8K) and mercury (234.3K); and the freezing points of tin (505.1K), aluminium (933.5K), gold (1337.3K) and copper (1357.8K). The relationship between the Kelvin, Celsius and Fahrenheit scales is shown (Table 1).
Comparison of the Kelvin, Celsius and Fahrenheit scales Temperature scale Celsius ( C)
Kelvin (K)
212 98.2 68
100 36.8 20
373.15 310 293
32 32 459.7
0.01 0 273.15
273.16 273.15 0
Steam point Body temperature Average room temperature Triple point of water Ice point Absolute zero
Table 1
Classification of thermometers Non-electrical C Touch C Liquid expansion thermometers (Mercury, Alcohol) C Gas thermometers C Bimetallic strip thermometers C Liquid crystal thermometers C Infrared thermometers Electrical C Resistance thermometers C Thermistors C Thermocouples Box 1
Thermometers and thermometric properties
expansion within a certain temperature range when heated. As the liquid warms it increases in volume and is forced up the capillary tube alongside a calibrated scale from which the temperature can be read. There is usually a constriction between the bulb and the capillary tube which delays the return of the liquid to the bulb on cooling, allowing the reader time to read the maximum temperature, after which the liquid may be returned to the bulb by shaking the thermometer. Liquid expansion thermometers are used to measure body temperature, usually from the oral, axillary and rectal regions. The effective range for ethanol is e100 C to 50 C (melting point e114 C, boiling point 78 C) and for mercury is e10 C to 350 C (melting point e39 C, boiling point 357 C); however, the range can be extended by adding gas or liquids such as toluene to the mercury. Accuracy depends on the diameter and length of the capillary tube. Mercury thermometers for clinical use normally have a scale range 35 Ce42 C.
A thermometer is a device for measuring the temperature of a substance or body. All thermometers rely on a thermometric property (i.e. a physical property that changes in a known, predictable, and reliable way with temperature). Ideally there should be a simple linear relationship between temperature and the thermometric property (e.g. the length of a mercury column), although some relationships are non-linear (e.g. thermistor). Based on their mechanism of action, thermometers can be classified as Non-electrical or Electrical (Box 1). Non-electrical thermometers Touch has the advantage of needing no equipment, but is extremely inaccurate and does not provide a quantitative measurement. Liquid expansion thermometers consist of a sealed glass capillary tube with a bulbous reservoir at one end which is filled with a liquid (usually mercury or alcohol) that undergoes linear
Advantages:
simple to use and read low cost accurate over body temperature range.
*
The SI unit Kelvin (K) should never be expressed or typeset as ‘degree kelvin’ or ‘ K’; in contrast to ‘degree Celsius ( C)’ and ‘degree Fahrenheit ( F)’.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 16:7
Fahrenheit ( F)
Disadvantages:
glass is fragile and can break, causing injury and toxicity if mercury used
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Ó 2015 Elsevier Ltd. All rights reserved.
PHYSICS
Slow response time (2e3 minutes) limited range due to boiling and freezing point of alcohol and mercury respectively.
Disadvantages
large and bulky slow response time. Bimetallic strip: thermometers consist of a strip of two different metals (e.g. brass and invar e an alloy of nickel and iron) bound together, which have different coefficients of expansion. As temperature changes, differential expansion causes the metal strip to bend. Bimetallic strips may therefore be may be employed in thermostatic switches and as a temperature compensation mechanism in anaesthetic vaporizers. In a bimetallic strip thermometer the strip is usually in the form of a coil or helix with one end fixed and the other attached to a pointer (Figure 1). As the temperature increases, the coil unwinds (and vice versa), which causes the pointer to move over a scale. Common applications include room thermometers and cooking thermometers.
Gas thermometers: The Universal Gas Law states: PV ¼ nRT Where P is pressure (Pa), V is volume (m3), n is number of moles of a gas, R is the universal gas constant, and T is the absolute temperature (K). Therefore at a fixed volume the pressure of a gas is directly proportional to its absolute temperature (Charles’s Law); and at a fixed pressure the volume of a gas is directly proportional to its absolute temperature (Gay-Lussac’s Law). We can use the above laws to derive temperature using either pressure changes of a gas at fixed volume or volume changes of a gas at constant pressure as temperature changes. A Hydrogen thermometer consists of a syringe filled with hydrogen gas, which has a known thermal expansion coefficient, and expands along a calibrated scale as temperature increases. A Bourdon thermometer consists of a gas held in a bulb which is connected to a Bourdon gauge by a fine bore tube. The gas volume is fixed. As temperature increases the pressure in this closed system increases, causing the Bourdon gauge to uncoil, moving an attached pointer along a calibrated dial. For increased precision, the Bourdon gauge can be replaced by a manometer.
Advantages:
cheap robust operate up to high temperatures (600 C). Disadvantages:
limited accuracy (readings recalibration) slow response time.
may
drift,
requiring
Liquid crystal thermometers employ liquid crystals: substances that can flow like liquids, but also have a regular layered crystalline arrangement of molecules like solids. They are used in thermometers because their optical properties change with temperature. As temperature increases, the space between the layers increases, causing a change in the wavelength of light
Advantages:
robust sensitive and accurate over a wide range of temperatures (up to 600 C).
A bimetallic strip thermometer 40
50
60
30 Free end
70
20
80
10
90 0
Spiral wound element
100
Rotating shaft
Free end attached to pointer shaft
Fixed end
Fixed end
Figure 1
ANAESTHESIA AND INTENSIVE CARE MEDICINE 16:7
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Ó 2015 Elsevier Ltd. All rights reserved.
PHYSICS
reflected from the crystals. This is visible as an apparent colour change. A liquid crystal thermometer for measurement of body temperature typically consists of a plastic strip containing cells of different liquid crystals that each change colour at different temperatures in the range 35 Ce40 C. The strip is placed on a person’s forehead, and the temperature is displayed by the colour change of the cells. Liquid crystal thermometers can also be used for measuring intravenous fluid, bath, and room temperatures.
Advantages:
Advantages:
Electrical thermometers Resistance thermometers: when metals are heated, the frequency and amplitude of vibration in their lattice structure increases, and impedes flow of electrons through the metal (i.e. electrical resistance increases). In a resistance thermometer, a fixed voltage is passed across a metal wire and the current flowing through the wire is measured. Using Ohm’s law (V ¼ IR) the resistance of the wire can be calculated and converted to temperature after calibration (Figure 3). Platinum is a particularly good choice of metal as it is inert, and its electrical resistance tends to vary linearly with a change of temperature over a wide temperature range (15 Ke900 K). Other metals (e.g. copper and nickel) can be used for different temperature ranges. A Wheatstone bridge is usually employed to measure the change in resistance to a high degree of accuracy.
fast response time accurate if directed properly at the tympanic membrane. Disadvantages:
can be inaccurate and give false low readings if the lens is not directed onto the tympanic membrane (poor user technique, unusual anatomy), or if there is a large amount of ear wax present, or debris on the lens.
easy to use cheap. Disadvantages:
slow response time (15e20 seconds) cannot usually display temperature to a resolution of <1 C. Infrared thermometers: all objects emit infrared radiation, with a wavelength and amplitude that depends on the temperature of the object. Pyroelectric crystals polarize temporarily when exposed to infrared radiation, generating a potential difference which is proportional to the temperature of the emitting object. An infrared tympanic thermometer (Figure 2) has a lens that can focus infrared radiation onto a sensor containing ceramic crystals with pyroelectric properties. The thermometer is inserted into the external auditory meatus so that the lens is exposed to the tympanic membrane, and the voltage generated across the crystals in response to the infrared radiation exposure is used to calculate the temperature. Alternatively, thermopiles can be used instead of ceramic crystals to convert thermal energy into electrical energy. The tympanic membrane has the same blood supply and similar location to the hypothalamus, so provides a good estimate of body core temperature.
Advantages:
accurate over a wide range of temperature can measure changes as small as 103 K. Disadvantages:
slow response time can be bulky some metals can corrode not as sensitive as thermistors.
Thermistor: (‘thermal resistor’) is a temperature-sensitive resistor made from a semiconductor metal oxide (e.g. oxide of manganese, cobalt, nickel, copper, iron or titanium). Resistance typically increases exponentially with decreasing temperature (negative temperature coefficient) (Figure 3); however, positive
The variation of resistance of a platinum wire and a thermistor with temperature
Thermistor
Resistance
Platinum wire
Temperature
Figure 2 A typical infrared tympanic membrane thermometer.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 16:7
Figure 3
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Ó 2015 Elsevier Ltd. All rights reserved.
PHYSICS
Thermocouples: the electrons in different metals have different energy levels. When two different metals are brought into contact, electrons move from a higher energy level to a lower one and a potential difference is generated which varies with temperature (the Seebeck effect). A thermocouple uses two such junctions between two dissimilar metals: a ‘measuring junction’ and a ‘reference junction’ that is held at a constant known temperature. The temperature at the measuring junction can be found by measuring the potential difference between the two junctions with a voltmeter (Figure 4). Typically the two metals used are iron and constantan (an alloy of copper and nickel), but other metals can be used (e.g. tungsten and rhenium). Depending on the metals used, temperatures between e200 C and 2000 C can be measured. Modern thermocouples include a cold junction compensator, which uses a thermistor to compensate the thermocouple output for any given reference junction temperature. Thermocouples are used in the food industry and for monitoring autoclave temperatures. Advantages:
fast response time can be made cheaply and small. Disadvantages:
low accuracy limiting use in clinical settings requires more complicated electronics and circuitry measured signal is very small and needs amplification. A
Figure 4
temperature coefficient thermistors also exist. A Wheatstone bridge is used to improve the accuracy of the measured resistance. Thermistors are small and can be used in tips of nasogastric temperature probes and pulmonary artery catheters. Some are very robust and can be steam sterilized and re-used.
FURTHER READING Middleton B, Phillips J, Thomas R, Stacey S. Physics in anaesthesia. Scion, 2012. Moseley M, Lynch J. The story of science: Power proof and passion. Mitchell Beazley, 2010. Robinson A. The story of measurement. Thames and Hudson, 2007. Sykes MK, Vickers MD, Hull CJ. Principles of measurement and monitoring ii anaesthesia and intensive care. Oxford: Blackwell Scientific, 1991.
Advantages:
fast response time highly sensitive and accurate can be made very small. Disadvantages:
non-linear change in resistance: needs a calibration equation output is prone to drift.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 16:7
362
Ó 2015 Elsevier Ltd. All rights reserved.
Measuring temperature
Learning objectives
E Byron Howells After reading this article, you should be able to: C define heat and temperature C describe fixed points and temperature scales in common use C explain the principles of action of thermometers in common use, with the particular advantages and limitations of each type
Abstract In the perioperative period patients tend to lose heat and become hypothermic. An understanding of the causes and prevention of heat loss is therefore important to the anaesthetist. Heat and temperature are measures of energy. Heat is a measure of the total kinetic energy (joules, J) of a body, and depends on the size of the body and its specific heat capacity. Temperature is a measure of the average kinetic energy, and describes the potential for heat transfer from a body at high temperature to one at lower temperature until both bodies reach equilibrium at the same temperature. Temperature is measured on a scale (e.g. Fahrenheit, Celsius, or Kelvin) which is defined by fixed points related to predictable physical events (e.g. the freezing point, steam point, and triple point of water).
Heat and temperature Heat is a measure of energy and therefore has the SI unit joule (J) (and non-SI unit calorie: 1 calorie ¼ 4.186 KJ). It is the quantity of thermal energy in a substance and represents the total kinetic energy of the molecules. The total amount of heat energy in a body depends on the mass of the body and its specific heat capacity. The heat capacity of a body is the heat required to produce a unit temperature rise, and has the SI unit joule per kelvin (JK1). Specific heat capacity is the heat capacity per unit mass of a substance, and has the SI unit joule per kelvin per kilogram (JK1Kg1). Temperature is the degree of hotness (or coldness) of a body. It is a measure of the average kinetic energy of the molecules within a substance, although individual molecules will have a wide range of kinetic energy. In a solid the molecules vibrate faster and with greater amplitude when heated. In liquids and gases where the molecules are free to move, they move with higher velocity when heated, resulting in more frequent collisions with each other and with the vessel walls. Temperature also represents the tendency of a substance to lose or gain heat relative to its surroundings. Heat tends to flow down a temperature gradient from a hot body towards a cold one, and given time this process will continue until they are both at the same temperature.
Keywords Fixed points; heat; temperature; thermometer; thermometric properties Royal College of Anaesthetists CPD matrix: 1A03
Introduction Humans are homeothermic, that is, they control core body temperature within a narrow range (36.8 0.4 C). When a patient undergoes surgery under anaesthesia they will inevitably lose heat due to vasodilatation, exposure of tissues, cold intravenous fluids and ventilation with dry cold gases. The mechanisms of heat loss are: convection, conduction, radiation and evaporation. It is important for the anaesthetist to make every effort to minimize heat loss as hypothermia has many adverse effects, including impairment of cardiac output, oxygen delivery, coagulation and drug metabolism. Further, shivering in the postoperative period increases pain and oxygen consumption. It is therefore very important to monitor patients’ body temperature to diagnose hypothermia or hyperthermia (due to, for example, fever or malignant hyperpyrexia). Core body temperature can be measured in the pulmonary artery, oesophagus, nasopharynx and tympanic membranes, while peripheral body temperature can be measured sublingually, in the axilla, rectum, bladder and on the forehead, and is usually within 0.5 C of core temperature. Heat loss can be minimized by: maintaining theatre ambient temperature >22 C use of blood/fluid warmers pre-warming intravenous fluids in warming cabinets forced air convection heaters/blankets warming mattresses hats for small children and babies.
Temperature scales In order to establish a temperature scale it is necessary to make use of fixed points. A fixed point is the single temperature at which a particular physical event always occurs, for example: The ice point: The temperature at which ice exists in equilibrium with water at standard pressure (101.3 kPa). This is designated 0 C or 32 F. The steam point: The temperature at which pure water exists in equilibrium with its vapour at standard pressure, that is, the temperature at which the saturated vapour pressure of pure water equals atmospheric pressure. This is designated 100 C or 212 F. The triple point of water: The temperature at which pure water, water vapour, and ice can exist in equilibrium. This is designated 0.01 C and can only occur at 611 Pa (0.006 atm.). The Fahrenheit scale was devised by German physicist Daniel Gabriel Fahrenheit in 1724. There is no particular logic to the scale and its evolution, and it has been revised several times. Fahrenheit defined the temperature of a mixture of ice, salt and sal ammoniac (ammonium chloride) as 0 F, the ice point as 32 F and the boiling point of water as 212 F; and divided the
E Byron Howells MB Bch (Wales) FRCA is a Consultant Anaesthetist at the Princess of Wales Hospital, Bridgend, UK. Conflict of interest: none.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 16:7
358
Ó 2015 Elsevier Ltd. All rights reserved.
PHYSICS
ice and boiling points into 180 divisions or ‘degrees’ ( ). His temperature scale was widely adopted because it was used in his accurate and commercially successful mercury thermometers. The Celsius scale was devised by Swedish astronomer Anders Celsius shortly after Fahrenheit’s death in 1736. Celsius divided the fixed points of the boiling and freezing points of water into 100 (hence until 1948 this was sometimes also called the ‘centigrade scale’). Celsius originally represented the boiling point as 0 C and the freezing point as 100 C; but this convention was reversed after his death by one of his students. It became known at the ‘Celsius scale’ from the early part of the 19th century, but was only officially recognized after the 9th Conference on Weights and Measurements in 1948. The Kelvin scale was proposed by Irish physicist William Thomson, First Lord Kelvin. It is also called the absolute or thermodynamic scale. His scale uses the fixed points of absolute zero and the triple point of water. Absolute zero is defined as the theoretical temperature at which the molecules in a substance have no kinetic energy. Absolute zero has been approached experimentally, but is impossible to achieve and therefore cannot be measured directly. The scale has the same intervals as the Celsius scale (1 C ¼ 1 K) with absolute zero representing 0 K and the triple point of water representing 273.16 K; therefore 1 K (kelvin) is defined as the fraction 1/273.16 of the triple point of water.* The International Temperature Scale of 1990 (ITS-90) is the current standard used to calibrate modern thermometers. ITS-90 ranges from 0.65 K to >1300 K and uses several fixed points including the triple points of water (273.16K), neon (24.56K), oxygen (54.36K), argon (83.8K) and mercury (234.3K); and the freezing points of tin (505.1K), aluminium (933.5K), gold (1337.3K) and copper (1357.8K). The relationship between the Kelvin, Celsius and Fahrenheit scales is shown (Table 1).
Comparison of the Kelvin, Celsius and Fahrenheit scales Temperature scale Celsius ( C)
Kelvin (K)
212 98.2 68
100 36.8 20
373.15 310 293
32 32 459.7
0.01 0 273.15
273.16 273.15 0
Steam point Body temperature Average room temperature Triple point of water Ice point Absolute zero
Table 1
Classification of thermometers Non-electrical C Touch C Liquid expansion thermometers (Mercury, Alcohol) C Gas thermometers C Bimetallic strip thermometers C Liquid crystal thermometers C Infrared thermometers Electrical C Resistance thermometers C Thermistors C Thermocouples Box 1
Thermometers and thermometric properties
expansion within a certain temperature range when heated. As the liquid warms it increases in volume and is forced up the capillary tube alongside a calibrated scale from which the temperature can be read. There is usually a constriction between the bulb and the capillary tube which delays the return of the liquid to the bulb on cooling, allowing the reader time to read the maximum temperature, after which the liquid may be returned to the bulb by shaking the thermometer. Liquid expansion thermometers are used to measure body temperature, usually from the oral, axillary and rectal regions. The effective range for ethanol is e100 C to 50 C (melting point e114 C, boiling point 78 C) and for mercury is e10 C to 350 C (melting point e39 C, boiling point 357 C); however, the range can be extended by adding gas or liquids such as toluene to the mercury. Accuracy depends on the diameter and length of the capillary tube. Mercury thermometers for clinical use normally have a scale range 35 Ce42 C.
A thermometer is a device for measuring the temperature of a substance or body. All thermometers rely on a thermometric property (i.e. a physical property that changes in a known, predictable, and reliable way with temperature). Ideally there should be a simple linear relationship between temperature and the thermometric property (e.g. the length of a mercury column), although some relationships are non-linear (e.g. thermistor). Based on their mechanism of action, thermometers can be classified as Non-electrical or Electrical (Box 1). Non-electrical thermometers Touch has the advantage of needing no equipment, but is extremely inaccurate and does not provide a quantitative measurement. Liquid expansion thermometers consist of a sealed glass capillary tube with a bulbous reservoir at one end which is filled with a liquid (usually mercury or alcohol) that undergoes linear
Advantages:
simple to use and read low cost accurate over body temperature range.
*
The SI unit Kelvin (K) should never be expressed or typeset as ‘degree kelvin’ or ‘ K’; in contrast to ‘degree Celsius ( C)’ and ‘degree Fahrenheit ( F)’.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 16:7
Fahrenheit ( F)
Disadvantages:
glass is fragile and can break, causing injury and toxicity if mercury used
359
Ó 2015 Elsevier Ltd. All rights reserved.
PHYSICS
Slow response time (2e3 minutes) limited range due to boiling and freezing point of alcohol and mercury respectively.
Disadvantages
large and bulky slow response time. Bimetallic strip: thermometers consist of a strip of two different metals (e.g. brass and invar e an alloy of nickel and iron) bound together, which have different coefficients of expansion. As temperature changes, differential expansion causes the metal strip to bend. Bimetallic strips may therefore be may be employed in thermostatic switches and as a temperature compensation mechanism in anaesthetic vaporizers. In a bimetallic strip thermometer the strip is usually in the form of a coil or helix with one end fixed and the other attached to a pointer (Figure 1). As the temperature increases, the coil unwinds (and vice versa), which causes the pointer to move over a scale. Common applications include room thermometers and cooking thermometers.
Gas thermometers: The Universal Gas Law states: PV ¼ nRT Where P is pressure (Pa), V is volume (m3), n is number of moles of a gas, R is the universal gas constant, and T is the absolute temperature (K). Therefore at a fixed volume the pressure of a gas is directly proportional to its absolute temperature (Charles’s Law); and at a fixed pressure the volume of a gas is directly proportional to its absolute temperature (Gay-Lussac’s Law). We can use the above laws to derive temperature using either pressure changes of a gas at fixed volume or volume changes of a gas at constant pressure as temperature changes. A Hydrogen thermometer consists of a syringe filled with hydrogen gas, which has a known thermal expansion coefficient, and expands along a calibrated scale as temperature increases. A Bourdon thermometer consists of a gas held in a bulb which is connected to a Bourdon gauge by a fine bore tube. The gas volume is fixed. As temperature increases the pressure in this closed system increases, causing the Bourdon gauge to uncoil, moving an attached pointer along a calibrated dial. For increased precision, the Bourdon gauge can be replaced by a manometer.
Advantages:
cheap robust operate up to high temperatures (600 C). Disadvantages:
limited accuracy (readings recalibration) slow response time.
may
drift,
requiring
Liquid crystal thermometers employ liquid crystals: substances that can flow like liquids, but also have a regular layered crystalline arrangement of molecules like solids. They are used in thermometers because their optical properties change with temperature. As temperature increases, the space between the layers increases, causing a change in the wavelength of light
Advantages:
robust sensitive and accurate over a wide range of temperatures (up to 600 C).
A bimetallic strip thermometer 40
50
60
30 Free end
70
20
80
10
90 0
Spiral wound element
100
Rotating shaft
Free end attached to pointer shaft
Fixed end
Fixed end
Figure 1
ANAESTHESIA AND INTENSIVE CARE MEDICINE 16:7
360
Ó 2015 Elsevier Ltd. All rights reserved.
PHYSICS
reflected from the crystals. This is visible as an apparent colour change. A liquid crystal thermometer for measurement of body temperature typically consists of a plastic strip containing cells of different liquid crystals that each change colour at different temperatures in the range 35 Ce40 C. The strip is placed on a person’s forehead, and the temperature is displayed by the colour change of the cells. Liquid crystal thermometers can also be used for measuring intravenous fluid, bath, and room temperatures.
Advantages:
Advantages:
Electrical thermometers Resistance thermometers: when metals are heated, the frequency and amplitude of vibration in their lattice structure increases, and impedes flow of electrons through the metal (i.e. electrical resistance increases). In a resistance thermometer, a fixed voltage is passed across a metal wire and the current flowing through the wire is measured. Using Ohm’s law (V ¼ IR) the resistance of the wire can be calculated and converted to temperature after calibration (Figure 3). Platinum is a particularly good choice of metal as it is inert, and its electrical resistance tends to vary linearly with a change of temperature over a wide temperature range (15 Ke900 K). Other metals (e.g. copper and nickel) can be used for different temperature ranges. A Wheatstone bridge is usually employed to measure the change in resistance to a high degree of accuracy.
fast response time accurate if directed properly at the tympanic membrane. Disadvantages:
can be inaccurate and give false low readings if the lens is not directed onto the tympanic membrane (poor user technique, unusual anatomy), or if there is a large amount of ear wax present, or debris on the lens.
easy to use cheap. Disadvantages:
slow response time (15e20 seconds) cannot usually display temperature to a resolution of <1 C. Infrared thermometers: all objects emit infrared radiation, with a wavelength and amplitude that depends on the temperature of the object. Pyroelectric crystals polarize temporarily when exposed to infrared radiation, generating a potential difference which is proportional to the temperature of the emitting object. An infrared tympanic thermometer (Figure 2) has a lens that can focus infrared radiation onto a sensor containing ceramic crystals with pyroelectric properties. The thermometer is inserted into the external auditory meatus so that the lens is exposed to the tympanic membrane, and the voltage generated across the crystals in response to the infrared radiation exposure is used to calculate the temperature. Alternatively, thermopiles can be used instead of ceramic crystals to convert thermal energy into electrical energy. The tympanic membrane has the same blood supply and similar location to the hypothalamus, so provides a good estimate of body core temperature.
Advantages:
accurate over a wide range of temperature can measure changes as small as 103 K. Disadvantages:
slow response time can be bulky some metals can corrode not as sensitive as thermistors.
Thermistor: (‘thermal resistor’) is a temperature-sensitive resistor made from a semiconductor metal oxide (e.g. oxide of manganese, cobalt, nickel, copper, iron or titanium). Resistance typically increases exponentially with decreasing temperature (negative temperature coefficient) (Figure 3); however, positive
The variation of resistance of a platinum wire and a thermistor with temperature
Thermistor
Resistance
Platinum wire
Temperature
Figure 2 A typical infrared tympanic membrane thermometer.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 16:7
Figure 3
361
Ó 2015 Elsevier Ltd. All rights reserved.
PHYSICS
Thermocouples: the electrons in different metals have different energy levels. When two different metals are brought into contact, electrons move from a higher energy level to a lower one and a potential difference is generated which varies with temperature (the Seebeck effect). A thermocouple uses two such junctions between two dissimilar metals: a ‘measuring junction’ and a ‘reference junction’ that is held at a constant known temperature. The temperature at the measuring junction can be found by measuring the potential difference between the two junctions with a voltmeter (Figure 4). Typically the two metals used are iron and constantan (an alloy of copper and nickel), but other metals can be used (e.g. tungsten and rhenium). Depending on the metals used, temperatures between e200 C and 2000 C can be measured. Modern thermocouples include a cold junction compensator, which uses a thermistor to compensate the thermocouple output for any given reference junction temperature. Thermocouples are used in the food industry and for monitoring autoclave temperatures. Advantages:
fast response time can be made cheaply and small. Disadvantages:
low accuracy limiting use in clinical settings requires more complicated electronics and circuitry measured signal is very small and needs amplification. A
Figure 4
temperature coefficient thermistors also exist. A Wheatstone bridge is used to improve the accuracy of the measured resistance. Thermistors are small and can be used in tips of nasogastric temperature probes and pulmonary artery catheters. Some are very robust and can be steam sterilized and re-used.
FURTHER READING Middleton B, Phillips J, Thomas R, Stacey S. Physics in anaesthesia. Scion, 2012. Moseley M, Lynch J. The story of science: Power proof and passion. Mitchell Beazley, 2010. Robinson A. The story of measurement. Thames and Hudson, 2007. Sykes MK, Vickers MD, Hull CJ. Principles of measurement and monitoring ii anaesthesia and intensive care. Oxford: Blackwell Scientific, 1991.
Advantages:
fast response time highly sensitive and accurate can be made very small. Disadvantages:
non-linear change in resistance: needs a calibration equation output is prone to drift.
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Ó 2015 Elsevier Ltd. All rights reserved.