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Measuring temperature
Physics
Measuring temperature
Learning objectives
Ian Dyer
After reading this article, you should be able to: • Define heat, temperature, the triple point and absolute zero • Classify thermometers into electrical and non-electrical types, and describe examples of each • Define core and peripheral temperature, and describe appropriate ways of measuring them.
Latha Srinivasa
Abstract
Absolute zero is the coldest absolute temperature, predicted from the ideal gas law (see below). Plotting volume against absolute temperature for an ideal gas yields a straight line. When extrapolated to zero volume, at which point the gas molecules have no kinetic energy, the line crosses the temperature axis at absolute zero, or −273.15 °C. During the phase change from solid to liquid, or liquid to gas, a substance gains heat energy without changing in temperature. The energy gained causes intermolecular bonds to break, thus increasing the potential energy of the molecules without an increase in kinetic energy. This potential energy is released when the phase change is reversed, which is why a burn from steam at 100 °C can be more severe than that from water at the same temperature. The triple point defines the unique temperature and pressure at which the above three phases of a pure substance coexist. The substance is enclosed within a sealed glass tube forming a triple point cell. Such cells are used as calibration standards since the triple point temperature can be reproduced to an accuracy of ± 0.00015 °C. The triple point of water is an important standard, occurring at 273.16 K and a partial vapour pressure of 611 pascals.
Heat and temperature are both measures of the energy possessed by molecules of a substance. Heat refers to total kinetic and potential en ergy; temperature refers to average kinetic energy. The coldest predicted temperature is absolute zero. The triple point defines the unique tem perature and pressure at which the solid, liquid and vapour phases of a pure substance coexist. The international temperature scale of 1990 attempts to standardize temperature measurement by defining various fixed points. Temperature may be measured mechanically by the expan sion of solids, liquids or gases. Electrical measurement methods include thermocouples, thermistors, semiconductors and resistance thermo meters. Liquid crystal and electromagnetic radiation thermometers are also used. Measuring body temperature is important as it tends to fall during anaesthesia. Core temperature measurements are more useful than peripheral temperature measurements. Active warming devices are often used to prevent perioperative hypothermia. For safety reasons the temperature of these should always be monitored.
Keywords heat; hypothermia; temperature; thermometer
The concepts of heat and temperature can best be explained at a molecular level. Molecules of any substance possess kinetic energy owing to their random vibration and potential energy due to intermolecular forces of attraction. Heat is a form of energy equal to the total kinetic and potential energies of all the molecules. For a given substance, the amount of heat energy it has depends on its mass, specific heat capacity and temperature. Temperature is a measure of the average kinetic energy of the molecules. There is a theoretical minimum temperature at which kinetic energy is zero. Temperature can also be regarded as the tendency of a substance to gain or lose heat relative to its surroundings. The direction of heat flow is governed by temperature, since for two objects in thermal contact heat will flow from the hotter to the colder one until both are at the same temperature.
Temperature scales Under the International System of Units, absolute temperature is measured in kelvin. Two fixed points are required to define measurement intervals, with absolute zero representing 0 K and the triple point of water representing 273.16 K. Thus, 1 K is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The International Temperature Scale of 1990 (ITS-90) attempts to standardize temperature measurement. It begins at 0.65 K and extends to the highest temperature practically measurable by means of electromagnetic radiation (see Infrared thermometry). Fixed points and their method of measurement, corresponding to the vapour pressure, triple point and freezing point of various elements, are specified. From 0.65 K to 5 K the vapour pressure of helium is used. From 3 K to 25 K gas thermometers are used. Platinum resistance thermometers are specified for the range 14–1235 K, and above this temperature a radiation thermometer is required. For everyday measurements the Celsius scale provides more convenient numbers. At sea level the freezing point of pure water corresponds to 0 °C and the boiling point 100 °C. In the USA the Fahrenheit scale is still used, with water freezing at 32 °F and boiling at 212 °F. Thermodynamic calculations usually require absolute temperatures in kelvin. This is because a proportional temperature change
Ian Dyer FRCA is a Consultant Anaesthetist at Princess of Wales Hospital, Bridgend. His interests include regional and orthopaedic anaesthesia and medical education. Conflicts of interest: none declared. Latha Srinivasa FRCA is a Specialist Registrar in Anaesthesia at Morriston Hospital, Swansea, with an interest in critical care. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:5
256
© 2009 Elsevier Ltd. All rights reserved.
Physics
on one scale is not equivalent to the same change on the other, although the Kelvin and Celsius scales have identical intervals.
Constant volume gas thermometer
Measurement devices
Bourdon gauge
Liquid in glass thermometers These work on the principle that the volume of most liquids increases with temperature. They consist of a sealed glass capillary tube, with a bulb-shaped reservoir at one end containing liquid. As the liquid expands it is forced up the capillary tube, indicating temperature on an attached scale. The liquid is usually ethanol (melting point −114 °C, boiling point 78 °C) or mercury (melting point −39 °C, boiling point 357 °C), as both remain liquid over a wide range of environmental temperatures. A mercury thermometer for clinical use has a scale from only 35 °C to 42 °C, allowing increased accuracy over this range. It also has a small constriction in the capillary tube near the bulb. This breaks the column of mercury when the temperature falls, preserving the maximum reading until the thermometer is reset by shaking. Mercury thermometers have a slow response time of 2–3 minutes owing to their high specific heat capacity. Mercury is also hazardous to health and the thermometers are fragile, so their use is rapidly declining.
Fine-bore tubing
Gas-filled bulb
Gas thermometers The ideal gas law states that:
Figure 1
PV = nRT
full-scale deflection. However, the reading drifts over time so periodic recalibration is needed.
where P is the pressure in pascals; V is the volume in m3; n is the number of moles of gas; R is the universal gas constant; and T is the temperature in kelvin. Using this law, temperature can be derived from either pressure changes at constant volume or volume changes at constant pressure. The simplest form of gas thermometer consists of a rigid gasfilled bulb (Figure 1). The gas pressure within the bulb is proportional to its absolute temperature. A bourdon gauge connected to the bulb measures the pressure; by using a suitable scale the gauge can indicate temperature instead. This type of gas thermometer is robust and operates up to 600 °C, making it suitable for industrial use. The accuracy is typically ± 1% of full-scale deflection but can be improved by increasing bulb size. For high-precision measurements the bourdon gauge is replaced by a manometer.
Thermocouples A thermocouple is an electrical junction formed from two dissimilar metals such as iron and constantan (a copper–nickel alloy). Depending on the metals used, temperatures between −200 °C
Bimetallic strip and thermometer
Metal Y
Bimetallic thermometers A bimetallic strip is formed from two metals with different expansion coefficients that are bonded together. Typically, brass and invar (an alloy of iron and nickel with a low expansion coefficient) are used. Differential thermal expansion of the metals causes the strip to bend. This feature may be used to activate microswitches in thermostats and for temperature compensation in some vaporizers. Dial thermometers use a long strip wound into a spiral or helix, which increases sensitivity without making the instrument too bulky. The spiral has one end fixed, the other being attached to a pointer. Temperature changes cause the pointer to rotate round a scale (Figure 2). Room thermometers and meat thermometers used during cooking are examples of this type. Bimetallic thermometers are robust, relatively inexpensive, operate at up to 600 °C and have an accuracy of ± 1% of
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:5
Metal X
Differential thermal expansion (arrows) causes strip to bend
Bimetallic strip Temperature scale
Figure 2
257
© 2009 Elsevier Ltd. All rights reserved.
Physics
and 2000 °C may be measured (Figure 3). Thermocouples make use of the Seebeck effect, in which a temperature difference between junctions will generate a voltage. The voltage increases nonlinearly with temperature and is only a few millivolts. This limits the accuracy of thermocouples to between ± 0.5 °C and ± 4 °C. As thermocouples measure the temperature difference between junctions, the reference junction must be kept at a constant temperature. Alternatively, the temperature of the reference junction can be measured by another device and a correction factor applied. Thermocouples have the advantages of rugged construction, low cost, small size and rapid response times. They are widely used in the food industry and to monitor autoclave temperatures. The low accuracy of thermocouples limits their use in clinical settings.
Resistance
Resistance–temperature curves for a platinum wire and a thermistor
Thermistors A thermistor is an electrical resistor whose resistance changes with temperature (Figure 4). Negative temperature coefficient thermistors show an exponential fall in resistance as temperature rises. They are formed from oxides of metals such as cobalt, manganese and nickel. Their small size gives them a rapid response time and allows incorporation into pulmonary artery catheters for measurement of blood and injectate temperatures. Other clinical uses include oesophageal temperature probes and digital thermometers. Some probes are re-useable, since thermistors can safely be steam sterilized. Thermistors have an operating range of −80 °C to 150 °C, are highly sensitive and have accuracies of up to ± 0.05 °C. Positive temperature coefficient types made from barium titanate are also available. However, the resistance of these increases acutely over a narrow temperature range and they are more commonly used as current-limiting devices.
NTC
Temperature NTC, negative temperature coefficient thermistor
Figure 4
of electronic components such as the central processing unit within a computer. Typical sensors have an operating range from −40 °C to 110 °C with a linear output voltage of 10 mV/°C. Platinum resistance thermometers The change in resistance of a metal wire can be used to measure temperature (Figure 4). If measurements are to be consistent then the metal must be pure and not degrade over time. Platinum is ideal as it is relatively inert and has an almost linear temperature response. Platinum resistance thermometers are used to define part of the International Temperature Scale; these have accuracies of ± 2 × 10−3 K but are too fragile for commercial use. Industrial platinum resistance thermometers are capable of measuring temperatures up to 1000 °C, with accuracies ranging from ± 0.01 °C to ± 0.2 °C. Copper or nickel wires are a less expensive alternative, but these metals are more susceptible to corrosion and oxidation so suitable only for temperatures up to about 300 °C.
Semiconductors A semiconductor sensor uses a transistor-based circuit incorporated within a silicon chip. It is often used to measure the temperature
A thermocouple Reference junction
Metal X
V
Liquid crystal thermometers Liquid crystals are substances that flow like a liquid but whose molecules have a regular layered crystalline arrangement. In thermochromic liquid crystals the spacing between layers is affected by temperature. This alters the wavelengths of light that are reflected. At certain temperatures, some liquid crystals reflect light in the visible spectrum, causing an apparent colour change. Above and below this temperature they appear transparent. Thermochromic liquid crystals are often made from esters of cholesterol, the colour change temperature being determined by the molecular composition. Liquid crystal thermometers are cheap to produce and often used as bath or room temperature monitors. Clinically, they are used to measure the temperature of skin and warmed intravenous fluids (Figure 5). Disadvantages are their slow response time of 15–20 seconds and accuracy of only ± 0.5 °C.
Metal Y
Measurement junction Figure 3
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:5
Platinum wire
258
© 2009 Elsevier Ltd. All rights reserved.
Physics
Figure 6 Thermogram showing right breast carcinoma. Reproduced, with permission, from Meditherm Inc (www.meditherm.com).
voltage proportional to their rate of temperature change. The voltage from either sensor is converted to a temperature reading for display. A dirty lens can cause temperature errors; however, ear wax has little effect as it is translucent to infrared. Tympanic thermometers are accurate to within 0.1 °C and have a response time of 1–2 seconds.
Relevance to anaesthetists
Figure 5 Liquid crystal thermometer clipped around an intravenous fluid line. The green segment indicates the fluid temperature.
Body temperature tends to fall perioperatively because most anaesthetic agents impair thermoregulation, lower metabolic rate and cause vasodilatation. Surgical exposure increases heat loss via convection, radiation and evaporation. A fall in body temperature impairs cardiac output and reduces oxygen delivery. Blood coagulation becomes less effective, drug metabolism is slowed and postoperative shivering increases pain and oxygen consumption. Consequently, it is important to monitor the temperature of the patient, the theatre environment and any active warming devices in use.
Infrared thermometry The thermal radiation emitted by an object is proportional to the fourth power of its absolute temperature and its thermal emissivity. Several methods of temperature measurement make use of this principle. A thermographic camera is sensitive to radiation in the infrared region of the spectrum, usually between 3 and 5 μm or between 7 and 14 μm. The image is displayed as a thermogram, which uses different colours to represent different surface temperatures and can be thought of as a ‘heat picture’ (Figure 6). Care is required when interpreting thermograms since objects with different thermal emissivity, such as wood and metal, may show as different colours even if they are at the same temperature. For accurate temperature readings the camera needs to be calibrated with an appropriate emissivity value. Temperatures between −50 °C and 2000 °C can be measured, with the advantage that direct contact with the object is not required. Medical uses of thermography include detection of malignancy and muscle injuries, which show up as hot spots. Thermography can assess heat loss from buildings and is also used by the emergency services to locate survivors. Tympanic thermometers are inserted into the auditory meatus and measure infrared emission from the tympanic membrane. Within the thermometer a lens focuses radiation onto a sensor, which may be either a thermopile or a pyroelectric crystal. A thermopile consists of between 10 and 100 thermocouples in series, magnifying the output voltage. Pyroelectric crystals generate a
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:5
Patient’s temperature Body temperature may be measured at various sites, representing core or peripheral temperature. The core comprises the brain and other internal organs and is usually maintained at an average temperature of 37 °C, although early morning temperature is around 0.5 °C lower than evening temperature. Core temperature may be measured in the pulmonary artery, oesophagus, nasopharynx or from the tympanic membrane. The oesophagus only provides accurate core temperatures in its distal portion as the proximal segment is cooled by inspired gases in the adjacent trachea. The nasopharynx is also cooled by inspired gases unless the patient is breathing through an orotracheal tube. Sublingual, axillary, rectal and bladder temperatures are usually within 0.5 °C of core temperature. However, rectal and bladder temperatures are slow to equilibrate and will not be representative of core temperature during rapid changes, such as rewarming following hypothermic cardiopulmonary bypass. In addition, rectal temperature measurement carries the risk of bowel perforation. Core temperature is used perioperatively as a guide to patient warming. It is also used to confirm the diagnosis of malignant hyperpyrexia 259
© 2009 Elsevier Ltd. All rights reserved.
Physics
and monitor the effectiveness of treatment. The Association of Anaesthetists states that temperature monitoring must be available throughout each anaesthetic and in the recovery area. More recent guidelines from the National Institute for Health and Clinical Excellence suggest that the patient’s temperature must be measured and documented before induction of anaesthesia and then every 30 minutes until the end of surgery. They also state that, unless there is clinical urgency, induction of anaesthesia should not begin unless the patient’s temperature is 36.0 °C or above. The peripheral temperature is typically 2–4 °C below core and is taken as the skin temperature in the arms or legs. It is subject to wide variation and is used only as an indication of peripheral perfusion.
electrode increases with temperature. Consequently, oxygen analysers usually incorporate a thermistor to compensate for temperature changes and maintain accuracy.
Theatre environment Operating theatre temperature is commonly measured using a bimetallic thermometer, thermistor or liquid crystal thermometer. To strike a balance between minimizing heat loss from the patient and comfort of the staff, current recommendations are for a theatre temperature of 21–23 °C. This may be increased for neonates as they have a higher thermoneutral temperature. Ambient temperature affects the reading from some monitoring devices. For example, the reading from a fuel cell oxygen
Further reading Childs PRN. Practical temperature measurement. Oxford: ButterworthHeinemann, 2001. Davis PD, Kenny GNC. Basic physics and measurement in anaesthesia, 5th edn. London: Butterworth-Heinemann, 2003. National Institute for Health and Clinical Excellence. NICE clinical guideline 65: inadvertent perioperative hypothermia. London: NICE publications, 2008 Also available at: http://www.nice.org. uk/nicemedia/pdf/CG65Guidance.pdf (accessed 27 Nov 2008).
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:5
Warming devices Heated mattresses, warm-air blowers and intravenous fluid warmers help to maintain normothermia. Each device should include a temperature sensor to indicate safe operation. A thermal cut-out should prevent further use if the device overheats. Hyperthermia is a risk when using active warming devices, so the patient’s temperature must always be monitored. ◆
260
© 2009 Elsevier Ltd. All rights reserved.
Measuring temperature
Learning objectives
Ian Dyer
After reading this article, you should be able to: • Define heat, temperature, the triple point and absolute zero • Classify thermometers into electrical and non-electrical types, and describe examples of each • Define core and peripheral temperature, and describe appropriate ways of measuring them.
Latha Srinivasa
Abstract
Absolute zero is the coldest absolute temperature, predicted from the ideal gas law (see below). Plotting volume against absolute temperature for an ideal gas yields a straight line. When extrapolated to zero volume, at which point the gas molecules have no kinetic energy, the line crosses the temperature axis at absolute zero, or −273.15 °C. During the phase change from solid to liquid, or liquid to gas, a substance gains heat energy without changing in temperature. The energy gained causes intermolecular bonds to break, thus increasing the potential energy of the molecules without an increase in kinetic energy. This potential energy is released when the phase change is reversed, which is why a burn from steam at 100 °C can be more severe than that from water at the same temperature. The triple point defines the unique temperature and pressure at which the above three phases of a pure substance coexist. The substance is enclosed within a sealed glass tube forming a triple point cell. Such cells are used as calibration standards since the triple point temperature can be reproduced to an accuracy of ± 0.00015 °C. The triple point of water is an important standard, occurring at 273.16 K and a partial vapour pressure of 611 pascals.
Heat and temperature are both measures of the energy possessed by molecules of a substance. Heat refers to total kinetic and potential en ergy; temperature refers to average kinetic energy. The coldest predicted temperature is absolute zero. The triple point defines the unique tem perature and pressure at which the solid, liquid and vapour phases of a pure substance coexist. The international temperature scale of 1990 attempts to standardize temperature measurement by defining various fixed points. Temperature may be measured mechanically by the expan sion of solids, liquids or gases. Electrical measurement methods include thermocouples, thermistors, semiconductors and resistance thermo meters. Liquid crystal and electromagnetic radiation thermometers are also used. Measuring body temperature is important as it tends to fall during anaesthesia. Core temperature measurements are more useful than peripheral temperature measurements. Active warming devices are often used to prevent perioperative hypothermia. For safety reasons the temperature of these should always be monitored.
Keywords heat; hypothermia; temperature; thermometer
The concepts of heat and temperature can best be explained at a molecular level. Molecules of any substance possess kinetic energy owing to their random vibration and potential energy due to intermolecular forces of attraction. Heat is a form of energy equal to the total kinetic and potential energies of all the molecules. For a given substance, the amount of heat energy it has depends on its mass, specific heat capacity and temperature. Temperature is a measure of the average kinetic energy of the molecules. There is a theoretical minimum temperature at which kinetic energy is zero. Temperature can also be regarded as the tendency of a substance to gain or lose heat relative to its surroundings. The direction of heat flow is governed by temperature, since for two objects in thermal contact heat will flow from the hotter to the colder one until both are at the same temperature.
Temperature scales Under the International System of Units, absolute temperature is measured in kelvin. Two fixed points are required to define measurement intervals, with absolute zero representing 0 K and the triple point of water representing 273.16 K. Thus, 1 K is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The International Temperature Scale of 1990 (ITS-90) attempts to standardize temperature measurement. It begins at 0.65 K and extends to the highest temperature practically measurable by means of electromagnetic radiation (see Infrared thermometry). Fixed points and their method of measurement, corresponding to the vapour pressure, triple point and freezing point of various elements, are specified. From 0.65 K to 5 K the vapour pressure of helium is used. From 3 K to 25 K gas thermometers are used. Platinum resistance thermometers are specified for the range 14–1235 K, and above this temperature a radiation thermometer is required. For everyday measurements the Celsius scale provides more convenient numbers. At sea level the freezing point of pure water corresponds to 0 °C and the boiling point 100 °C. In the USA the Fahrenheit scale is still used, with water freezing at 32 °F and boiling at 212 °F. Thermodynamic calculations usually require absolute temperatures in kelvin. This is because a proportional temperature change
Ian Dyer FRCA is a Consultant Anaesthetist at Princess of Wales Hospital, Bridgend. His interests include regional and orthopaedic anaesthesia and medical education. Conflicts of interest: none declared. Latha Srinivasa FRCA is a Specialist Registrar in Anaesthesia at Morriston Hospital, Swansea, with an interest in critical care. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:5
256
© 2009 Elsevier Ltd. All rights reserved.
Physics
on one scale is not equivalent to the same change on the other, although the Kelvin and Celsius scales have identical intervals.
Constant volume gas thermometer
Measurement devices
Bourdon gauge
Liquid in glass thermometers These work on the principle that the volume of most liquids increases with temperature. They consist of a sealed glass capillary tube, with a bulb-shaped reservoir at one end containing liquid. As the liquid expands it is forced up the capillary tube, indicating temperature on an attached scale. The liquid is usually ethanol (melting point −114 °C, boiling point 78 °C) or mercury (melting point −39 °C, boiling point 357 °C), as both remain liquid over a wide range of environmental temperatures. A mercury thermometer for clinical use has a scale from only 35 °C to 42 °C, allowing increased accuracy over this range. It also has a small constriction in the capillary tube near the bulb. This breaks the column of mercury when the temperature falls, preserving the maximum reading until the thermometer is reset by shaking. Mercury thermometers have a slow response time of 2–3 minutes owing to their high specific heat capacity. Mercury is also hazardous to health and the thermometers are fragile, so their use is rapidly declining.
Fine-bore tubing
Gas-filled bulb
Gas thermometers The ideal gas law states that:
Figure 1
PV = nRT
full-scale deflection. However, the reading drifts over time so periodic recalibration is needed.
where P is the pressure in pascals; V is the volume in m3; n is the number of moles of gas; R is the universal gas constant; and T is the temperature in kelvin. Using this law, temperature can be derived from either pressure changes at constant volume or volume changes at constant pressure. The simplest form of gas thermometer consists of a rigid gasfilled bulb (Figure 1). The gas pressure within the bulb is proportional to its absolute temperature. A bourdon gauge connected to the bulb measures the pressure; by using a suitable scale the gauge can indicate temperature instead. This type of gas thermometer is robust and operates up to 600 °C, making it suitable for industrial use. The accuracy is typically ± 1% of full-scale deflection but can be improved by increasing bulb size. For high-precision measurements the bourdon gauge is replaced by a manometer.
Thermocouples A thermocouple is an electrical junction formed from two dissimilar metals such as iron and constantan (a copper–nickel alloy). Depending on the metals used, temperatures between −200 °C
Bimetallic strip and thermometer
Metal Y
Bimetallic thermometers A bimetallic strip is formed from two metals with different expansion coefficients that are bonded together. Typically, brass and invar (an alloy of iron and nickel with a low expansion coefficient) are used. Differential thermal expansion of the metals causes the strip to bend. This feature may be used to activate microswitches in thermostats and for temperature compensation in some vaporizers. Dial thermometers use a long strip wound into a spiral or helix, which increases sensitivity without making the instrument too bulky. The spiral has one end fixed, the other being attached to a pointer. Temperature changes cause the pointer to rotate round a scale (Figure 2). Room thermometers and meat thermometers used during cooking are examples of this type. Bimetallic thermometers are robust, relatively inexpensive, operate at up to 600 °C and have an accuracy of ± 1% of
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:5
Metal X
Differential thermal expansion (arrows) causes strip to bend
Bimetallic strip Temperature scale
Figure 2
257
© 2009 Elsevier Ltd. All rights reserved.
Physics
and 2000 °C may be measured (Figure 3). Thermocouples make use of the Seebeck effect, in which a temperature difference between junctions will generate a voltage. The voltage increases nonlinearly with temperature and is only a few millivolts. This limits the accuracy of thermocouples to between ± 0.5 °C and ± 4 °C. As thermocouples measure the temperature difference between junctions, the reference junction must be kept at a constant temperature. Alternatively, the temperature of the reference junction can be measured by another device and a correction factor applied. Thermocouples have the advantages of rugged construction, low cost, small size and rapid response times. They are widely used in the food industry and to monitor autoclave temperatures. The low accuracy of thermocouples limits their use in clinical settings.
Resistance
Resistance–temperature curves for a platinum wire and a thermistor
Thermistors A thermistor is an electrical resistor whose resistance changes with temperature (Figure 4). Negative temperature coefficient thermistors show an exponential fall in resistance as temperature rises. They are formed from oxides of metals such as cobalt, manganese and nickel. Their small size gives them a rapid response time and allows incorporation into pulmonary artery catheters for measurement of blood and injectate temperatures. Other clinical uses include oesophageal temperature probes and digital thermometers. Some probes are re-useable, since thermistors can safely be steam sterilized. Thermistors have an operating range of −80 °C to 150 °C, are highly sensitive and have accuracies of up to ± 0.05 °C. Positive temperature coefficient types made from barium titanate are also available. However, the resistance of these increases acutely over a narrow temperature range and they are more commonly used as current-limiting devices.
NTC
Temperature NTC, negative temperature coefficient thermistor
Figure 4
of electronic components such as the central processing unit within a computer. Typical sensors have an operating range from −40 °C to 110 °C with a linear output voltage of 10 mV/°C. Platinum resistance thermometers The change in resistance of a metal wire can be used to measure temperature (Figure 4). If measurements are to be consistent then the metal must be pure and not degrade over time. Platinum is ideal as it is relatively inert and has an almost linear temperature response. Platinum resistance thermometers are used to define part of the International Temperature Scale; these have accuracies of ± 2 × 10−3 K but are too fragile for commercial use. Industrial platinum resistance thermometers are capable of measuring temperatures up to 1000 °C, with accuracies ranging from ± 0.01 °C to ± 0.2 °C. Copper or nickel wires are a less expensive alternative, but these metals are more susceptible to corrosion and oxidation so suitable only for temperatures up to about 300 °C.
Semiconductors A semiconductor sensor uses a transistor-based circuit incorporated within a silicon chip. It is often used to measure the temperature
A thermocouple Reference junction
Metal X
V
Liquid crystal thermometers Liquid crystals are substances that flow like a liquid but whose molecules have a regular layered crystalline arrangement. In thermochromic liquid crystals the spacing between layers is affected by temperature. This alters the wavelengths of light that are reflected. At certain temperatures, some liquid crystals reflect light in the visible spectrum, causing an apparent colour change. Above and below this temperature they appear transparent. Thermochromic liquid crystals are often made from esters of cholesterol, the colour change temperature being determined by the molecular composition. Liquid crystal thermometers are cheap to produce and often used as bath or room temperature monitors. Clinically, they are used to measure the temperature of skin and warmed intravenous fluids (Figure 5). Disadvantages are their slow response time of 15–20 seconds and accuracy of only ± 0.5 °C.
Metal Y
Measurement junction Figure 3
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:5
Platinum wire
258
© 2009 Elsevier Ltd. All rights reserved.
Physics
Figure 6 Thermogram showing right breast carcinoma. Reproduced, with permission, from Meditherm Inc (www.meditherm.com).
voltage proportional to their rate of temperature change. The voltage from either sensor is converted to a temperature reading for display. A dirty lens can cause temperature errors; however, ear wax has little effect as it is translucent to infrared. Tympanic thermometers are accurate to within 0.1 °C and have a response time of 1–2 seconds.
Relevance to anaesthetists
Figure 5 Liquid crystal thermometer clipped around an intravenous fluid line. The green segment indicates the fluid temperature.
Body temperature tends to fall perioperatively because most anaesthetic agents impair thermoregulation, lower metabolic rate and cause vasodilatation. Surgical exposure increases heat loss via convection, radiation and evaporation. A fall in body temperature impairs cardiac output and reduces oxygen delivery. Blood coagulation becomes less effective, drug metabolism is slowed and postoperative shivering increases pain and oxygen consumption. Consequently, it is important to monitor the temperature of the patient, the theatre environment and any active warming devices in use.
Infrared thermometry The thermal radiation emitted by an object is proportional to the fourth power of its absolute temperature and its thermal emissivity. Several methods of temperature measurement make use of this principle. A thermographic camera is sensitive to radiation in the infrared region of the spectrum, usually between 3 and 5 μm or between 7 and 14 μm. The image is displayed as a thermogram, which uses different colours to represent different surface temperatures and can be thought of as a ‘heat picture’ (Figure 6). Care is required when interpreting thermograms since objects with different thermal emissivity, such as wood and metal, may show as different colours even if they are at the same temperature. For accurate temperature readings the camera needs to be calibrated with an appropriate emissivity value. Temperatures between −50 °C and 2000 °C can be measured, with the advantage that direct contact with the object is not required. Medical uses of thermography include detection of malignancy and muscle injuries, which show up as hot spots. Thermography can assess heat loss from buildings and is also used by the emergency services to locate survivors. Tympanic thermometers are inserted into the auditory meatus and measure infrared emission from the tympanic membrane. Within the thermometer a lens focuses radiation onto a sensor, which may be either a thermopile or a pyroelectric crystal. A thermopile consists of between 10 and 100 thermocouples in series, magnifying the output voltage. Pyroelectric crystals generate a
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:5
Patient’s temperature Body temperature may be measured at various sites, representing core or peripheral temperature. The core comprises the brain and other internal organs and is usually maintained at an average temperature of 37 °C, although early morning temperature is around 0.5 °C lower than evening temperature. Core temperature may be measured in the pulmonary artery, oesophagus, nasopharynx or from the tympanic membrane. The oesophagus only provides accurate core temperatures in its distal portion as the proximal segment is cooled by inspired gases in the adjacent trachea. The nasopharynx is also cooled by inspired gases unless the patient is breathing through an orotracheal tube. Sublingual, axillary, rectal and bladder temperatures are usually within 0.5 °C of core temperature. However, rectal and bladder temperatures are slow to equilibrate and will not be representative of core temperature during rapid changes, such as rewarming following hypothermic cardiopulmonary bypass. In addition, rectal temperature measurement carries the risk of bowel perforation. Core temperature is used perioperatively as a guide to patient warming. It is also used to confirm the diagnosis of malignant hyperpyrexia 259
© 2009 Elsevier Ltd. All rights reserved.
Physics
and monitor the effectiveness of treatment. The Association of Anaesthetists states that temperature monitoring must be available throughout each anaesthetic and in the recovery area. More recent guidelines from the National Institute for Health and Clinical Excellence suggest that the patient’s temperature must be measured and documented before induction of anaesthesia and then every 30 minutes until the end of surgery. They also state that, unless there is clinical urgency, induction of anaesthesia should not begin unless the patient’s temperature is 36.0 °C or above. The peripheral temperature is typically 2–4 °C below core and is taken as the skin temperature in the arms or legs. It is subject to wide variation and is used only as an indication of peripheral perfusion.
electrode increases with temperature. Consequently, oxygen analysers usually incorporate a thermistor to compensate for temperature changes and maintain accuracy.
Theatre environment Operating theatre temperature is commonly measured using a bimetallic thermometer, thermistor or liquid crystal thermometer. To strike a balance between minimizing heat loss from the patient and comfort of the staff, current recommendations are for a theatre temperature of 21–23 °C. This may be increased for neonates as they have a higher thermoneutral temperature. Ambient temperature affects the reading from some monitoring devices. For example, the reading from a fuel cell oxygen
Further reading Childs PRN. Practical temperature measurement. Oxford: ButterworthHeinemann, 2001. Davis PD, Kenny GNC. Basic physics and measurement in anaesthesia, 5th edn. London: Butterworth-Heinemann, 2003. National Institute for Health and Clinical Excellence. NICE clinical guideline 65: inadvertent perioperative hypothermia. London: NICE publications, 2008 Also available at: http://www.nice.org. uk/nicemedia/pdf/CG65Guidance.pdf (accessed 27 Nov 2008).
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:5
Warming devices Heated mattresses, warm-air blowers and intravenous fluid warmers help to maintain normothermia. Each device should include a temperature sensor to indicate safe operation. A thermal cut-out should prevent further use if the device overheats. Hyperthermia is a risk when using active warming devices, so the patient’s temperature must always be monitored. ◆
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