EBME & Clinical Engineering Articles

Temperature measurement - Thermometers

A thermometer is a device that measures temperature or temperature gradient

 The word thermometer is derived from two smaller word fragments: thermo from the Greek for heat and meter from Greek, meaning to measure. A thermometer has two important elements, the temperature sensor (e.g. the bulb on a mercury thermometer) in which some physical change occurs with temperature, plus some means of converting this physical change into a value (e.g. the scale on a mercury thermometer).

Thermometers can be divided into two groups according to the level of knowledge about the physical basis of the underlying thermodynamic laws and quantities.

1. Primary thermometers; the measured property of matter is known so well that temperature can be calculated without any unknown quantities. Examples of these are thermometers based on the equation of state of a gas, on the velocity of sound in a gas, on the thermal noise, voltage or current of an electrical resistor, and on the angular anisotropy of gamma ray emission of certain radioactive nuclei in a magnetic field.
2. Secondary thermometers are most widely used because of their convenience. Also, they are often much more sensitive than primary ones. For secondary thermometers knowledge of the measured property is not sufficient to allow direct calculation of temperature. They have to be calibrated against a primary thermometer at least at one temperature or at a number of fixed temperatures. Such fixed points, for example, triple points and superconducting transitions, occur reproducibly at the same temperature.

Internationally agreed temperature scales are based on fixed points and interpolating thermometers. The most recent official temperature scale is the International Temperature Scale of 1990. It extends from 0.65 K to approximately 1358 K (-272.5 °C to 1085 °C).

Thermometry timeline.

• 1592 - Galileo Galilei builds a crude thermometer using the contraction of air to draw water up a tube
• 1612 - Santorio Sanctorius puts thermometer to medical use
• 1629 - Joseph Solomon Delmedigo describes in a book an accurate sealed-glass thermometer which uses brandy
• 1643 - Evangelista Torricelli invents the mercury barometer
• 1714 - Daniel Gabriel Fahrenheit invents the mercury-in-glass thermometer
• 1821 - Thomas Johann Seebeck invents the thermocouple
• 1864 - Henri Becquerel suggests an optical pyrometer
• 1885 - Calender-Van Duesen invented the platinum resistance temperature device
• 1892 - Henri-Louis Le Châtelier builds the first optical pyrometer

A basal thermometer

is an ultra-sensitive thermometer used to take the basal [base] body temperature, the lowest normal body temperature. This temperature typically occurs during sleep, so the basal temperature is normally measured immediately upon waking. Whereas ordinary household thermometers express body temperature to only one decimal place (e.g. 98.6 degrees Fahrenheit), basal thermometers express body temperature to two decimals places (e.g. 98.56 degrees Fahrenheit). Basal thermometers are useful for charting a woman's fertility cycle, since basal body temperature typically rises when ovulation occurs. This information can be used to help maximize or minimize the chance of pregnancy. Basal thermometers are used as part of the Sympto-Thermal method of Natural family planning.

A bi-metallic strip is used to convert temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper. The strips are joined together throughout their length by rivets, by brazing or by welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its normal temperature. The metal with the higher expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled. The sideways displacement of the strip is much larger than the small lengthways expansion in either of the two metals. This effect is used in a range of mechanical and electrical devices. In some applications the bi-metal strip is used in the flat form. In others, it is wrapped into a coil for compactness. The greater length of the coiled version gives improved sensitivity.

A Galileo thermometer or thermoscope is a thermometer made of a sealed glass cylinder containing a clear liquid. Suspended in the liquid are a number of weights. Commonly those weights are themselves sealed glass bulbs containing coloured liquid for an attractive effect. As the liquid in the cylinder changes temperature its density changes and those bulbs which are free to move, rise or fall to reach a position where their density is either equal to that of the surrounding liquid or where they are brought to a halt by other bulbs. If the bulbs differ in density by a very small amount and are ordered such that the least dense is at the top and densest at the bottom, they can form a temperature scale.

The temperature is typically read from an engraved metal disc on each bulb. Usually a gap would separate the top bulbs from the bottom bulbs and then the temperature would be between the tag readings on either side of the gap. If a bulb is free-floating in the gap, then its tag reading would be closest to the ambient temperature. To achieve this requires manufacturing the weights to a tolerance of less than 1/1000 of a gram (1 milligram).

A gas thermometer measures temperature by the variation in volume or pressure of a gas. One common apparatus is a constant volume thermometer. It consists of a bulb connected by a capillary tube to a manometer. The bulb is filled with a gas and in thermal contact with the body whose temperature is to be measured. A reservoir of mercury is raised or lowered such that the volume of the gas in the bulb remains constant. The pressure of the gas in the bulb can be obtained by measuring the level difference in the two arms of the manometer. Gas thermometers are often used to calibrate other thermometers.

Infrared thermometers

measure temperature using blackbody radiation (generally infrared) emitted from objects. They are sometimes called laser thermometers if a laser is used to help aim the thermometer, or non-contact thermometers to describe the device's ability to measure temperature from a distance. By knowing the amount of infrared energy emitted by the object and its emissivity, the object's temperature can be determined. The most basic design consists of a lens to focus the infrared energy on to a detector, which converts the energy to an electrical signal that can be displayed in units of temperature after being compensated for ambient temperature variation. This configuration facilitates temperature measurement from a distance without contact with the object to be measured. As such, the infrared thermometer is useful for measuring temperature under circumstances where thermocouples or other probe type sensors cannot be used or do not produce accurate data for a variety of reasons. Some typical circumstances are where the object to be measured is moving; where the object is surrounded by an electromagnetic field, as in induction heating; where the object is contained in a vacuum or other controlled atmosphere; or in applications where a fast response is required.
Infrared thermometers can be used to serve a wide variety of temperature monitoring functions that include:

• Detecting clouds for remote telescope operation
• Checking mechanical equipment or electrical circuit breaker boxes or outlets for hot spots
• Checking heater or oven temperature, for calibration and control purposes
• Detecting hot spots / performing diagnostics in electrical circuit board manufacturing
• Checking for hot spots in fire fighting situations
• Monitoring materials in process of heating and cooling, for research and development or manufacturing quality control situations

There are many varieties of infrared temperature sensing devices available today, including configurations designed for flexible and portable handheld use, as well many designed for mounting in a fixed position to serve a dedicated purpose for long periods. Specifications of portable handheld sensors available to the home user will include ratings of temperature accuracy (usually plus or minus a degree or two) and other parameters. The distance-to-spot ratio (D:S) is the ratio of the distance to the object and the diameter of the temperature measurement area. For instance if the D:S ratio is 12:1, measurement of an object 12 inches away will average the temperature over a 1-inch diameter area. The sensor may have an adjustable emissivity setting, which can be set to measure the temperature of reflective (shiny) and non-reflective surfaces. A non-adjustable thermometer can be used to measure the temperature of a shiny surface by applying a non-shiny paint or tape to the surface. The most usual infrared thermometer is the Spot Infrared Thermometer or Infrared Pyrometer, which measures the temperature at a spot on a surface.

A liquid crystal thermometer or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change colour to indicate different temperatures. Liquid crystals possess the mechanical properties of a liquid, but have the optical properties of a single crystal. Temperature changes can affect the colour of a liquid crystal, which makes them useful for temperature measurement. The resolution of liquid crystal sensors is in the 0.1°C range. Disposable liquid crystal thermometers have been developed for home and medical use. Liquid crystal thermometers portray temperatures as colours and can be used to follow temperature changes caused by heat flow. They can be used to observe that heat flows by conduction, convection, and radiation.

Liquid crystals are very often used when someone is ill and other medical reasons because they can read body temperature by being placed against someone's forehead and are safer than a mercury-in-glass thermometer. Unfortunately they don't always give an exact result especially the smaller ones.

A mercury-in-glass thermometer, invented by German physicist Daniel Gabriel Fahrenheit, is a thermometer consisting of mercury in a glass tube. Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the tube, which varies according to the temperature. To increase the sensitivity, there is usually a bulb of mercury at the end of the thermometer which contains most of the mercury; expansion and contraction of this volume of mercury is then amplified in the much narrower bore of the tube. The space above the mercury may be filled with nitrogen or it may be a vacuum.

Phosphor thermometry is an optical method for surface temperature measurement. The method exploits luminescence emitted by phosphor material. Phosphors are fine white or pastel-coloured inorganic powders which may be stimulated by any of a variety of means to luminesce, i.e. emit light. Certain characteristics of the emitted light change with temperature, including brightness, colour, and afterglow duration. The latter is most commonly used for temperature measurement.

Resistance thermometers, also called resistance temperature detectors (RTDs), are temperature sensors that exploit the predictable change in electrical resistance of some materials with changing temperature. As they are almost invariably made of platinum, they are often called platinum resistance thermometers (PRTs). They are slowly replacing the use of thermocouples in many industrial applications below 600 °C. A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. Thermistor is a combination of the words thermal and resistor. The Thermistor was invented by Samuel Ruben in 1930.

The silicon bandgap temperature sensor is an extremely common form of temperature sensor (thermometer) used in electronic equipment. Its main advantage is that it can be included in a silicon integrated circuit at very low cost. The principle of the sensor is that the forward voltage of a silicon diode is temperature-dependent.

Thermocouples: In 1821, the German-Estonian physicist Thomas Johann Seebeck discovered that when any conductor (such as a metal) is subjected to a thermal gradient, it will generate a voltage. This is now known as the thermoelectric effect or Seebeck effect. In electronics, thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal potential difference into electric potential difference. They are cheap and interchangeable, have standard connectors, and can measure a wide range of temperatures. The main limitation is precision; system errors of less than 1 °C can be difficult to achieve.

Medical thermometers were traditionally mercury-in-glass thermometers used for measuring human body temperature, with the tip of the thermometer being inserted either into the mouth (oral temperature), under the armpit (axillary temperature), or into the rectum via the anus (rectal temperature). The human body has the remarkable capacity for regulating its core temperature somewhere between 98°F and 100°F when the ambient temperature is between approximately 68°F and 130°F according to Guyton. This presumes a nude body and dry air.

The external heat transfer mechanisms are radiation, conduction and convection and evaporation of perspiration. The body takes a very active role in temperature regulation.

The temperature of the body is regulated by neural feedback mechanisms which operate primarily through the hypothalamus in the brain. The hypothalamus contains not only the control mechanisms, but also the key temperature sensors. Under control of these mechanisms, sweating begins almost precisely at a skin temperature of 37°C and increases rapidly as the skin temperature rises above this value. The heat production of the body under these conditions remains almost constant as the skin temperature rises. If the skin temperature drops below 37°C a variety of responses are initiated to conserve the heat in the body and to increase heat production.

These include:

• Vasoconstriction to decrease the flow of heat to the skin.
• Cessation of sweating.
• Shivering to increase heat production in the muscles.
• Secretion of norepinephrine, epinephrine, and thyroxine to increase heat production
• In lower animals, the erection of the hairs and fur to increase insulation.

The normal core body temperature of a healthy, resting adult human being is stated to be at 98.6 degrees Fahrenheit or 37.0 degrees Celsius. Though the body temperature measured on an individual can vary, a healthy human body can maintain a fairly consistent body temperature that is around the mark of 37.0 degrees Celsius.

The normal range of human body temperature varies due to an individual's metabolism rate, the higher (faster) it is the higher the normal body temperature or the slower the metabolic rate the lower the normal body temperature. Other factors that might affect the body temperature of an individual may be the time of day or the part of the body in which the temperature is measured at. The body temperature is lower in the morning, due to the rest the body received, and higher at night after a day of muscular activity and after food intake. Body temperature also varies at different parts of the body. Oral temperatures, which are the most convenient type of temperature measurement, are at 37.0 °C. This is the accepted standard temperature for the normal core body temperature.

Maxillary temperatures are an external measurement taken in the armpit or between two folds of skin on the body. This is the longest and most inaccurate way of measuring body temperature, the normal temperature falls at 97.6 °F or 36.4 °C. Rectal temperatures are an internal measurement taken in the rectum, which fall at 99.6 °F or 37.6 °C. It is the least time consuming and most accurate type of body temperature measurement, being an internal measurement. But it is not the most comfortable method to measure the body temperature of an individual.

The traditional mercury-filled medical thermometer works in the same way as a meteorological maximum thermometer. The thermometer consists of a bulb containing mercury attached to a small tube. As the temperature rises, the mercury expands and flows up the tube. The temperature is obtained by reading the scale inscribed on the side of the thermometer. There is a constriction in the neck close to the bulb. As the temperature rises, the mercury is forced up through the constriction by the force of the expansion. When the temperature falls, the column of mercury breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. To reset the thermometer, it must be swung sharply. The break in the column of mercury is visible.

In the 1990s, mercury thermometers were found too risky to handle and have largely been replaced with electronic thermometers, or, more rarely, thermometers based on liquids other than mercury. In some places, it may be illegal to sell products which contain mercury, such as thermometers. Both kinds may be used orally, axillary, or rectally.

Oral

Oral temperature may only be taken from a patient who is capable of holding the thermometer in their mouth correctly and securely, which generally excludes small children or people who are overcome by coughing or vomiting. (This is less of a problem with fast-reacting digital thermometers, but was certainly an issue with mercury thermometers, which took several minutes to register a temperature.) Another counter-indication is if the patient has drunk a hot or cold liquid beforehand, in which case one has to wait or use another method.

Rectal

Rectal temperature-taking, especially if performed by a person other than the patient, should be facilitated with the use of lubricant (such as petroleum jelly (now discouraged) or a water-based personal lubricant). Although rectal temperature is the most accurate, this method may be considered embarrassing in some countries or cultures, especially if used on patients older than young children; and, if not taken the correct way, a rectal temperature-taking can be uncomfortable and in some cases painful for the patient. Rectal temperature-taking is considered the method of choice for infants for the general public; however, the rectal route is least desirable in infants from a nursing point of view.

Ear and forehead

Other kinds of medical thermometers exist, such as the tympanic thermometer that measures the temperature of the tympanum by infrared measurement, and the band thermometer that is applied to the patient's brow.

Sources:

http://hypertextbook.com/facts/LenaWong.shtml
http://en.wikipedia.org/
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatreg.html

Compiled and Edited by John Sandham IEng MIET MIHEEM