Medical thermometer
Encyclopedia
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
under the tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...
(oral or sub-lingual temperature), under the armpit
Underarm
-Axillary intertrigo:Excessive perspiration can result in axillary intertrigo. Intertrigo is an inflamed skin condition caused by heat, friction, and moisture. A warm, wet armpit may accommodate the growth of pathogenic bacteria, yeasts, and fungi. The condition is responsible for rash-like...
(axillary temperature), or into the rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
via the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
(rectal temperature).
Liquid-filled
The traditional thermometer is a glass tube with a bulb at one end containing a liquid which expands in a uniform manner with temperature. The tube itself is narrow (capillary) and has calibration markings along it. The liquid is often mercuryMercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
, but alcohol thermometer
Alcohol thermometer
The Alcohol thermometer or spirit thermometer is an alternative to the mercury-in-glass thermometer, and functions in a similar way. But unlike mercury-in-glass thermometer, the contents of an alcohol thermometer are less toxic and will evaporate away fairly quickly...
s use a coloured alcohol. Medically, a maximum thermometer is often used, which indicates the maximum temperature reached even after it is removed from the body.
To use the thermometer, the bulb is placed in the location where the temperature is to be measured and left long enough to be certain to reach thermal equilibrium
Thermal equilibrium
Thermal equilibrium is a theoretical physical concept, used especially in theoretical texts, that means that all temperatures of interest are unchanging in time and uniform in space...
—typically three minutes. Maximum-reading is achieved by means of a constriction in the neck close to the bulb. As the temperature of the bulb rises, the liquid expands up the tube through the constriction. When the temperature falls, the column of liquid breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. After reading the value, the thermometer must be reset by repeatedly swinging it sharply to shake the liquid back through the constriction.
Mercury
Mercury-in-glass thermometer
A mercury-in-glass thermometer, also known as a mercury thermometer, was invented by German physicist Daniel Gabriel Fahrenheit in 1724 and 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...
s have long been considered the most accurate liquid-filled types. However, mercury is a toxic heavy metal, and mercury has only been used in clinical thermometers if protected from breakage of the tube.
The tube must be very narrow to minimise the amount of mercury in it—the temperature of the tube is not controlled, so it must contain very much less mercury than the bulb to minimise the effect of the temperature of the tube—and this makes the reading rather difficult as the narrow mercury column is not very visible. Visibility is less of a problem with a coloured liquid.
In the 1990s it was decided that mercury-based thermometers were too risky to handle; the vigorous swinging needed to "reset" a mercury maximum thermometer makes it easy to accidentally break it and spill the moderately poisonous
Mercury poisoning
Mercury poisoning is a disease caused by exposure to mercury or its compounds. Mercury is a heavy metal occurring in several forms, all of which can produce toxic effects in high enough doses...
mercury. Mercury thermometers have largely been replaced by electronic digital thermometers, or, more rarely, thermometers based on liquids other than mercury (such as galinstan
Galinstan
Galinstan is a family of eutectic alloys mainly consisting of gallium, indium, and tin, which are liquids at room temperature, typically freezing at . Due to the low toxicity and low reactivity of its component metals, it finds use as a replacement for many applications that previously employed...
, coloured alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
s and heat-sensitive liquid crystals).
Electronic
Since compact and inexpensive methods of measuring and displaying temperature became available, electronic thermometers (often called digital, because they display numeric values) have been used. Many display readings to great precisionAccuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...
(0.1 °C or 0.2 °F, sometimes half that), but this should not be taken as a guarantee of accuracy: specified accuracy must be checked in documentation and maintained by periodical recalibration. A typical inexpensive electronic ear thermometer for home use has a displayed resolution of 0.1 °C, but a stated accuracy within ±0.2 °C when new.
Contact
Electronic thermometers may work by contact (the electronic sensor is placed in the location where temperature is to be measured, and left long enough to reach equilibrium). They typically reach equilibrium faster than mercury thermometers; the thermometer may beep when equilibrium has been reached, or the time may be specified in the manufacturer's documentation.Remote
Other electronic thermometers work by remote sensing: an infrared sensor responds to the radiation spectrumSpectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
emitted from the location. Although they are not in direct contact with the area being measured, they may touch part of the body (a thermometer which senses the temperature of the eardrum without touching it is inserted into the ear canal). To eliminate the risk of patient cross-infection, disposable probe covers and single-use clinical thermometers of all types are used in clinics and hospitals.
Basal thermometer
A basal thermometer is a thermometerThermometer
Developed during the 16th and 17th centuries, a thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor Developed during the 16th and 17th centuries, a thermometer (from the...
used to take the basal (base) body temperature
Basal body temperature
Basal body temperature is the lowest temperature attained by the body during rest . It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature...
, the temperature upon waking. Basal body temperature
Basal body temperature
Basal body temperature is the lowest temperature attained by the body during rest . It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature...
is much less affected than daytime temperature by environmental factors such as exercise and food intake. This allows small changes in body temperature to be detected, such as those caused by ovulation
Ovulation
Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum . Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from the menstrual cycle...
or changes in thyroid function.
Glass fever thermometers typically have markings every 0.1 °C or 0.2 °F. Basal temperature is stable enough to require accuracy of at least 0.05 °C or 0.1 °F, so special glass basal thermometers are distinct from glass fever thermometers. Digital thermometers which have sufficient resolution (0.05 °C or 0.1 °F is sufficient) may be suitable for monitoring basal body temperatures; the specification should be checked to ensure absolute accuracy, and thermometers (like most digital instruments) should be calibrated at specified intervals. If only the variation of basal temperature is required, absolute accuracy is not so important so long as the readings do not have large variability (e.g., if real temperature varies from 37.00 °C to 37.28 °C, a thermometer which inaccurately but consistently reads a change from 37.17 °C to 37.45 °C will indicate the magnitude of the change). Some digital thermometers are marketed as "basal thermometers" and have extra features such as a larger display, expanded memory functions, or beeping to confirm the thermometer is placed properly.
Classification by location
The temperature can be measured in various locations on the body which maintain a fairly stable temperature (mainly sub-lingual, axillary, rectal, vaginal, forehead, or temporal artery). The normal temperature varies slightly with the location; an oral reading of 37 °C does not correspond to rectal, temporal, etc. readings of the same value. When a temperature is quoted the location should also be specified. If a temperature is stated without qualification (e.g., typical body temperature) it is usually assumed to be sub-lingual. The differences between core temperature and measurements at different locations, known as clinical bias, is discussed in the article on normal human body temperatureNormal human body temperature
Normal human body temperature, also known as normothermia or euthermia, is a concept that depends upon the place in the body at which the measurement is made, and the time of day and level of activity of the person...
. Measurements are subject to both site-dependent clinical bias and variability between a series of measurements (standard deviation
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
s of the differences). For example, one study found that the clinical bias of rectal temperatures was greater than for ear temperature measured by a selection of thermometers under test, but variability was less.
Oral
Oral temperature may only be taken from a patient who is capable of holding the thermometer securely under the tongue, which generally excludes small children or people who are unconscious or overcome by coughing, weakness, 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 stabilise their reading.) If the patient has drunk a hot or cold liquid beforehand time must be allowed for the mouth temperature to return to its normal value.The typical range of a sub-lingual thermometer for use in humans is from about 35°C to 42°C or 90°F to 110°F.
Rectal
Rectal temperature-taking, especially if performed by a person other than the patient, should be facilitated with the use of a water-based personal lubricant. Although rectal temperature is the most accurate, this method may be considered unpleasant, or embarrassing in some countries or cultures, especially if used on patients older than young children; also, if not taken the correct way, rectal temperature-taking can be uncomfortable and in some cases painful for the patient. Rectal temperature-taking is considered the method of choice for infantInfant
A newborn or baby is the very young offspring of a human or other mammal. A newborn is an infant who is within hours, days, or up to a few weeks from birth. In medical contexts, newborn or neonate refers to an infant in the first 28 days after birth...
s.
Ear
Other kinds of medical thermometers exist, such as the tympanic thermometer that measures the temperature of the tympanumEardrum
The eardrum, or tympanic membrane, is a thin membrane that separates the external ear from the middle ear in humans and other tetrapods. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles...
by infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
measurement, The thermometer has a projection (protected by a one-time hygienic sheath) which contains the infrared probe; the projection is gently placed in the ear canal and a button pressed; the temperature is read and displayed within about a second. These thermometers are used both in the home (models are available for prices starting at around U$D 20) and in medical facilities.
Temporal artery
A newer development is the Temporal artery thermometer, which uses the infrared principle to accurately report a patient's temperature, with comparable accuracy to rectal thermometryRectal thermometry
Rectal thermometry is an umbrella term covering the practice, widely used in modern medicine and science, of taking a mammal's temperature by inserting a thermometer into the aforementioned mammal's rectum via the anus. This is generally regarded as the most accurate means of temperature-taking,...
.
Forehead
The band thermometer is applied to the patient's brow. It is typically a band coated with different temperature-sensitive markings using liquid-crystalLiquid crystal thermometer
A liquid crystal thermometer or plastic strip thermometer is a type of thermometer that contains heat-sensitive liquid crystals in a plastic strip that change color to indicate different temperatures....
or similar technology; at a given temperature the markings (numerals indicating the temperature) in one region are at the right temperature to become visible. This type gives an indication of fever, but is not considered accurate.