A fever thermometer (also known as a fever meter ) can be used to measure the body temperature of a person and thus diagnose any fever . According to the European standard, such a thermometer must meet the following requirements:
- Measuring range at least 35 ° C to 42 ° C with a resolution of 0.1 ° C
- Record the maximum temperature reached during the measurement
Mercury fever thermometers meet these requirements by expanding a certain amount of mercury in a very thin capillary . The display of the maximum temperature is achieved by a glass spike located in the capillary, which can tear off the mercury thread that retracts when it cools down, whereby the maximum temperature reached continues to be displayed. To reset, the glass mandrel must be pushed back onto the mercury thread by "shaking it back" or tapping it.
Since the use of mercury is very problematic both when the thermometer is damaged and when it is disposed of, Galinstan has established itself as a thermometer liquid . These thermometers work on the same principle and look very similar to the old mercury thermometers. Therefore, they were often wrongly classified as "containing mercury" and disposed of.
In the EU, the sale of measuring devices containing mercury in the health sector has been prohibited since April 2009.
The use of glass clinical thermometers with Galinstan still has advantages today in the age of digital clinical thermometers. The thermometers are 100% waterproof and can therefore be easily cleaned and disinfected. No battery is required. They are also suitable for patients with nickel contact allergy , for whom the use of electronic probes is problematic.
The first clinical thermometers go back to Daniel Gabriel Fahrenheit , but were difficult to handle due to their length of around 60 cm.
Medical thermometer of today's form
The English doctor Thomas Clifford Allbutt invented the short clinical thermometer in 1867, which at around 15 cm was roughly the size it is today. In 1868, Karl Ehrle described the first maximum thermometer for measuring fever (the mercury thread being torn off by an air bubble). This enabled the hospital staff to carry out the measurement, but the reading could only be taken later by the doctor. Previously, the thermometer often had to remain with the patient for hours until the doctor's round so that it could be read reliably.
In 1890 the chemist Wilhelm Uebe developed the modern, closed clinical thermometer as we know it today. He came up with the idea of melting the top end of the glass thermometer instead of sealing it with a plaster stopper. This further development increased the safety of use and the hygiene standard of the clinical thermometer. Uebe Medical developed from this small approach . For years, their medical thermometers were the best-selling tools for self-diagnosis worldwide.
As a further development, Kitasato Shibasaburō has brought the first reliably functioning clinical thermometer onto the market. In 1921 he was co-founder and namesake of the Japanese company Terumo . This company name goes back to the Japanese pronunciation of the German word thermometer.
The basal thermometer was developed by Uebe Medical in 1959 to measure the fine temperature differences that accompany a woman's cycle . With the help of a basal thermometer, the fertile and infertile days of the woman can be determined from the recorded temperature curve.
Digital clinical thermometers
Inexpensive and at the same time accurate electronic temperature measurement technology have also helped the digital thermometer (also called LCD thermometer after the display method ) as a clinical thermometer to gain a considerable market share. The only disadvantage here is the requirement of a sufficiently fresh battery , which must also be disposed of at the end of its service life. The maximum value display is reached by ending the measurement as soon as the measured temperature no longer increases within a certain time. The digital clinical thermometer generates an acoustic signal to indicate the end of the measurement.
With the digital clinical thermometer, the temperature is recorded by an electronic component ( sensor ) that changes its electrical resistance according to its temperature. This change in resistance is evaluated by an electronic circuit, which takes the exact temperature-resistance relationship of the sensor into account, and displayed in ° C (degrees Celsius) or ° F ( degrees Fahrenheit ). In the measuring range of the sensor, the relationship between temperature and resistance is almost linear .
The accuracy of the digital clinical thermometer depends on which tolerance is accepted in the manufacture of the sensor and whether the 37 ° C point is set precisely. The DIN EN 12470-3 standard , for example, has precise requirements .
Infrared clinical thermometer
Infrared clinical thermometers measure the infrared radiation emitted by the eardrum or forehead . This is transmitted to a sensor by means of a lens , converted into a temperature value and displayed. The advantage of infrared clinical thermometers compared to conventional thermometers is the short measurement time, which is only a few seconds. The range of inexpensive infrared clinical thermometers has increased in recent years, so that they are now also used in medical practices and clinics. The devices for household use now also have sufficiently precise measurement results, which is why they are particularly popular for measuring fever in small children.
If infrared clinical thermometers are used for non-contact measurements, protective covers or disinfectants are unnecessary, even when measuring across people; This type of clinical thermometer is therefore also suitable for large-scale random checks, for example if there is a suspicion of infection in public spaces.
Rapid fever test
Retailers offer so-called quick test strips for quick temperature measurement. These are plastic strips in which thermochromic dyes are embedded. If the body temperature exceeds a certain value, z. B. the letter F visible.
The following types of measurement apply to classic clinical thermometers with a measuring tip.
- In the armpit (axillary): Here the clinical thermometer is clamped under the arms in the armpit. This measuring method is the most convenient, but also the least accurate.
- In the oral cavity (oral): This measurement method is much more accurate than the axillary method. However, it must be ensured that the measuring tip has good contact with the tissue in the mouth. It is recommended to bring the measuring tip under the tongue ( sublingually ).
- In the anus (rectal): The measuring tip of the clinical thermometer is inserted rectally into the anus. This measurement method is the most accurate and is particularly used for infants and young children.
Evaluation of the measurement results
When evaluating the measurement results of the three classic measurement types rectal, oral and axillary, it must first be noted that these are different temperature zones of the body. In the axillary measurement, for example, a surface temperature is measured, while the rectal measurement, through the probe inserted into the anus, delivers values that come very close to the core body temperature . The oral measurement (more precisely: under the tongue) is also carried out "in the body", but it is not measured directly in the trunk . Due to the different temperature zones, the definition of when a fever is actually present differs. The information varies slightly depending on the source. Typical values for normal temperature are
- axillary: 34.7 - 37.7 ° C
- rectal: 36.6 - 38.0 ° C
- oral: 35.5-37.5 ° C
Furthermore, the different possible interfering influences must be taken into account in the measurement types. In the case of axillary measurements, for example, incorrect measurements can already occur due to damp skin or a measuring probe not firmly attached to the body. In the case of oral measurements, breathing air, incorrect positioning of the probe in the oral cavity or cold or warm drinks consumed before the measurement can affect the measurement result. With the rectal measurement, the probe is largely protected from interference, so that this method is the safest from a metrological point of view. The rectal method is therefore the best way to determine reproducible and precise values close to the core body temperature.
A calculation of rectal temperature values, for example from the axillary measured value, should be examined very critically. Different sources have different temperature differences between the methods. The range for the difference between axillary and rectal measurements is between 0.5 and 1.5 ° C. Together with possible measurement errors, the result is nowhere near an exact temperature value.
Infrared clinical thermometers for measuring fever in the ear basically measure according to a safe measuring method. Here the greatest measurement uncertainty lies in the incorrect handling of the thermometer. Since the infrared beam has to hit the sensor directly from the eardrum, ear wax and incorrect positioning of the infrared sensor are the greatest sources of error. There are also different specifications for the temperature difference between values measured in the rectum and in the ear. Typical values are around 0.5 ° C.
- Instructions for handling broken mercury thermometers. In: bcp.fu-berlin.de
- Taking fever measurements for children, toddlers and babies. In: familie-und-tipps.de
- EU bans mercury clinical thermometers ( Memento of July 14, 2007 in the Internet Archive ), Netzeitung, July 10, 2007
- Test winner: Digital clinical thermometer ( memento from July 9, 2012 in the web archive archive.today ), ORF1, February 25, 2008
- Frequently asked questions. In: geratherm.com (PDF; 1.1 MB)
- Company Uebe history. In: uebe.com , accessed November 26, 2016
- Frequently asked questions about the correct temperature measurement. In: fieber.aponorm.de , accessed on May 12, 2020
- Temperature difference between bottom and armpit. In: navigator-medizin.de , accessed on April 1, 2017
- Correct temperature measurements in children. In: elternwissen.com , accessed April 2, 2017