Thermometer

A thermometer is a testing and measuring device for determining the temperature of various media. Thermometers are available in many different shapes and types for a wide variety of applications. Depending on the type of the thermometer, it works on a different physical principle. Here, also the accuracy of the thermometer is defined. Electronic thermometers often work according to the resistance measuring method. These thermometers use an NTC (thermistor), PTC (PTC thermistor), platinum sensors such as Pt100, Pt500 or Pt1000 for the exact determination of the temperature. A more common measurement method for the thermometer is the measurement of temperature via thermocouples. These thermometers use the Seebeck effect of 2 different metallic conductors to measure the temperature.

A thermometer, which electronically determines the temperature via the resistance, uses the temperature sensitivity of the special materials. These thermometers use thermistors or PTC thermistors to measure the temperature. But what does it exactly mean? If a thermometer with an NTC, or thermistor, is used, it can be said that the resistance from the temperature sensor decreases with the increase of temperature. As a result, a higher current can flow at a constant voltage provided by the thermometer. With a PTC, or PTC thermistor, the resistance increases almost proportionally to the temperature. Thus, a lower current flow. The current that flows and that the thermometer can measure is the measure of the temperature. Thermometers that work according to the resistance measuring method are often very precise, since the measuring resistance can be very large (Pt1000: 1000 ohms at 0 ° C) and, due to the characteristic curve, better measurable current flows arise. Platinum sensors are often used because they are very clean so that they work very precisely. Platinum thermometers are often used in industrial processes because these sensors are installed enclosed in stainless steel. These thermometers are then used to measure the process temperature of foodstuffs, liquids, oils, etc.

Thermometers with thermocouples use the Seebeck effect of two different metallic conductors. The most widespread are type K thermocouples. These are made of Ni-CrNi (nickel-chrome-nickel). In the thermometers of this type, the voltage is measured that is generated when the two materials that touch each other are heated. The voltages generated in a thermocouple are very low and are a few µV per ° C. Even at high temperatures, the thermocouple thermometer often measures voltages that are < 0.1 V. If x thermocouples are connected in series to a thermometer, the thermometer will display a temperature by a factor of x times. Thermometers of this type are often used in mass applications because the measuring technology is inexpensive and the type of measurement is relatively easy to implement. Thermocouple thermometers are used for a wide variety of applications, depending on the design of the sensor for air temperatures, surface temperatures or core temperatures.

Similar to resistance thermometers, thermometers with semiconductor sensors are useful for the electrical resistance of different materials. If more current flows, the measured temperature is also higher. Thermometers with semiconductor sensors are often used for measuring the air temperature or in applications where the accuracy of the measured value plays a minor role.

The thermometers with different types of sensors and of different design and shapes present the measured values on a display. The measured value is usually shown on the thermometer in the unit °C (degrees Celsius). In the American area, the display on the thermometer in °F (degrees Fahrenheit) is common. In the scientific field, when it comes to temperature differences and at very low temperatures, the measured value of the thermometer is often given in K (Kelvin). Where do the different temperature scales come from? The origin of the thermometer scales lies in the discoverer of the respective scale. Anders Celsius (1701… 1744) determined that the temperature that a thermometer measures in ice water is 0 ° C and that boiling water has a temperature of 100 ° C. The Daniel Gabriel Fahrenheit made various tests and wanted to set its own scale taking into account the body temperature, a mixture of ice, water and ammonia and the lowest temperature in Gdansk at the time. So, the thermometers with the Fahrenheit scale show a temperature of 32 ° F for an ice-water mixture and a temperature of 212 ° F for boiling water.