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.

Infrared temperature sensors
A temperature measuring device with an infrared temperature sensor can determine surface temperatures without a direct contact with the measuring point. The infrared sensor picks up the infrared waves emitted by the object to be studied and uses them to determine the temperature. The sensors can be optimised for different areas of application. They differ, for example, in the measurable temperature range and in the measuring spot ratio. The measuring spot ratio describes the diameter of the detected object surface in relation to its distance from the sensor. A spot ratio of 15:1 corresponds to a spot diameter of three centimetres at a distance of 45 centimetres, while a spot ratio of 60:1 means that the temperature of a surface with a diameter of only one centimetre can be monitored from a distance of 60 centimetres.

Resistance temperature sensors made of platinum or nickel
At the temperature change the electrical resistance of many electrically conductive materials changes as well. This resistance can be captured by the digital temperature measuring device and evaluated in relation to the temperature. The elements made of platinum and nickel are often used as resistance temperature detectors (RTD). As the temperature rises, their resistance increases and the conductivity decreases. Platinum shows a uniform change in resistance over a very wide temperature range. This enables platinum sensors for temperatures from - 200 to 1000 degrees Celsius. In contrast, temperature sensors made of nickel only have resistances that increase linearly with temperature for temperatures between - 60 and 300 degrees Celsius. Nickel sensors, however, react with higher sensitivity. Compared to platinum, they therefore show a greater change in resistance at the same temperature difference. In addition to the material, in the designation of the sensor there is also an indication of the value for the nominal resistance at zero degrees Celsius. A NI500 is therefore a nickel resistor with 500 Ohm nominal resistance and a PT1000 is a platinum resistor with 1000 Ohm nominal resistance. To avoid the measurement errors due to the influence of the long connecting cables on the measurement, the resistance sensors are also available with 3- and 4-wire technology.

Thermistors – semiconductor sensors for temperature measurement
Thermistors are temperature sensors based on semiconductors that are only electrically conductive under certain conditions. They are divided into NTC thermistors with a negative temperature coefficient and PTC thermistors with a positive temperature coefficient. The resistance of NTC thermistors decreases when the temperature increases. These NTC thermistors therefore conduct better at high temperatures, which is why they are also called thermistors. PTC thermistors, on the other hand, conduct better at lower temperatures. They are therefore PTC thermistors because their resistance increases with the temperature. In the digital temperature measuring device, both types of semiconductor sensors can be used for temperatures up to about 250 degrees Celsius. However, the thermistors can also be used as a component for automatic current limitation in electrically driven machines. In this case, the NTC thermistors are used to smoothly start the machines when they are switched on, and the PTC thermistors are used as overload protection, which reduces and finally blocks the current flow when the temperature rises too high.

Thermocouples for temperature measurement
Thermocouples consist of two wires of different metals connected at one end. If the unconnected ends have a different temperature than the connected ends, a voltage difference can be measured between two wires. To determine the temperature at the junction of the wires from this voltage difference, the temperature at the free ends must be known. There are more than 300 known material pairings for temperature measurement with thermocouples. Some of them, which are suitable for industrial measurements, have been standardised in DIN EN 60584. The thermoelectric voltages of these material combinations vary considerably from 5 mV/°C to 100 mV/°C. To simplify the use of the thermocouples, different types have been defined according to their properties: the types T, E, J, K and N as well as S/R, B, C and A for particularly high temperatures even above 1,000 degrees Celsius. The voltage measured at the thermocouple must be linearized in the temperature measuring device with the characteristic curve of the thermocouple used. If the temperature measuring device has connections for the standardised plugs of a certain type of the thermocouple, these thermocouples can be directly connected with the plug and used. If the compensating leads are required up to the measuring point, care must be taken that the compensating leads correspond to the type of the thermocouple.