Digital Thermometer

Digital thermometers are devices for temperature measurement that digitally display or transmit the measurement value. In the digital thermometer, the measurement and display of temperature can be carried out separately. The actual temperature sensor delivers analog signals depending on the temperature at the measuring point. The temperature sensors, in which the analog signal, before being passed on, is converted into a digital signal with an analog-digital converter, are defined as digital sensors. The digital signal is less susceptible to interference, but the measuring range is limited because of the temperature-sensitive electronics. Digital thermometers enable easy storage and evaluation of the measurement data. Even in the simple digital thermometers, there are such special functions as fixation of the current measurement value on the screen and display of the minimum and maximum values of a series of measurements. Some digital thermometers give an alarm (either as warning sounds or additional visual notifications) if the set limit values are exceeded.

Those digital thermometers that can record the measured values over longer periods of time are also called temperature data loggers. These simplify the recording and evaluation of temperatures. The storage interval can be set in the device menu or via the software belonging to the digital thermometer. In addition to the temperature, the date and time of the measurement are also saved. To do this, these values in the most digital thermometers must be set correctly when they are started for the first time. In some special data loggers it can be controlled via the analog inputs that the storage is started or stopped when certain values are reached.

Some digital thermometers record additional measurement parameters. The digital thermometers that also measure air humidity are called thermo-hygrometers. Many digital thermometers can calculate the dew point from the values of air humidity and temperature. The dew point is the temperature at which the moisture from the air builds on cold surfaces.

Depending on its temperature, each material emits energy in the form of light waves in the infrared range. The higher the temperature is, the more energy is emitted. Digital thermometers with radiation sensors are also called pyrometers. You can determine the temperature of the heat-emitting surface from the incident radiation. The measurement of these digital thermometers works in a contactless way and very quickly. There are infrared digital thermometers that calculate a current individual value for the targeted area, as well as infrared cameras for the pictorial representation of the temperatures by means of the infrared evaluation.

When measuring with the infrared digital thermometer, the following should be noted:
- the digital thermometer has had time to measure the ambient temperature
- the sensor lens is not covered, dirty or misty
- the infrared radiation to the digital thermometer is not affected by dust or steam
- the size of the measuring spot corresponds to the desired measuring point
- the emission value corresponds to the material of the surface to be measured.

The material-specific emissivity between 0 and 1 describes how well a surface emits the infrared radiation. Many digital thermometers allow you to set the emissivity in the device menu. This value is then taken into account for the temperature determination. If the emissivity in the infrared digital thermometer is permanently set to 0.95 or is not known for the material, the surface can be made measurable by painting or with the help of the measuring point sticker.

The size of the measuring spot in the infrared digital thermometer depends on the design. It describes the circular area, the infrared radiation of which is detected by the sensor during the measurement. Further the infrared digital thermometer is removed from the target point, the larger the diameter of the measuring spot is. The ratio of the distance to the measuring point diameter is device-specific and is given as a numerical value. Thus, 30:1 means that the measuring point diameter is 1 cm at the distance of 30 cm and 2 cm – at the distance of 60 cm. Many infrared digital thermometers have laser markings to make the target point visible. A simple laser point shows only the middle, a cross laser shows the diameter of the measuring spot. The circumference of the measuring spot is displayed with the so-called circular laser marking.

Thermocouples for digital thermometers consist of two wires made of different metals that are welded at one end. Many thermocouples are defined in DIN EN 60 584 and DIN 43 710 with type designation, material information, color code, temperature limits, voltage series and permissible limit deviations.

The temperature difference between the connected and the free wire ends creates a tension. This voltage depends on the metals used and does not increase linearly with the temperature. The temperature at the measuring point can only be determined if the temperature at the free ends of the thermocouple is known or if it is registered with an additional sensor on the digital thermometer. Besides, the measured voltage must be linearized by using the characteristic curve of the thermocouple type used. Digital thermometers with the connections for thermocouples either have their characteristic curves stored or allow the entry of the required characteristic values. Thermocouples of the same type can easily be replaced by each other. The digital thermometers with plug connections standardized for the corresponding type are particularly easy to use. In case of the extension of the thermocouples, the same type of the thermocouple and a compensating cable must be used. This applies to all the connections up to terminal point compensation in the digital thermometer.

When it goes about platinum resistance sensors for digital thermometers, the conductivity over wide rangeы is linear to the temperature. Platinum resistance sensors for temperature measurements are defined in EN 60751. The platinum resistor PT100, which is often used for digital thermometers, has a nominal resistance of 100 Ω at 0 ° C, the PT50 50 Ω, the PT500 500 Ω and the PT1000 even 1 kΩ.
Since the line resistance of the connecting cable is also registered by the digital thermometer during the measurement, the measurement errors can occur. The influence on the measurement result is small for short connection cables and for PT1000 sensors. However, the line resistance can be compensated by connecting additional lines to the sensor. The additional lines in the 3 and 4 wire technology eliminate the effect of the line resistance on the measurement result.

Platinum resistors for digital thermometers can be manufactured in different accuracy classes. Four accuracy classes are defined in the EN 60751 standard. Class AA is identical to the designation 1/3 DIN B. With the accuracy 1/10 DIN B, platinum resistors are marked, in which the temperature deviation is only 1/10 of class B.

Class AA: tg = 0.10 ° C + 0.0017 · | t | (corresponds to 1/3 DIN B)
Class A: tg = 0.15 ° C + 0.0020 · | t |
Class B: tg = 0.30 ° C + 0.0050 · | t |
Class C: tg = 0.60 ° C + 0.0100 · | t |

The digital thermometers with thermistors use the special properties of semiconductors. Thermistors are semiconductor components the electrical resistance of which is influenced by temperature. The semiconductors with the electrical resistance decreasing with the increasing temperature have a negative temperature coefficient. They are called thermistors or NTC thermistors. PTC thermistors, with the electrical resistance increasing with the temperature, are called PTC thermistors because of their positive temperature coefficient.

Digital thermometers with NTC or PTC thermistors offer very high accuracies in the temperature range from approximately -50 ° C to 150 ° C. These digital thermometers are suitable for many applications, for example, in the medical field, in food control, for weather stations, automotive electronics, heating, ventilation and air conditioning technology.