Temperature Measurements
To check circuits and prototypes a temperature measurement is often necessary. In many cases the failure of semiconductors, transformers and other passive components due to thermal overload. This is to prevent by observing the temperature of critical components during the test phase.
Due to a little understanding of temperature measurements, there is little attention paid to these measurements. Because it is often unknown how errors occur, measurement errors are very commonplace. Errors of a few tens of degrees Celsius are no exception.
Thermocouple
Fig. 1: Thermocouple thermometer.
Thermometers on the basis of thermocouples exist in various designs. They can run as an independent instrument as in the photo on the right, integrated into a multimeter, as a measuring card for a computer etc.. The actual sensor consists of that wires of different metals who welded at one side, this is the sensor, also known as hot-junction. On the other side the voltage is measured at a temperature compensated measurement block, the cold-junction.
There are several types of thermocouples available. The distinction in the various types lies in the two types of metal their are made of. Each sensor has its own temperature range, sensitivity and linear deviation.
| Type | Material | Temperature range [°C] | Sensitivity [µV/K] |
| T | Copper-Constantan Cu-CuNi | -40...+350 | 40 |
| J | Iron-Constantan Fe-CuNi | -40...+750 | 50 |
| E | Chromel-Constantan NiCr-CuNi | -40...+900 | 60 |
| K | Chromel-Alumel NiCr-NiAl | -40...+1200 | 40 |
| N | Nicrosi-Nisil NiCrSi-NiSi | -40...+1200 | 36 |
| S | Platina-PtRh10 Pt-PtRh 10% | 0...+1600 | 9 |
| R | Platina-PtRh13 Pt-PtRh 13% | 0...+1600 | 9 |
| B | PtRh6-PtRh30 | +600...+1700 | 5 |
The different types are distinguished by their color-coded. There are multiple systems (BS, ANSI, DIN and IEC) that each have their own color indications. The most used is the K-type thermocouple that has a linear output characteristic. This is usually carried out with yellow connectors. The used sensor and processing/reading unit should match. So a type-K thermocouple can only be connected with a device there destined for.
Thermal coupling of the sensor
Fig. 2: Heat flow at a thermocouple on a surface.
The standard thermocouple is performed with a spherical weld to the end. These sensors are only suitable for measurements of liquids and gases. The spherical end can't have a good thermal contact with hard surfaces.
Here is a schematic representation drawn by the situation in the photo below.
The thermocouple weld has only by a very small surface thermal contact with the measured object. The weld has a relatively large cooling surface to the environment. By a measurement at a surface of 65 °C, the thermometer recorded a temperature of 42 °C. A measurement error of 23 °C!
Fig.3: Heat flow at a thermocouple placed in a hole.
If the measure object has a deep hole, it's favourable to put the thermocouple inhere. The connecting wires have through the surrounding air thermal contact with the measuring object. The error will now be greatly reduced.
To do a temperature measurement on objects, a good thermal contact must be made. There are special sensors with ring terminals that are better suitable on surfaces. These can be bolted sturdy to the object to be measured.
Fig. 4: Measurment errors occurs by poor heat coupling between object and sensor.
IR thermometers
Fig. 5: Infrared thermometer with LED-pointer.
Any object warmer than 0 K (-273 °C) transmitting electromagnetic waves, especially in the infrared region. The intensity depends on the temperature and type of surface of the object.
This infrared radiation can be measured with an IR thermometer. The big advantage of these thermometers is that they do not need to make contact with the measured surface. This makes it possible to measure temperatures on moving objects or parts with a hazardous voltage.
| Material | Emission coefficient |
| Skin, human | 0,98 |
| Water | 0,92...0,96 |
| Ice | 0,96...0,98 |
| Asphalt, tar, pitch | 0,95...1,00 |
| Plastic | 0,90...1,00 |
| Black rubber | 0,94 |
| Paint | 0,80...0,97 |
| Glass | 0,85...1,00 |
| Ceramics | 0,80...0,94 |
| Cement | 0,96 |
| Concrete | 0,94 |
| Wood | 0,80...0,90 |
| Steel, oxidized | 0,80 |
| Iron, oxidized | 0,78...0,84 |
| Red copper, oxidized | 0,78 |
| Brass oxidized | 0,56...0,64 |
| Rolled stainless steel | 0,45 |
| Aluminum, oxidized | 0,40 |
| Zinc, galvanized | 0,28 |
| Red copper, diffuse | 0,22 |
| Brass, polished | 0,05 |
| Iron, non-oxidized | 0,05 |
| Red copper, polished | 0,02...0,07 |
Emission coefficient
Before an object is measured the emission coefficient must be set. Table 2 shows the emission coefficient of some materials.
Many IR thermometers have not the possibility to set the emission factor, and are at a fixed value, 0.95 is typical. In determining the temperature of surfaces with a different emission factor will not be given the right temperature. There is special paint on the market with a well-defined emission coefficient. This allows a surface to be treated before it is measured.
If the absolute radiation constant C [W/(m2*K4] must be set, the emission coefficient must be multiplied by 5.67.
Radiation transparency and reflection
Fig. 6: A IR-thermometer "sees" no objects behind glass.
Glass is transparent to visible wavelengths, light transfers virtually unimpeded through. For the infrared range in which these thermometers measure (8 ... 16 micrometers), most types of glass are opaque. It is not possible to measure the temperature of objects located behind glass. An IR thermometer aimed on glass measures the temperature of the glass itself.
Fig. 7: From polished objects temperature can not be measured. The ambient radiation will be reflected.
From shiny objects like polished metal surfaces, the temperature can not be determined. They hardly radiate in the infrared range but reflect this very well. By targeting an IR thermometer on a shiny surface, the temperature of the object will not be measured but instead the temperature of the environment or a mirrored object is measured.
The measuring spot
Regarding the measuring spot, this is the area where the temperature is measured, can be divided into short and long distance ranges. IR thermometers are equipped with a pointer for easy targeting and to show the size of the measurement spot.
The short-distance meters are equipped with an LED pointer have a small minimum spot size in order of 3 mm. These are most suitable for electronics use. The small minimum spot size makes it possible to measure the temperature of SMD power components.
The long-distance thermometers are equipped with a laser pointer. They often called misleading "LASER thermometers". These thermometers are unsuitable for electronics purposes because of their large measurement spot. This makes it impossible to aim it at an individual component so not only the temperature of a single component is measured but also the surrounding area.
Fig. 8: A comparative example of the spot size versus distance of IR thermometers with LED and laser pointer.
Fig. 10: The small measuring spot of an IR thermometer with LED pointer may also take the temperature of small objects.
Fig. 9: De IR-thermometer met LED-pointer (detail van de bovenste figuur).
If the measurement spot is not focused on the measured component and covers a larger area than just a single component, they will not register the correct temperature. The thermometer measures not only the radiation of the target but also that of the surrounding area.
Example of measurement errors
Here is an example of the impact of a wrong emission coefficient settings on the registered temperature.
The target consists of a transistor in TO218 housing mounted on a pre-drilled SK18/75 black anodised heat sink. The heat flux was 15.2 W. Ambient temperature 24 °C. The emission constant of the thermometer in all measurements was set to 0.9. This setting corresponds to measurements on surfaces with a constant radiation of 5.1 W/m2*K4.
Fig. 11: Transistor in TO218 housing mounted on a heat sink SK18/75. The measurement locations are indicated.
In the figure on the right the measurement spots are shown in proportion and in the table below the measured temperature are listed. Measuring point "e" is a small piece of sanded heat sink. The other part is black anodised.
| measurement location | description | measured temperature |
| a. | plastic part transistor | 83 °C |
| b. | metal plate transistor | 48 °C |
| c. | screw head (M4) | 36 °C |
| d. | heat sink at transistor | 78 °C |
| e. | blank sanded part heat sink | 36 °C |
| f. | next to the white sanded part | 75 °C |
| g. | outer fin | 74 °C |
| h. | bottom side heat sink | 74 °C |
| - | red circled area | 56 °C |
The most striking error is that of point e, the blank made part of the heat sink. Measurement point f is located directly next to e and has the same temperature. The measured temperature difference between measurement e (white) and f (black) is caused by the different emission coefficients of these areas. The temperature differences must be related to the ambient temperature to have a good impression of the deviations:
- Blank: 36 °C - 24 °C = 12 K,
- Black: 75 °C - 24 °C = 51 K.
The transistor, we see this again. The shiny objects (metals b mounting plate and M4 screw c) measure a much lower temperature than the black plastic housing.
The red circled area is the area that is measured without a clear focus on a specific element. There is a kind of average. But because this field yields different surfaces with different emission coefficients there is nothing sensible measured.
Other thermometers
In addition to the above thermocouple and IR thermometers are also PT100, semiconductor and other thermometers on the market. These thermometers are like the thermocouple primarily intended for measurements in liquids and gases. In determining the temperature of objects with these devices counts that a good contact with the measurement objective must always certain.