CONTEXTO INVESTIGATIVO

The thermometers described in Chapter 8.2 are standard styles that are used primarily in the wide field of process engineering, such as the chemical and petrochemical industries, for example, or in power station construction. But these thermometers cannot be used for measurements in every situation. In many cases the overall dimensions are oversized for the measurement location at hand, as the thermometer is overloaded or the measurement requirements are increased because of mechanical vibration.

The following chapters introduce the various styles that have been optimized for specific applica- tions.

8.3.1

Resistance thermometers for strong vibration

Temperature probes used for engine monitoring and control must fulfil very different requirements. One of these is a high resistance to mechanical vibration over a period of more than 10 years. The vibration frequency is between 500Hz and 3000Hz, and the accelerations that occur are up to 50g (50 times the acceleration due to gravity). Such conditions are encountered on air compressors or refrigeration compressors, for example, used on road and rail transport vehicles (gearbox oil tem- perature, cooling water temperature, turbocharger air temperature), or in shipping.

As the probe is often externally mounted below the vehicle floor, very high temperature gradients occur within the temperature probe. Internally, the measurement temperature is 180°C, whereas externally the plug connector is exposed to ambient temperature (down to -30°C), very damp con- ditions or water spray (e.g. vehicle driven in rain).

Because of this, both the probe and the plug connector system must be designed to provide a high degree of protection up to IP67/IP69.

As well as high mechanical stresses, the temperature probe also has to meet increased measure- ment requirements. The response time must be very short (t₀₅ < 1.1sec in water), so that rapidly-in- creasing gearbox oil temperatures are detected and overtemperatures avoided. Because of the small immersion depths, steps must be taken at the design stage to ensure that the interior of the temperature probe incorporates measures to reduce the heat conduction error.

The illustration shows a selection of such temperature probes that have all these attributes, and that have been tried and tested over a number of years.

Fig. 50: Shock-proof resistance thermometers from the JUMO VIBROtemp range

8.3.2

Resistance thermometers for the food industry

In cooking, baking and curing processes in the foodstuffs industry, measurement of the core tem- perature of the product is vitally important for process control. Temperatures up to 260°C occur here. The situation is made worse by the fact that varying amounts of steam are also injected dur- ing the process. These thermometers are often cleaned by immediately immersing them complete- ly in cold cleaning fluids, whilst the thermometer is still hot. As the air inside it contracts, the probe starts to “breathe”, and fluid tries to penetrate into the probe. If moisture reaches the measurement circuit, a shunt path is established, and the thermometer indicates a lower temperature. This case is especially bad, as too low a temperature is transmitted to the control system. The cooking pro- cess is then extended or the process temperature increased, and as a result the food product is burned.

The use of an encapsulating method specially developed for this type of thermometer successfully prevents moisture from reaching the measurement circuit and sensor, despite severe stresses over several years. A development of this type of thermometer is the use of multiple resistance ther- mometers and thermocouples that measure the temperature at various fixed distances along the protection tube. This allows the variation of core temperature to be tracked at various points over time, to establish a uniform cooking process.

Fig. 51: Various JUMO insertion probes

8.3.3

Resistance thermometers for heat meters

In order to be able to calculate the heat given out or absorbed in a heating or cooling circuit, mea- surements must be taken. For this, a heat meter is often installed in the circulation system; the heat meter measures the quantities needed to calculate the amount of heat, i.e. the flow and tempera- ture of the flow and return lines of the heating system. The arithmetic unit calculates the amounts of heat, taking into account the heat capacity of the heat carrier (normally water in heating sys- tems); the value is stored and then displayed in statutory units. If heat meters are referred to for cal- culation of heating costs, they are subject to official certification and must meet certain error limits, known as the certification error limits. Before the heat meter is brought into commercial use, the components must be checked and certified for compliance with the certification error limits. The amount of heat given out is calculated as follows:

Formula 30:

Q = amount of heat,

k = temperature coefficient of the heat carrier,

V = volume,

∆Θ = temperature difference between flow and return.

The error limits for the individual components of a heat meter are defined in both Appendix 22 of the Certification Regulations (for Germany) and in European Standard EN 1434.

For temperature probes used to measure the temperature difference between the flow and return lines, the applicable relative error limit (E) is dependent on the temperature difference to be mea- sured, and is given by the following formula:

Formula 31:

E = relative error,

∆Θmin = smallest permissible measurable temperature difference (normally: 3°C),

∆Θ = temperature difference to be measured.

For compliance with the error limits for the temperature difference, the temperature probes must be paired in line with a mathematical procedure, taking into account the individual characteristics. For this, the individual characteristic parameters R₀, A and B of each individual temperature probe must be determined in the applicable operating range, as any two temperature probes, even with restricted tolerances (1/3 DIN, for example), do not automatically comply with the temperature dif- ference error limits. To clearly establish the characteristic parameters, each temperature probe must be calibrated at three temperature points, with a measurement uncertainty of 0.021°C. The heating systems used in housing generally consist of pipes with a diameter less than 25mm, resulting in a very small immersion depth of the temperature probe, between 15mm and 30mm, depending on the mounting method. For correct temperature acquisition in the pipeline, this tem- perature probe must be optimized for a very small heat conduction error. In accordance with the approval criteria, the heat conduction error at the minimum insertion depth of the temperature probe must not be more than 0.1°C. (see also Chapter 6 “Heat conduction error“).

Fig. 52: Various temperature probes from the JUMO HEATtemp range