Measurement of Temperature in the Plastics Industry

Transcription

1 Sawi Mess- und Regeltechnik AG CH 8405 Winterthur-Gotzenwil, Switzerland Telephone , Fax Measurement of Temperature in the Plastics Industry Johannes Wild, Dipl. Ing., Winterthur-Gotzenwil 1. Introduction The measurement of temperatures is an important discipline in the fields of science and research, as well as industry. Temperature is a standard value for the determination of the thermodynamic state or condition of a system. Temperature, however, can only be depicted by its effect. The determination of this action has to be conducted wherever possible independently of the individual, diverse sensitivity and lead to comparable and reproducible results. We can proceed from the fact that structures are able to exchange heat through conduction, convection or radiation. The exchange of energy through thermal conduction is utilized for all contact-measurement processes. In general, this is the only process that finds wide application in the plastics industry. This paper considers the contact-measurement processes in detail, by which there is a flow of temperature from the object to be measured through direct contact (coupling) to the temperature sensor. I have arranged my paper as follows: General remarks Measurement principles Thermocouples Resistance sensors Selection criteria Conclusion As a result of the continuous and progressive refinement of temperature-measurement technology, it is becoming increasingly difficult to acquire information about the actual state. Elementary knowledge of applied temperature-measurement technique is considered an essential for plastics technology.

2 The modern mechatronic offers a broad spectrum of precise, cost-effective, measuring and control devices, The user, however, must always consider his respective requirements: Maximum continuous operating temperature Minimum service-life requirement for the temperature sensor Type of measurement system Design of the temperature sensor Positioning and attachment of the sensor at the measuring point Employment of suitable connecting lead Selection of the optimum measuring and control device This paper is concerned primarily with the application of the temperature-measurement technique in the practice, and which is customary nowadays in plastics technology. Further processes, which deviate partly from those employed in the plastics industry, are used for research and scientific requirements, and also in other branches of industry. 2. Measurement principles In the main, two fundamentally different measuring systems are employed in the plastics industry: Thermocouples and Resistance sensors Thermocouples produce a temperature-dependent electromotive force, an EMF Resistance sensors indicate a temperature-dependent change in the electric resistance Example: Example: Thermocouple, type J Resistance sensor Pt 100 Thermoelectro voltage between Resistance at 000 C = Ohms 0 C and 100 C = mv Resistance at 100 C = Ohms Thermocouples produce a temperature-dependent electromotive force. Resistance sensors have a temperature-dependent electric resistance. 2

3 2.1 Thermocouples The use of thermocouples for the measurement of temperature is based on the socalled thermoelectric effect or the "Seebeck effect, which was discovered by the German physicist, Thomas J. Seebeck, in 1821 and named after him. A thermocouple consists of two electrical conductors, the thermopair, made of dissimilar metals, and the thermowires connected conductively at one end. This connection, the hot end, constitutes the measuring position. The other two ends, the cold ends, form the reference or comparative position. An electric voltage, the thermoelectro voltage, is applied in the millivolt range at the reference position, and is proportional to the difference in the temperature of the hot and cold end. In the practice, the length of the thermopair is extended with a compensation lead to the input terminals of the indicating instrument or control device. In this way, it is possible to compensate definable temperature values of the cold ends, respectively a defined value, for example, to 20 C. (The norm IEC584 is related to 0 C). The length of the thermopair has to be extended with the respective thermowire or by means of a suitable compensation lead. Thermocouples differentiate as a result of the employed thermomaterial with different thermoelectric force. Diverse characteristics result according to the paired materials. Of importance in our case are the pairing of pure iron (Fe) with a copper-nickel alloy (CuNi) and the pairing of nickel-chrome alloy (NiCr) with pure nickel (Ni). Fe-CuNi thermopairs are designated internationally as type "J. NiCr-Ni thermopairs as type "K. The two pairings are described in the norm IEC 584, and more recently in DIN EN Thermocouple or TC Fe-CuNi = Type J Thermocouple or TC NiCr-Ni = Type K In the material-related, written form Fe-CuNi or NiCr-Ni, the positive shank is always denoted first. By the thermopair "J, the positive shank (Fe) is magnetic. With the thermopair "K, it is negative (Ni). Persons with experience are able to differentiate quite clearly between these two thermopairs by means of a magnet. In the European economic area (influenced especially by Germany), it is well-nigh a tradition in the plastics industry to employ the type Fe-CuNi (formerly Fe-Konst) thermocouple. On the other hand, it is more the thermocouples, type NiCr-Ni (formerly Chromel- Alumel), that are used in the economic areas influenced by the USA. 3

4 In accordance with DIN EN , the marginal deviations for thermocouples are divided into two classes Class 2 permits a marginal deviation of 2.5 C for thermocouples, type J and K. Class 1 thermocouples have a maximum deviation of 1.5 C. For professional duties, however, deviations in excess of 1.5 C should not be accepted (Sawi, for example, only supplies Class 1 thermocouples). The international norms (IEC584, in Europe DIN EN ) include further thermoparings, but they are not of any real significance for the subject matter of this paper. Nevertheless, I should like to draw your attention to a new thermocouple with the designation "N. This is a modified pairing, similar to the K-type, by which the thermoshank is doped additionally with silicon. In some cases, an older thermopairing, type Fe-CuNi, is still being used in Germany. It is described in DIN and designated with the letter "L. This type has no further significance internationally. Although its characteristic curve is very similar to that of the type "J, hardly any notice is taken of this double-tracking in the practice Pro and contra thermocouples For many years, the type "J has been the most widely used measuring system in the plastics industry. This is not only attributable to its favourable price, but also the system-conditioned possibility of creating sensor applications with the smallest possible geometry. Without any doubt, the world-wide accepted standardization through the norm IEC584 has also helped the thermocouple to achieve the breakthrough. From the negative standpoint, however, the fact should be noted that the plus shank of this thermocouple is made of pure iron (Fe) and so it has a marked tendency to scale and rust during the course of manufacture and operation of the thermocouples. From today s standpoint, the type "K thermopairings have fewer disadvantages. 2.2 Resistance sensors All sensors, by which changes in temperature result in reproducible changes in resistance, are known as "resistance sensors. We differentiate, however, between the main groups "Metal-Resistance Sensors and "Semi-Conductor Sensors. Among the metal-resistance sensors, such as copper, nickel or platinum, it is only the platinum sensors that have any significance. They are standardized worldwide under the norm IEC

5 DIN IEC 751, and recently DIN EN , superceded the old DIN-norm with the number in A platinum sensor is a temperature-sensitive, electric resistance. Its conductor is made of platinum and exhibits a defined resistance of 100 Ohms, for example, at a temperature of 0 C. The classical Pt100-sensor consists of a ceramic body wound with platinum wire. Later designs include the platinum-layer resistances, which are subdivided in thick and thin-film technique. The later is the more popular of the two. In this case, the layer of platinum is deposited on a ceramic substrate by means of vaporization. The conducting paths, for the attainment of the desired nominal resistance, are created and calibrated by means of laser cut. Platinum-resistance sensors are characterized by their excellent stability and high precision. No special compensation lead is required for the extension from the measuring position to the indicating or control device. From a certain length, however, one has to decide in favour of either a three or four-wire circuit in order to eliminate the line resistance. Platinum resistance 100 Ohms = Pt100 In accordance with DIN IEC 752, or new DIN EN , the marginal deviations for platinum-measurement resistances are subdivided in two classes. Whereas the Class "B permits a marginal deviation of 1.8 C by a temperature of 300 C, the maximum permissible deviation by Class "A is 0.75 C. Both variants are used in the practice. However, Class "B has a considerably better price/performance ratio and, in general, fulfils the requirements of the plastics industry Pro and Contra Pt100 The Platinum 100-measurement system has established itself in the plastics industry over a period of many years. Its principal advantage is that Pt100-sensors provide absolute values, which are independent of the ambient influences. Further plus-points are the extremely good long-term stability and, in comparison with thermocouples, less difficult connection conditions that do not require any compensation leads. The world-wide acceptance of IEC 751 is also another positive factor. The noticeable higher price in comparison with thermocouples is a negative aspect, and a factor that can quickly escalate by some suppliers, especially with small and the smallest possible design shapes. 5

6 3. Selection criteria 3.1 Maximum continuous operating temperature Thermocouples, type "J and "K, as well as the resistance sensors Pt100 can be used without any reservation for the customary branch temperatures. 3.2 Minimum service-life requirement for the temperature sensor Whereas the initial outfitter frequently considers the price in preference to quality, the end user places higher demands on the quality and the service life of the temperature sensors for obvious reasons. Products from reputable manufacturers, for instance sawi, usually have a service life of three years. 3.3 Design of the temperature sensor Contact-temperature sensors, such as those that are customary here, operate according to the principle of heat flow from the object to be measured to the sensor. The smaller the own mass of the sensor to be heated, the quicker it will attain the temperature of the object 2. In other words, sensors with small own mass have shorter response times. Thermocouples and, in particular, the mineral-insulated, encased thermocouples with a diameter of 1.0 or 1.50 mm, have an optimum response behaviour and a good service life. They are now the latest state-of-the-art and employed primarily in complex forms and injection systems. There is no general, optimum type of design for temperature sensors. The design has to be determined from case to case. 3.4 Positioning and attachment of the sensor at the measuring point From what has been said under Para 3.3, it is quite clear that the coupling of heat between the object and the sensor is a matter of central importance. At the same time, one must be conscious of the fact that the plastic processor is actually interested in the temperature of his plastic mass. Whatever the case may be, he can only measure the temperature of the cylinder or the form in the practice. In other words, a comparative temperature, which then permits conclusions to be drawn as to the effective temperature of the material. Good results are obtained by placing the sensor as near as possible to the desired measuring point. For example, uniform coupling of the heat can be realized through flexible attachment of the sensor. 6

7 3.5 Employment of suitable connecting leads For the employment of thermocouples, one must be absolutely sure that the connection to the indicating or control devices is effected with the appropriate compensation lead. This is the only way to obtain reliable measurements. Resistance sensors do not require any special leads. Nevertheless, one must be conscious of the fact that the electric line resistance is completely emerged in the measurement result. A three or four-wire circuit serves as an aid here. 3.6 Selection of the optimum measuring and control device Whereas a decision had to be made between analogue and digital devices just a few years ago, the selection now is merely a choice of European or non European. With the latest state-of-the-art, these devices can be set by the user for any standardized sensor systems. As to the possibilities, they exceed all the requirements of the plastics industry. In my opinion, the most important thing is a good, easy to read, operating manual, which is also understandable for the operating and maintenance personnel and available at all times. 4. Conclusion Since the plastic material cannot be measured directly or only with difficulty in the most interesting phase for us the processing, the measurement of temperature in the plastics industry is a demanding discipline. A great deal of experience is needed to obtain satisfactory results and calls for the timely and good co-operation of all concerned Johannes Wild, sawi Mess-und Regeltechnik AG, CH-8405 Winterthur 7