Industrial tempera ture measurement ...because calibration is a matter of confidence AMETEK CALIBRATION INSTRUMENTS

AMETEK Denmark A/S • Gydevang 32-34 • 3450 Allerod • Denmark Phone: +45 4816 8000 • Fax: +45 4816 8080 • E-mail: [email protected] • www.jofra.com

Industrial temperature measurement AMETEK Denmark A/S • Gydevang 32-34 • 3450 Allerød • Denmark Phone: +45 4816 8000 • Fax: +45 4816 8080 • E-mail: [email protected] • www.jofra.com

Industrial temperature measurement AMETEK® CALIBRATION INSTRUMENTS 1. INTRODUCTION Temperature is one of the most measured parameters within industry and science. A correct measurement is of great importance to the quality of the product, as well as to security and to energy consumption. Therefore, it is very important to choose the right sensor for the actual application. However, a 100% ideal solution for a measuring job is difficult to find if not to say impossible. The choice will often be a compromise between the requirements of the user and the limitations set by the sensor types that are...

Industrial temperature measurement The definition of temperature The temperature of a medium is an expression of its content of thermodynamic energy. The thermodynamic energy represents the average velocity of the unarranged molecular movement in the material. To measure a temperature is therefore different from measuring one of the other basic units. Metres for example can be measured with an inch rule without affecting or changing the length. To measure temperature is different. Actually, the content of thermodynamic energy in the medium should be measured, based on the definition. This is...

Industrial temperature measurement ference and on a material parameter called the thermal power S. Seebeck proved the phenomenon in 1821. T1 The relation between temperature difference and voltage is ΔU = S ⋅ (T2 – T1) The thermal power S of a material is called the absolute Seebeck coefficient [μV/°C]. It is not possible to measure this thermal power unless combined with another material. To achieve values with a common starting point, the precious metal of platinum is used as reference. If two metals are connected to a closed circuit, the current running will be dependent on the difference...

Industrial temperature measurement The thermocouple voltage is: Umeas = ( Smat A – Smat B) ⋅ (T2 – T1) If you want to measure absolute temperature, one soldering point must be kept at a constant temperature. This soldering point is now called the cold junction point, and the other soldering point may be used as temperature sensor. As the ice point of water is fairly easy to realise, 0°C is often used as reference temperature. In tables on thermal voltages, 0°C is always used as reference. For industrial use, this method is not practical. In modern electronic instruments for temperature measuring,...

Industrial temperature measurement AMETEK® CALIBRATION INSTRUMENTS 5.1 Types of thermocouples Two metals or alloys welded/joint together form a thermocouple, so the number of possibilities is infinite. It is a serious job to select the proper materials for a thermocouple. The types that for one reason or another are selected are defined very precisely as far as the composition of the alloy is concerned. This, together with the requirements to the accuracy tolerance makes it possible for different manufacturers to produce even complicated alloys completely uniform themselves. The following thermocouples...

Industrial temperature measurement Temperature limits for thermocouple type N NiCroSil - NiSil type E NiCr - CuNi Normally recommended Normally not recommended AMETEK Denmark A/S • Gydevang 32-34 • 3450 Allerød • Denmark Phone: +45 4816 8000 • Fax: +45 4816 8080 • E-mail: [email protected] • www.jofra.com

Industrial temperature measurement AMETEK® CALIBRATION INSTRUMENTS 5.3 Tolerances We distinguish between three tolerance classes. Class 1 is the most accurate. Class 2 is standard, i.e. this is the quality that is delivered if nothing else is mentioned. Below you will find an overview. Note 1) : The highest value is valid. I.e. for example for type K class 2 the tolerance AMETEK Denmark A/S • Gydevang 32-34 • 3450 Allerod • Denmark Phone: +45 4816 8000 • Fax: +45 4816 8080 • E-mail:

Industrial temperature measurement Temperature-voltage-relation The temperature-voltage-relation is not based on a simple relation. The size of the thermal power or the thermal voltage has a most complicated relation to the temperature. This appears from the fact that it takes a high power polynomial to describe this relation. Additionally tables calculated from these with polynomial are available with thermal voltages per °C. Compensation cable The thermocouple goes from the hot junction in the sensor to the cold junction, which is normally found in the instrument, i.e. in principle a thermo...

Industrial temperature measurement AMETEK® CALIBRATION INSTRUMENTS Colour Codes for compensation cables 5.6 Applications and limitations A good knowledge of the conditions under which the measurement shall take place gives the best possibility for choosing the right type of thermocouple. For the thermocouples most often used the following should be observed as to applications and limitations: 5.6.1 Type J. Fe-CuNi thermocouple * Cannot be used below 0°C, because condensation on the Fe-conductor will cause rusting. * Most suitable for reducing atmospheres * May be used up to 500°C as a max. Above...

Industrial temperature measurement ∗ In the range from 200 to 500°C a so-called hysteresis effect may occur, i.e. you do not reach the same value in heating and cooling. There may be a difference of up to 5°C. ∗ To be used in neutral or oxidizing atmospheres. ∗ Shows too low temperature after ageing. ∗ Is not recommended to be used in reducing atmospheres. In this case chromium will come out of the NiCr-conductor, the so-called migration. The thermocouple will change thermal voltage and show too low temperature. This error effect is called Green-rot. ∗ Sulphurous atmospheres are poisonous, because...