PECM Issue 32 2018 | Page 10

In order to maintain the efficiency and prolong the service life of temperature meters, proper selection strategy must be followed. The wire metals can be an alloy or a very pure metal, and the measurement technique works on the principle of a thermo voltage being created when two different metals are brought into contact with one another. A variety of temperature limits and materials of construction are available to measure temperature for different applications. They are: copper/constantan; chromel/alumel; platinum/rhodium- platinum; and iron/ constantan. To achieve high accuracy, thermocouples used for process measurements must be protected by a thermowell. For thermal contact with the thermowell, a physical contact or thermally conductive lubricant is placed between the thermocouple and the thermowell. For installations at low temperatures, extreme precautions must be taken to reduce sources of moisture. With their ability to measure a wide range of temperatures, thermocouples are used in furnaces, salt baths, heat treating, molten metal, and ceramics applications. I nfrared Infrared (IR) thermometers are non- contact temperature measurement devices that have many industrial applications. They measure temperature by detecting the amount of radiation emitted by a surface. To function properly, an IR instrument must take into account the emissivity of the surface being measured. The most basic design consists of a lens to focus the infrared energy onto a detector, which converts the energy to an electrical signal that can be displayed in units of temperature after being compensated for ambient temperature variation. With the ability to measure temperature measurement from a distance without contact with the object, an IR thermometer is useful for measuring temperature under circumstances where thermocouples or other probe type sensors cannot be used or do not produce accurate data. The non-contact advantage is particularly relevant for food industry applications. R esistance (RTD) Resistance thermometers or RTDs (resistance temperature devices) work on the principle of the resistance of a material changing with temperature. Platinum, nickel and copper are regularly used for RTDs because the change in resistance with these metals is large. RTDs are used in applications where faster response and greater accuracies are required than may be obtained with thermocouples. Their relatively high electrical output also makes them suitable for direct connection to indicators, controllers, recorders, etc, and RTDs may also be more economical than thermocouples in some installations since the extension wires can be made of copper. In addition, a reference temperature source is not required for calibration S election strategy In order to maintain the efficiency and prolong the service life of temperature meters, proper selection strategy must be followed. Indeed, improper selection strategy is regarded as the most likely cause of measurement failure, which can also result in significant plant downtime. In a worst case scenario, an improperly specified temperature meter can fail to measure temperature during an over and under temperature event, compromising plant safety and damaging valuable equipment. In order to improve temperature meter operating efficiency and service life, as detailed below, there are certain basic selection criteria end users should consider: A ccuracy The closeness of a reading of the measurement device to the actual value of the quantity be ing measured, accuracy goes a long way in prolonging the efficiency of a process. Usually expressed as ± percent of full scale output or reading, accuracy specifications can be misleading because they are values usually published by the OEM for ideal boundary conditions. In practice, however, deviations from the ideal conditions are common, so that additional stipulations must be made for the instrument accuracies. It is also important to establish if the accuracy is based on the measured value (% of rate) or on the range end value (% of max). P rocess media In selecting a particular temperature meter for installation, the phase media compatibility with the device must be taken into consideration, otherwise, corrosion, wear, etc is likely will occur. Corrosion of parts will eventually cause 10 PECM Issue 32