Instrumentation Control: Resistance Temperature Detectors

Instrumentation Control Resistance Temperature DetectorsAbstractWithin manufacturing, process control is thorough in order to routinely produce high quality parts. Process control is an engineering discipline which can be defined as maintaining the output of a specific process within a desired range. 1Temperature regulation is omnipresent in the world we live, for example, e genuinely living creature has autonomous embodied functions to regulate temperature. Artificially, central heating systems work in the same way, the user sets the desired temperature and the system detects the current state and makes adjustments to the temperature.A opposition temperature detector (hereby referred to as an RTD) is a temperature sensor which exploits the basic principles of science to play a key role in process control.1. Literature ReviewAn RTD is a temperature sensor which contains a resistive element. As the elements temperature changes so does its value of electrical resistance. It is thi s predictable stead which enables us to control the temperature of a chosen substance or environment. RTDs are vigorous known for their truth, stability and repeatability. The electrical resistance related to the variation of temperature is given as the Callendar-Van Dusen equationWithin a basic RTD the change in resistance of the element is small in comparison to the change in temperature. Therefore a bridge circuit is used to allow the measuring of the voltage recede across the resistor when applying a small constant current. Once the sensing element (see figure 1, below) has been placed where the temperature needs to be monitored or controlled, it will do thermal equilibrium with its environment. At that temperature, the element will exhibit a certain value of resistance, this resistance is calculated and converted into a temperature which is fed prat to the end user. Any change to the temperature of the environment will be represented by the change in electrical resistance of the RTD.There are louver different construction variants of RTD elements, these overwhelmCarbon resistor elements- these elements are cheap and popular. At low temperatures these elements are at their well-nigh reliable and are exceedingly reproducible. Another advantage is that carbon resistor elements often dont suffer from hysteresis.Strain free elements- used in SPRTs (the highest accuracy of all RTDs) and can work up to 961.78C. These elements consist of a wire coil surrounded by inert gas.Thin film elements- a very thin layer of resistive stuff is deposited onto a ceramic substratum and coated in epoxy. These elements are not as stable as wire-wound or coiled elements, and they are only legal over a small temperature range. However they are smaller and cheaper than the standard wire-wound elements.Wire-wound elements- have a greater accuracy over wider temperature ranges. They consist of a duration of pure metal in the form of a wire, usually wrapped around a glass o r ceramic core. The elements are usually very fragile, and therefore, need protecting with a sheath. The diameter of the coil minimises the strain on the element by compromising between mechanical stability and expansion of the material. turn elements- currently the technology of choice, replacing wire-wound elements. They consist of a stress free design which lets the coil expand and contract freely as the temperature changes. The housing is do from ceramic oxide and the coils are threaded through bores, which are packed with fine ceramic powder. These elements are effective up to 850C.The most common materials used as RTD elements are platinum, atomic number 28, copper, balco and tungsten. Platinum is the most popular due to it being very repeatable and quite sensitive, balco and tungsten are very rarely used. Platinum, copper and nickel have temperature ranges of 650C, 300C and 120C respectively and exhibit strong linear properties over a wide range of temperature. (see figure 2, below).2. PT100 SensorsThe most common type of RTD used within industry is the PT100. This RTD has an element of pure platinum and has an electrical resistance of 100 at 0C, hence the key PT100. The resistance of a typical PT100 RTD changes at 0.39 per 1C.2.1 Wiring VariationsTwo wire castAs the name suggests, this PT100 RTD uses two wires. However this configuration does include a possible root word of error where the resistance of the connecting wires is added to the resistance of the sensor. Should a company or individual choose this configuration they can combat this source of error by mounting a temperature transmitter close to the element. This configuration is best used when high accuracy is not required.Three wire configurationThe three wire configuration is the most widely used minimises the effects of the lead resistances. The two leads to the sensor are on adjoining arms, in each arm there is a lead resistance which is cancelled out if the two lead resistances are a ccurately the same.Four wire configurationThe accuracy of the measurement of resistance is increased by using the four-wire configuration. Voltage drop in the measuring leads is eliminated using four- terminalinal sensing.2.2 Properties of PT100 RTDs2.2.1 PrecisionPT100 RTDs are extremely precise. A leeway of 0.75C is applied up to 300C, for temperatures between 700C and 800C a tolerance of 1% is applied.2.2.2 StabilityA PT100 RTD has a long term stability of around 0.2% of the 0% value over a 1000 hour (one year span). This test was conducted in special ovens with look heated to 800C. This property is arguably the greatest property of an RTD.2.2.3 Speed of ResponseSpeed of response is measured by submerging the PT100 sensor in water or air moving at 1 m/s with a 63.2% step change (see figure x). Thermo-electra show that the response metre for one of their PT100 sensors are as follows 5Resistance -element mineral insulatedResistance-element with protection tube and insertDiameter3 -6mm9mmInsertion length100-500mm100-150mmResponse time in water (s)0.6-430Response time in air (s)26-551403. References1 B.R. Mehta, Y. Jaganmohan Reddy. (2014). Batch Automation Systems. In B.R. Mehta, Y. Jaganmohan Reddy Industrial Process Automation Systems Design and Implementation. - Butterworth-Heinemann. 135.2 Correge. (). Head- or cable type mineral insulated probes. purchasable http//www.correge.fr/rtd-sensor-mineral-insulated.html. Last accessed 11th Dec 2016.3 Various. (2016). Resistance Thermometer. Available https//en.wikipedia.org/wiki/Resistance_thermometer. Last accessed 21/12/2016.4 Acromag. (2011). CRITERIA FOR TEMPERATURE SENSOR SELECTION OF T/C AND RTD SENSOR TYPES . Available https//www.acromag.com/sites/ thoughtlessness/files/RTD_Temperature_Measurement_917A.pdf. Last accessed 21/12/2016.5 Thermo-electra. (-). Sensor Response Times. Available http//www.thermo-electra.com/en/producten/technische-informatie/response-times. Last accessed 21/12/2016.6 electrical4u . (2012). Resistance Temperature Detector or RTD Construction and working(a) Principle. Available http//www.electrical4u.com/resistance-temperature-detector-or-rtd-construction-and-working-principle/. Last accessed 22/12/2016.