Nanmac Theory Presentation
- 1. NANMAC Temperature Sensor Theory Thermocouples And RTDs
- 2. General Thermocouple Theory Thermocouple circuit is made from two metals of different materials being electrically connected. This connection is called the thermocouple junction. The output of a thermocouple is a milliVolt signal. The higher the temperature, the higher the milliVolt output, this is called Electro Motive Force (EMF)
- 3. General Thermocouple Theory The type of metals used to make the thermocouple element determines what the temperature range of the thermocouple is. Each thermocouple Type , has a specific accuracy as determined by the US National Institute of Standards (NIST). Accuracy is based on a percentage of the temperature being measured.
- 4. General Thermocouple Theory Type T, Copper/Constantan: (-)185 to 370’C. Accuracy is +/- ¾ % Type J, Iron/Constantan: 0 to 760’C. Accuracy is +/- ¾ % Type E, Chromel/Constantan: 0 to 870’C. Accuracy is +/- ½ % Type K, Chromel/Alumel: 0 to 1230’C. Accuracy is +/- ¾ % Type N, Nicrosil/Nisil: 0 to 1230’C. Accuracy is +/- ¾ % Type S, Pt/Pt-10%Rhodium (Rh): 0 to 1480’C. Accuracy is +/- ¼ % Type B, Pt-6%Rh / Pt-30%Rh: 870 to 1705’C. Accuracy is +/- ½ % Type C, Tungsten-5%Rhenium / Tungsten-26%Rhenium: 426 to 2315’C. Accuracy is +/- 1%
- 5. General Thermocouple Theory The design of the thermocouple determines how the thermocouple will react to the temperature it is measuring. The larger the thermocouple junction is, the slower the thermocouple response time is The smaller the thermocouple junction is, the faster is it is, and the more delicate it is. Protection tubes such as stainless steel, alumina or molybdenum prolong the life of the thermocouple, and slow down the response time.
- 6. General Thermocouple Theory Several factors must be accounted for when designing a thermocouple. What is the expected temperature: Lowest and highest? Is it a long-term measurement such as in an oven or furnace, or is it a short term test? What is the response time requirement? What is the environment; air, neutral, vacuum, flame? How will the thermocouple be mounted? What is the size and shape of the thermocouple – do you have a drawing?
- 7. General Thermocouple Theory In the situation of special tests: What is the material that is being tested? What is the specific temperature of interest:? Surface temperature of an object such as an exhaust nozzle Gas temperature of a flame or explosion Fluid or gases flowing within a pipeline
- 8. General RTD theory Resistive Temperature Detectors (RTDs) are temperature sensors that change resistance as temperature changes. Temperature range is from –200 to 600 Deg C. Accuracy is +/- ½ % of the temperature being measured, and can be even better for high accuracy RTD More delicate and sometimes more expensive than thermocouple. RTDs are usually either Platinum or Nickel and have several different resistances, such as: 100 Ohm Pt or 1000 Ohm Pt 100 Ohm Nickel or 200 Ohm Nickel The same design questions must be asked for RTDs.