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Temp.
| Question | Answer |
|---|---|
| Thermocouple | electrical thermometer consisting of 2 dissimilar metal wires joined @ one end, and a volt meter at the other end of the 2 wires |
| Thermocouple junction | the point where the 2 dissimilar wires are joined |
| hot junction | (measuring) end of the thermocouple that is exposed to the process to be measured |
| Cold junction | (reference) end of the thermocouple that is kept at a constant temperature to provide a reference point |
| Seeback effect | a thermoelectric effect where continuous current is generated in a current where the junctions of 2 dissimilar metals are kept @ different temperatures |
| Peltier effect | thermoelectric effect where heating and cooling occurs at the junctions of 2 dissimilar conductive materials when a current flows through the junctions |
| Thomson effect | thermoelectric effect where heat is generated or absorbed when an effective current passes through a conductor in which there is a temperature gradient. There are 3 types. |
| Positive thomson effect | heat is generated as a current flows through the object from hot to cold; heat is absorbed from cold to hot. Copper & zinc |
| Negative thomson effect | heat is absorbed as current flows through the object from hot to cold; heat is generated as current flows from cold to hot. Iron, nickel, cobalt |
| Zero thomson effect | no heat is generated or absorbed. Lead only |
| seeback voltage | peltier effect voltage @ junctions & Thomson effect voltage along dissimilar wires |
| law of intermediate temperatures | temperature @ the end of wires determines the electrical potential regardless of the intermediate temperatures |
| laws of intermediate metals | other metals may be used in a thermocouple circuit as long as the junctions are at the same temperature |
| Constantan | Copper-Nickel |
| Chromel | Nickel-Chromium |
| Alumel | Nickel-Aluminum; (magnetic) |
| Type J Thermocouple | iron / constantan (most common – can’t be used above 1400oF) |
| Type K Thermocouple | chromel / alumel |
| Type T thermocouple | copper / constantan |
| type E thermocouple | chromel /constantan |
| Types R, S, B thermocouples | contain platinum |
| thermopile | electrical thermometer consisting of several thermocouples connected in series to provide a higher voltage output. Voltages of each thermocouple are added together |
| Grounded Thermocouples have a faster temperature response time than ungrounded | because the heat is conducted through the metal walls |
| If one of the wires in a thermocouple breaks | the voltage @ the cold joint will read zero |
| When the hot & cold junctions are the same | the reading will be zero |
| RTD | consists of a high-precision resistor with resistance that varies with temperature, a voltage or current source, & a measuring circuit. Resistance increases with an increase in temperature |
| wheatstone bridge | circuit used to measure the resistance change of an RTD |
| thermistor | a temperature-sensitive resistor consisting of solid-state semi-conductors made from sintered metal oxides & lead wires, hermetically sealed in glass. Higher resistance than RTDs |
| Infrared Radiation Thermometer | measures IR radiation emitted by an object to determine the object's temperature |
| IR detector | provides an electrical output proportional to the amount of infrared radiation focused on it |
| Emissivity | the ability of a body to reflect thermal energy |
| Transmisivity | the ability of objects – like glass – to allow IR radiation to pass through. (Ex. Feeling the sun trough a window on a sunny day) |
| Pyrometer | used to measure temperatures beyond the range of a mercury thermometer |
| thermal imager | an infrared device hat uses a two-dimensional array of IR detectors to generate a thermal image, and may be non-radiometric or radiometric |
| infrared radiation thermometers | are calibrated with a blackbody calibrator. Heated or unheated surfaces whose emissivity is nearly 1.0. Often measured with a certified RTD |
| dry well calibrator | temperature-controlled well or box where a thermometer can be inserted & the output compared to the known dry well temperature. Uses a reference thermometer made of a platinum RTD. Range: -50°F to almost 2200°F |
| microbath | small tank containing a stirred liquid used to calibrate thermometers. Range: -20°F to 400°F |
| Many transmitters include an optical isolator | to eliminate an electrical path for ground currents & other electrical path for ground currents & other electrical noise |
Created by:
bugmenot
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