| Measuring temperature electricallyElectrical | | | | generate a voltage, RTD's require a power source |
| temperature measurement is found in a wide | | | | to operate and use electrical resistance. Ideally, |
| variety of industries. From melting steel to baking | | | | the resistance will vary linearly with temperature. |
| cookies, the range of temperatures can vary | | | | Resistance thermocouples are most often made |
| greatly and the type of sensor required to | | | | using platinum, due to its linear |
| measure these temperatures must be chosen | | | | resistance-temperature relationship as well as its |
| accordingly. | | | | chemical inertness. The platinum detecting wire |
| ThermocouplesA thermocouple is a | | | | needs to be guarded from contamination to |
| temperature-sensing element which converts | | | | remain stable. Commercial platinum grades are |
| thermal energy directly into electrical energy. In its | | | | produced which exhibit a change of resistance of |
| basic form it consists of two dissimilar metallic | | | | 0.385 ohms/°C (European Fundamental |
| conductors connected in a closed loop. Each | | | | Interval) The sensor is usually made to have a |
| junction forms a thermocouple. If one | | | | resistance of 100 ? at 0 °C. |
| thermocouple is maintained at a temperature | | | | Resistance thermometers require a small current |
| different from that of the other, an electrical | | | | to be passed through in order to determine the |
| voltage proportional to this temperature | | | | resistance. This can cause self-heating so it is |
| difference will be produced by the circuit. | | | | important to minimize the current to reduce |
| Thermocouples are interchangeable, cheap, have | | | | self-heating errors. Care should also be taken to |
| standard connectors, and can measure a wide | | | | avoid any strains on the resistance thermometer |
| range of temperatures. The main limitation of | | | | in its application. Lead wire resistance should be |
| thermocouples is their accuracy; system errors of | | | | considered, and adopting three and four wire |
| less than 1 °C can be difficult to achieve. | | | | connections can eliminate connection lead |
| The thermoelectric effect (Seebeck effect) | | | | resistance effects from measurements. |
| In 1822, physicist Thomas Johann Seebeck | | | | RTD wiring Configurations |
| accidentally discovered that when any conductor | | | | - Two-wire configuration The simplest resistance |
| is subjected to a thermal gradient, it will generate | | | | thermometer configuration uses two wires. It is |
| a voltage. Any attempt to measure this voltage | | | | only used when high accuracy is not required as |
| involves connecting another conductor to the | | | | the resistance of the connecting wires is always |
| "hot" end. This additional conductor will then also | | | | included with that of the sensor, thus leading to |
| experience the temperature gradient, and develop | | | | errors in the signal. This applies equally to balanced |
| a voltage of its own which will oppose the original; | | | | bridge and fixed bridge systems. The values of |
| the magnitude of this effect depends upon the | | | | the lead resistance can only be determined in a |
| metal in use. If there is the same temperature at | | | | separate measurement without the resistance |
| the two junctions there is no flow of current | | | | thermometer sensor and therefore a continuous |
| since the partial voltages produced at the two | | | | correction during the temperature measurement |
| points cancel each other. Using a dissimilar metal | | | | is not possible. |
| to complete the circuit will have a different | | | | - Four-wire configuration The four wire resistance |
| voltage generated, leaving a small difference | | | | thermometer configuration even further increases |
| voltage available for measurement, which | | | | the accuracy and reliability of the resistance being |
| increases with temperature. This difference can | | | | measured. A standard two terminal RTD is used |
| typically be between 1 to 70 µV/C? for the | | | | with another pair of wires. One pair carries the |
| available range of metal combinations. Certain | | | | excitation current while the other pair carries the |
| combinations have become popular as industry | | | | reulting RTD voltage back to the measurement |
| standards, driven by cost, availability, convenience, | | | | system. No current flows in the second pair, so |
| melting point, chemical properties, stability, and | | | | lead resistnace has no effect on accuracy. |
| output. | | | | ThermistorsThermistors are a type of resistor |
| Connecting a thermocouple | | | | with an electrical resistance that possesses either |
| When choosing a thermocouple, consideration | | | | a negative or positive temperature coefficient of |
| should be given to the insulation, thermocouple | | | | resistivity. Thermistors are composed of solid |
| type, and probe construction. | | | | semiconducting materials with a resistance that |
| RTD's Resistance Temperature Detectors | | | | decreases 4% per °C. They are constructed |
| (RTDs), also referred to as platinum resistance | | | | in a variety of sizes and may be obtained with |
| thermometers (PRTs) or resistance | | | | thermal time constants of a millisecond or less. |
| thermometers, are temperature sensors that | | | | Thermistors produce a non-linear voltage and |
| change resistance at a predetermined rate in | | | | because of this are limited to a useful |
| response to variation in temperatures. RTD's are | | | | temperature span of only about 100°C. |
| rapidly replacing thermocouples in many industrial | | | | Thermistors are the most accurate of the |
| applications below 600? C due to their higher | | | | temperature sensors, ~±0.02 °C, as |
| sensitivity and accuracy over thermocouples. | | | | well as the most sensitive. Their response time is |
| RTDs types | | | | short in relation to RTD's, and about the same as |
| - Film thermometers Film thermometers have a | | | | thermocouples. |
| thin layer of platinum (as thin as 1 micrometer) on | | | | Connecting a Thermistor The resistance of |
| a substrate. Advantages of this type of RTD are | | | | thermistors is normally several orders of |
| relatively low cost and fast response. Such | | | | magnitude greater than any lead resistance. The |
| devices have improved in performance although | | | | lead resistance therefore, has a negligible effect |
| the different expansion rates of the substrate | | | | on the temperature reading and thermistors are |
| and platinum give "strain gauge" effects and | | | | almost always connected in a 2-wire configuration. |
| stability problems. | | | | In an industrial environment, it is not always |
| - Wire-wound thermometers Have greater | | | | practical to measure temperature locally. Many |
| accuracy, especially for wide temperature ranges. | | | | times the temperature will need to be measure |
| The coil diameter provides a compromise | | | | remotely at one location and then relayed back to |
| between mechanical stability and allowing | | | | a computer for processing and recording. To |
| expansion of the wire to minimize strain and | | | | provide the electrical signal necessary for these |
| consequential drift. | | | | measurements it is common to use |
| How do RTD's work? Resistance thermometers | | | | thermocouples, RTD's, and thermistors interfaced |
| offer greater stability, accuracy and in some | | | | to a remote device such as an Ethernet data |
| cases repeatability than thermocouples. While | | | | acquisition system to capture the electrical signal |
| thermocouples use the thermoelectric effect to | | | | and relay it back to a user interface. |