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Monitoring Temperature |
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Index
ThermocouplesPrincipals of operation Thermocouples work by the thermoelectric effect, whereby a small voltage is generated when a temperature difference is applied across two junctions of dissimilar metals. The reference junction which is usually called the cold junction (although is can be hotter than the "hot" junction) is either kept a constant temperature by a temperature controlled oven, or (more often), its temperature is monitored and the difference used to calculate the temperature at the hot junction. This cold-junction-compensation technique is the one used for the NP4000 instrumentation. One particular feature of thermocouples is that the hot junction can be soldered. In fact a reading is given even if the junction is separated by another piece of metal. They are available in the following types:
InterfacingThe correct input module for the particular thermocouple should be used. This is because the cold junction compensation is done in the analogue side by an RTD and therefore the correct slope for room temperature should be used. Thermocouple input modules have a green connector and should be used with the green terminal blocks. These have a built in RTD, so the distance between the junction and the compensation device is a small as possible.
Sources of supply
RTDPrincipals of operation Resistor-temperature-devices are a good way of measuring temperature using the principle that the resistivity of certain materials varies with temperature. Platinum is the most common form of material, and the resistance of the sensor is usually 100 or 1000 ohms. If the sensor is some distance from the NP4000 data acquisition unit, then either the resistance of the wire should be taken into account, or a 4 wire RTD should be used where the two outer wires are used for excitation (passing a constant current), and the inner wires are used to measure the voltage across the sensor. Interfacing To interface to the NP4000 instrument, you should use a NPAIxxRT module. This gives a constant current of 0.25 milliamperes as a constant current source, thus giving an output of 25 millivolts at 0C, and therefor negligible self heating.
Accuracy Stability is the primary advantage platinum has over all other temperatures sensors. The standards which are in force are IEC 751 which defines repeatability, DIN ???? which defines initial accuracy. Accuracy is normally stated by the DIN???? standard. DIN-B .. 0.1% or 0.3 deg C (at room temperature) The range of platinum RTD's is normally form -200 to +850 deg C Self Heating All RTD's will heat up when a current is passed through them. When using the NP4000 instrumentation, this power is less than 10 microwatts, which will not cause any significant heating for the majority of devices. The self heating effect can be used to good use for example in anemometry, where the amount of air passing across the sensor can be calculated using the formula for cooling. The degree of self heating is usually given in the data sheet for the device. Packaging Sensors come in a wide range of shapes and sizes from thin film devices for OEM applications to devices encapsulated in sealed stainless steel tubes for food measurement and other Sources RTD's are available form the following sources
Non-Contact MeasurementNon-Contact measurement relies on radiometry. The sensor and the source are "radiatively Connected". That is radiation from the source is focused onto a sensor. This can be a small thermocouple, or a pyroelectric detector. Pyroelectric detectors are more sensitive than thermocouples, and are best for long range measurement. The temperature of the sensor is independent of the temperature of the source, and therefore an emissivity adjustment is required. Emissivity is the proportion of radiation power emitted compared to a perfect black body radiator, for example, skin has an emissivity of 0.98, silver has an emissivity of 0.05.
Other temperature sensorsOther temperature sensing methods include Pyrometry using pyroelectric detectors, Lead sulfide, Lead selenide, or in rare cases Mercury cadmium telluride. Another, very precise measurement technique is to use the triple point of some chemical. Water/Ice is the most obvious example. There are two semiconductor worth mentioning: NTC thermistors have a rapidly changing resistance with temperature. The effect is exponential, so is not good for measurement over a wide range of temperatures. Silicon semiconductor devices can be very precise, and have a good variation with temperature. Examples of this type are Analogue Devices LM???, or Linear Technology LT???. They are restricted over their operational range to around -40 to +150C.
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