Heat energy is generated while applying current to excite the RTD element in order to measure its signal. The disadvantage is that they get damaged easily if dropped and the battery powering them eventually runs out. Temperature compensation is not necessary. current source is needed. However, thermistors do not have the accuracy of RTD’s and this probably accounts for thermistors being limited in process instrumentation. These wires are called the compensating lead.The industrial type thermometer is shown in the figure below. To minimize thermal EMF errors, select a Also, using alternating current circuitry and appropriately selected transmitters can eliminate EMF influences.The time constant of an RTD refers to the speed with which its element changes resistance in response to a change in contact temperature.A rapid time constant helps to reduce error in a measurement system that encounters rapid changes in temperature.When we consider the construction of an RTD, we can infer that response time will have a strong dependence on the mass of the sensor element and its insulating structure, as well as the heat transfer capability to the material being sensed.This directly affects the rate at which heat transfers from the outer sensing surface to the core sensing element.Comparatively, because an RTD measures temperature over a larger area, rather than small point of contact like the thermocouple, and because the RTD sensing element must be insulated, it has a much slower response time than a thermocouple.A sensor in a solidly bonded internal assembly would respond twice as fast as one with a single loose interface in the same assembly.A surface RTD will respond more quickly to a surface temperature change.The response time for a given sensor is typically defined as the time it takes the sensor to reach 63% of its final value at thermal equilibrium in response to a step-change in contact temperature.These times are typically expressed as measured in water flowing at 1m/sec (3ft/sec), and/or in air flowing at 3m/sec (10ft/sec).Although less common, sometimes the response time will refer to the time interval for the Platinum RTD to reach 90% of its final value (as opposed to 63%).Be sure to make note of this distinction when making comparisons between sensor types.A time response-related error can be produced during temperature transients if the PRT (RTD) cannot respond to the change in temperature fast enough.During steady-state or near-steady-state operation, this error is zero.
fragile. It cannot record temperature automatically or electronically. easily calibrated. The output of a thermocouple is very small, typically only around 0.001 volt at room temperature, increasing as temperature rises. The self-heating that occurs will drive error in temperature measurement.
slow response. As RTD elements are fragile, they are often housed in protective probes.The following are the main disadvantages of Resistance Temperature Detectors are :Heat energy is generated while applying current to excite the RTD element in order to measure its signal.The self-heating that occurs will drive error in temperature measurement. It gives more accurate readings. The melting point of the thermometer is 1800° Celcius. The advantages of digital thermometers are that they are inexpensive, easy to read, require very little maintenance and give an accurate reading. But when platinum measures the temperature higher than 1200°C they start evaporating. There are some advantages of resistance temperature detector (RTD) which are given below. The platinum sensing coil is enclosed inside a bulb which is either made of glass or Pyrex. The terminals of the copper wire are joined together with the help of the thick copper lead. Alcohol thermometer (compared to a mercury thermometer) lower freezing point (-114 ºC) less durable (alcohol evaporates) larger coefficient of expansion. The material has an accurate resistance/temperature relationship which is used to provide an indication of temperature.