Hottest resistance temperature detector RTD

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Resistance temperature detector (RTD)

RTD has proved to be a stable and accurate temperature measuring device, but it is also the most expensive, whether it is configured with 2-wire, 3-wire or 4-wire. Understanding the advantages and disadvantages of RTD can help you make temperature measurement decisions

among many temperature measurement methods, resistance temperature detector (or resistance thermometer, usually referred to as RTD) is the most accurate method. In RTD, the resistance of the device is positively proportional to the temperature. Although some RTDS use nickel or copper, platinum is the most commonly used resistance material for RTDS. RTD has a wide temperature measurement range. According to its structure, RTD can measure the temperature range of -270 ~ 850 ℃

rtd needs external excitation (usually a current source) to work properly. However, the current will also generate heat in the resistance element, resulting in measurement errors. The temperature error can be calculated by the following formula: △ t = P x s (where t is the temperature, P is the generated power I2R, and the unit of S is ℃/mw)

I. measurement methods

there are many methods of measuring temperature with RTD. One is a 2-wire method to let the current pass through RTD and measure the voltage on it. Its advantage is that only two wires are needed, so it is easy to connect and realize. The disadvantage is that the lead resistance will participate in the temperature measurement, thus introducing some errors

An improvement of the 2-line method is the 3-line method. Although the method of letting the current pass through the resistance and measuring its voltage is also adopted, the third wire can be used to compensate the lead resistance. This requires a third line compensation measurement unit, or it needs to measure the temperature value on the third line and subtract it from the total temperature measurement value

Figure 1 Schematic diagram of typical 2-wire resistance temperature measurement method

Figure 2 typical 4-wire resistance measurement scheme helps to eliminate most of the noise and uncertainty in temperature measurement

the third method is the 4-wire method. Like the other two methods, the 4-wire method also adopts the method of letting the current pass through the resistance and measuring its voltage. However, the current is introduced from one end of the lead and the voltage is measured at the other end. The voltage is measured on the resistance element (RTD) rather than at the same point as the source current, which means that the resistance of lead Qi Jun is completely excluded from the temperature measurement path. In other words, the lead resistance is not part of the measurement, so there will be no error

for example, if the lead resistance is about 100 milliohms and the RTD is 100 ohms, the lead resistance will produce a measurement error of about 0.1%. In the 4-wire method, the lead resistance is not part of the measurement, so it is a more accurate method to measure RTD resistance, because it completely excludes the lead resistance

II. Advantages and disadvantages

rtd has some obvious advantages that it will encourage chlorination and CO production of sulfuric acid to clean the titanium dioxide production process, and eliminate the heavy pollution production process of traditional sulfuric acid method, which is superior to other temperature measuring devices. For example, it is the most stable and accurate of all temperature measuring devices, and its linearity is better than that of thermal couple. However, RTD also has some disadvantages. For example, it is more expensive than thermistors and thermocouples, and requires the use of a current source. Its DR is also small, which means that the resistance used to measure temperature changes is also small. For example, when the temperature changes by 1 ℃, RTD may only change by 0.1 ohm. However, if the 2-wire method is adopted, the lower absolute resistance will also cause measurement error

Although RTD is the most stable and accurate of all temperature measuring devices, it also has some disadvantages. RTD is more expensive than thermistors and thermocouples, and requires a current source. When using RTD, there are several common phenomena that are often not considered by people, and the most serious one is self heating. If RTD generates self heating due to the flow of measuring current, it may cause error. For example, when measuring low temperature (e.g. below 0 ℃), the self heating of RTD will increase the expected measurement temperature. In addition, if the lead (resistance) is not compensated, it will cause greater error. The 4-line method is helpful to eliminate this error. Another phenomenon is that an appropriate RTD temperature range is not selected. Trying to measure the temperature outside the RTD temperature range can cause greater error and even damage the sensor (RTD). Therefore, we should always choose the appropriate RTD for the expected measurement. (end when installing the handsaw)

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