What is rtd temperature

As a result, only the small RTD element itself is made of platinum. As a practical matter the resistance value of the RTD element would be useless without a means to communicate that resistance to an instrument. Accordingly, insulated copper wires typically connect the RTD element to the measuring instrument. Like platinum, copper has a resistance value. Resistance along the copper lead wires can impact the resistance measurement determined by the instrument connected to the RTD.

Two wire RTDs do not have a practical means for accounting for the resistance associated with the copper lead wires which reduces the extent to which the resistance measured can be accurately correlated to the temperature of the RTD element.

As a result, two wire RTDs are least commonly specified and are generally used where only an approximate value for temperature is needed. Three wire RTDs are the most common specification for industrial applications. Three wire RTDs normally use a Wheatstone bridge measurement circuit to compensate for the lead wire resistance as shown below. These lengths are significant because the intention of the Wheatstone bridge is to make the impedances of wires A and B, each acting as an opposite leg of the bridge, cancel the other out, leaving Wire "C" to act as a sense lead carrying a very small microamperage range current.

This can provide significantly increased accuracy at the relatively low cost of increased copper extension wire. A protective sheath normally stainless steel houses the sensing element. Platinum is typically the material of choice in an RTD as this allows for a larger temperature range and it is a more stable material. Nickel and copper can also be used in RTD construction if a lower temperature range is required.

Table 1. Outlines usable temperature ranges for each material. Table 2A. Table 2B. Typical choices for wire insulation include nickel, nickel alloys, tined copper, silver plated copper or nickel plated copper.

For RTD construction, there are two methods most used. Each type is best suited for use in certain environments and applications. The invention of a resistance thermometer was made possible by the discovery that the conductivity of metals decreases predictably with increases in their temperatures. The first-ever resistance thermometer was assembled from insulated copper wire, a battery and a galvanometer in However, its inventor, C.

Siemens, soon discovered that a platinum element yielded more accurate readings at a much wider range of temperatures. Platinum remains the most commonly used material in temperature measurement using RTD sensing elements today. Learn More. Because every Pt element in the circuit containing the sensing element—including the lead wires, connectors and the measuring instrument itself—will introduce additional resistance into the circuit. Since the lead wire used between the resistance element and the measuring instrument has a resistance itself, we must also supply a means of compensating for this inaccuracy.

There are three types of wire configurations, 2 wire, 3 wire, and 4 wire, that are commonly used in RTD sensing circuits. A 2-wire configuration with a compensating loop is also an option. The RTD Pt and Pt are available in a similar range of tolerances, and both can have similar temperature coefficients, depending on the purity of the platinum used in the sensor.

When comparing the Pt vs Pt in terms of resistance, keep in mind that resistance value readings for the Pt will be higher by a factor of ten than resistance value readings for the Pt at the same temperature. For most applications, the Pt and Pt can be used interchangeably depending on the instrument used. In some cases the Pt will work better and be more accurate.

The same year that Seebeck made his discovery about thermoelectricity, Sir Humphrey Davy announced that the resistivity of metals showed a marked temperature dependence. Fifty years later, Sir William Siemens proffered the use of platinum as the element in a resistance thermometer. His choice proved most propitious, as platinum is used to this day as the primary element in all high-accuracy resistance thermometers.

Platinum is especially suited to this purpose, as it can withstand high temperatures while maintaining excellent stability. As a noble metal, it shows limited susceptibility to contamination. The classical resistance temperature detector RTD construction using platinum was proposed by C.

Meyers in He wound a helical coil of platinum on a crossed mica web and mounted the assembly inside a glass tube. This construction minimized strain on the wire while maximizing resistance. Meyers RTD Construction Although this construction produces a very stable element, the thermal contact between the platinum and the measured point is quite poor.