2 Wire RTD-Resistance Temperature Dector
2 Wire RTD
The two wire RTD is the simplest RTD wire configuration. One wire is attached to each side of the element. A measure can be taken by any device equipped to measure resistance, including basic volt Ohm Meter. This is the least accurate way of measuring temperature, due to the fact that the lead wire resistance is in series with the sensing element. The lead wire is at a different temperature than the sensing element and also has a different resistance verses temperature characteristics. The longer the lead wire the greater the effect on the temperature measurement. A typical two wire configuration looks like the following:
As stated previously, a Resistance Temperature Detector (RTD) element generally appears in a sheathed form. Obviously, all of the criteria applicable to resistance elements also apply here, but rather than element size, the construction and dimensions of the entire RTD assembly must be considered. 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. Refer to Figure 2 for the 2-wire configuration.The circle represents the resistance element boundaries to the point of calibration. 3- or 4-wire configuration must be extended from the point of calibration so that all uncalibrated resistances are compensated.
The resistance RE is taken from the resistance element and is the value that will supply us with an accurate temperature measurement. Unfortunately, when we take our resistance measurement, the instrument will indicate RTOTAL:
RT = R1 + R2 + RE
This will produce a temperature readout higher than that actually being measured. Many systems can be calibrated to eliminate this. Most RTD's incorporate a third wire with resistance R3. This wire will be connected to one side of the resistance element along with lead 2.