rtd wheatstone bridge circuits
Westton Bridge in PT100: Each type of metal has a unique composition and has a different resistance to electric current. For most metals, the change in electrical resistance is directly proportional to the change in temperature and is linear over a wide range of temperatures. This constant factor is called the temperature coefficient of electrical resistance. RTD can actually be considered as a high-precision resistor whose resistance varies with different temperatures. By measuring the strength of a metal, its temperature can be determined.
Several different pure metals (such as platinum, nickel, and copper) can be used to make RTD. A typical RTD probe contains a coil of very fine metal wire that allows for high resistance without the need for large space. Platinum RTDs are commonly used as process temperature monitors due to their accuracy and linearity.
A Wheatstone bridge consisting of an RTD, three resistors, a voltmeter and a voltage source is shown in the figure above. In this circuit, when the current in the meter is zero (the voltage at point A is equal to the voltage at point B), the bridge is called empty in equilibrium. This can be a zero point or set on the RTD temperature output. As the RTD temperature increases, the voltage read by the voltmeter increases.
If a voltage converter replaces a voltmeter, a 4-20 mA signal can be generated that is appropriate for the temperature range. As in the case of thermocouples, there is a problem when installing the RTD a short distance from the transmitter . Because the connecting wires are long, the resistance of the wires changes as the air temperature changes. Changes in the resistance of the wire cause an error in the transmitter. To solve this problem, three-wire RTD is used.
The connecting wires (w1, w2, w3) are made of the same length and therefore have the same resistance. The power supply is connected to one end of the RTD and above the Wheatstone Bridge. It can be seen that the resistance of the right foot of the Wheatstone bridge is R1 + R2 + RW2. The left foot resistance of the bridge is R3 + RW3 + RTD. Since RW1 = RW2, the result is that the resistance of the wires is lost and therefore the effect of the connecting wires is lost.
Advantages and disadvantages of RTD
- Response time is very fast compared to thermocouples – in a fraction of a second.
- RTD will not experience thrust problems because it is not a force.
- It is more accurate in its amplitude and is more sensitive to thermocouples.
- In installations that require long wires, the RTD does not require a special format cable.
- Unlikethermocouples , radioactive radiation (beta, gamma and neutron) has the least effect on RTD because the measured parameter is resistance, not voltage.Loss of power or shortening in RTD will cause the temperature to read low.
- Because the metal used for RTD must be in its purest form, they are much more expensive than thermocouples.
- In general, RTD is not able to measure a range of temperatures as a thermocouple.
- Disconnecting the power supply can cause the reading to be incorrect
- Small changes in resistance are measured, so all connections must be tight and free of corrosion, which can cause errors.
- An open circuit in the RTD or wiring between the RTD and the bridge causes a high temperature reading.
- Failure modes:
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