Why Pt100 have three wires.

Can anyone tell me why PT100 must use three whereas two of the wire is internal linked.Why can not connect like thermocouple only two wires.

How is the PT100 measure the temperature ?

Vincent

Reply to
Vincent
Loading thread data ...

By 'PT100' I *think* you mean a platinum RTD. If so, then here is some information, otherwise, just ignore all this ;-)

A Resistance Temperature Detector (RTD) is used to measure the temperature of something. The electrical resistance of most materials changes with temperature. In many substances, the change is not very linear, so calculating the temperature based on the measured resistance is a little complicated. Platinum is one material where the resistance changes almost perfectly linearly with temperature over a range from 0 to several hundred degrees. So it is often used for RTDs as this makes it easier to calculate the measured temperature.

But measuring the resistance of a piece of platinum wire that is located some distance from the measuring equipment is complicated by the electrical leads used to connect the wire to the equipment. These leads are made from

*something* (usually copper wire), and that *something* changes its resistance with temperature as well. So to improve the accuracy of the measurement, it is best to try and compensate for changes in these lead's resistance.

One way to accurately measure the resistance of anything, is with a circuit know as a 'Wheatstone Bridge'. (no, I don't know where the name came from, but since it is usually capitalized, it is probably the name of the inventor). This arranges two fixed resistors, a calibrated variable resistor and the unknown resistance in a 'diamond' configuration with a 'null detector' (often just a galvanometer) connected across the left and right points of the 'diamond'. When a voltage is applied across the top and bottom of the diamond, the variable resistor is adjusted until the 'null detector' shows the left and right sides are at equal potential. With knowledge of the fixed and variable resistors, the unknown resistance can be calculated.

Now, why three leads. As you say, two of the leads are 'internally linked'. Take one of these and connect it so the voltage source of a Wheatstone bridge. Take the other of the two internally linked leads, and connect it to the variable resistor of your 'bridge'. Finally, take the third lead and connect it to 'null detector' and fixed resistor on one side of the 'diamond'. You now have one lead in the 'unknown resistance' leg, and one lead in the 'variable resistor' leg of the bridge. The third lead isn't in either leg of the bridge, it is in the power lead coming into the bridge. The 'top' of the bridge is at the 'internally linked' connection of those two leads.

If you have made all the leads the same length, and they are the same temperatures (as they would be if in the same cable), then the lead resistance in one side of the bridge legs is exactly the same as the lead resistance in the other. So any changes in lead resistance occurs in *both* sides and has no effect on your reading.

Some RTDs come with four leads. Two are 'internally linked' on one side of the resistance element, and the other two are 'internally linked' on the other side. This type can be used in different circuits to provide for even greater accuracy in measuring the resistance of the element.

Thermocouples work on an entirely different principle than RTDs Dissimilar metals,when joined create a minute voltage potential that is a function of the junction's temperature. But I'll save that explanation for another time.

daestrom

Reply to
daestrom

Leadwire (resistance) compensation. There are a variety of schemes.

At the instrument, the equivalent circuit is this:

+----------------o | | | / .-. Rx |/| | | /'-' | | .-. | | Rt | | '-' Rx | __/ +---|_/_|--------o | / .-./ Rx |/| | | /'-' | +----------------o

Wher Rx is the resistance of the leadwires, and Rt varies according to temperature as per the tables. The DIN standard alpha = 0.00385 is the most common these days, though you could run into old US standard 0.00392 units conceivably.

Anyway, the instrument uses the matching of the 3 Rx resistances to cancel out the effect of the leadwires, assuming they are matched. In some cases the resistance might be quite significant. If the leadwires are only a few inches long, it's not necessary.

The resistance varies fairly linearly with temperature, according to the tables. The instrument uses current sources and such like to put constant or first-order constant currents through the element and leadwires and then applies signal conditioning. Often linearization and break protection are requirements as well.

Best regards, Spehro Pefhany

Reply to
Spehro Pefhany

You also get 2 wire & 4 wire PT100's

formatting link
Embedded software/hardware/analog Info for designers:
formatting link

Reply to
ripper

True. 3-wire is pretty much the standard for industrial applications, including process control, wheras scientific applications tend to prefer 4-wire. They're all 2-wire at the business end. ;-)

Best regards, Spehro Pefhany

Reply to
Spehro Pefhany

What is the different between 3-wires and 4-wires PT100 in term of its advantages and disadvantages.Also,when must use 3 wires and not 4 wires and vice vessus.

Thanks

Vincent

formatting link
Embedded software/hardware/analog Info for designers:
formatting link

Reply to
Vincent

Four wire types are theoretically better**, but just about nobody uses them in industry hence you'll find few industrial instruments that support the extra wire (of course you can ignore it). Also the extra wire costs money to run. Neophytes and scientists tend to be attracted to it because it's theoretically perfect (3 wire setups can introduce double the error for some contact resistance and wire mis-match effects).

Depends on accuracy requirements vs. cost etc.. Now, in 2003, I think I'd rather see a loop-powered transmitter out in the field and two non-critical wires run back to the instrumentation rather than 2, 3 or

4 low-level wires. If you need IS barriers then that might affect the choice directly or through the cost equation as well. ** They eliminate effect of all contact resistance, leadwire resistance even if the wires are not matched in resistance or are at different temperatures, and so on. This is because the energizing current is completely seperate from the measuring connections. Google on "Kelvin connection" for more information.

Best regards, Spehro Pefhany

Reply to
Spehro Pefhany

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.