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How one can find common wire in 3-wire RTD ,with the help of multimeter.
<pre>
----------- (1)
|
|
-----
| |
| |
| | 10 ohm Copper RTD
| |
| |
-----
|\
| \
| \
| ------- (2)
|
|
----------- (3)
</pre>
The above drawing is of a typical three-wire RTD. Two leads (#2 and #3) are "commoned" at one side of the RTD.
If you set the multimeter to resistance, and with the RTD disconnected from any monitor or other circuit, you apply the leads to (1) and (2), you will read something around 10 ohms, the value of the RTD resistance because you are effectively reading "through" the RTD.
If you apply the meter to leads (1) and (3) you will obtain the same reading, because you are still reading "through" the RTD.
If you apply the meter to leads (2) and (3), you will read a very low resistance (something around 1 ohm or less), because those two leads are commoned at one side of the RTD and you are <b>not</b> reading "through" the RTD.
The two commoned leads usually have the same color wire insulation, and the single lead usually has a different color wire insulation.
Either of the two leads that are commoned can be considered to be the "common" wire; they effectively the same wire for all intents and purposes. There is no difference between them as far as the RTD or circuit is concerned, because the RTD is nothing more than a variable resistance. A three wire RTD just has two wires on one side of the variable resistance, and one wire on the other side of the variable resistance.
Use your preferred Internet search engine and you will find many references to RTDs on the world wide web.
----------- (1)
|
|
-----
| |
| |
| | 10 ohm Copper RTD
| |
| |
-----
|\
| \
| \
| ------- (2)
|
|
----------- (3)
</pre>
The above drawing is of a typical three-wire RTD. Two leads (#2 and #3) are "commoned" at one side of the RTD.
If you set the multimeter to resistance, and with the RTD disconnected from any monitor or other circuit, you apply the leads to (1) and (2), you will read something around 10 ohms, the value of the RTD resistance because you are effectively reading "through" the RTD.
If you apply the meter to leads (1) and (3) you will obtain the same reading, because you are still reading "through" the RTD.
If you apply the meter to leads (2) and (3), you will read a very low resistance (something around 1 ohm or less), because those two leads are commoned at one side of the RTD and you are <b>not</b> reading "through" the RTD.
The two commoned leads usually have the same color wire insulation, and the single lead usually has a different color wire insulation.
Either of the two leads that are commoned can be considered to be the "common" wire; they effectively the same wire for all intents and purposes. There is no difference between them as far as the RTD or circuit is concerned, because the RTD is nothing more than a variable resistance. A three wire RTD just has two wires on one side of the variable resistance, and one wire on the other side of the variable resistance.
Use your preferred Internet search engine and you will find many references to RTDs on the world wide web.
S
Sam
Hi there,
Very well explained by CSA.
Something else to keep in mind is that a RTD is also sometimes refer to as a PT100 since that is the most common type in use. This refer to the fact that at 0 degrees C the resistance of the RTD is 100 Ohm. So at ambient temperature you normally would read about 110 to 113 Ohm.
a Good method in the beginning is to make a drawing as per CSA's post and write in all the values as you read them with the multimeter. Later on you will be able to do it without a drawing and within a couple of seconds in the field.
Very well explained by CSA.
Something else to keep in mind is that a RTD is also sometimes refer to as a PT100 since that is the most common type in use. This refer to the fact that at 0 degrees C the resistance of the RTD is 100 Ohm. So at ambient temperature you normally would read about 110 to 113 Ohm.
a Good method in the beginning is to make a drawing as per CSA's post and write in all the values as you read them with the multimeter. Later on you will be able to do it without a drawing and within a couple of seconds in the field.
