Maker Pro
Maker Pro

Why Pt100 have three wires.

V

Vincent

Jan 1, 1970
0
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
 
D

daestrom

Jan 1, 1970
0
Vincent said:
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 ?


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
 
S

Spehro Pefhany

Jan 1, 1970
0
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.

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.
How is the PT100 measure the temperature ?

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
 
R

ripper

Jan 1, 1970
0
You also get 2 wire & 4 wire PT100's

Spehro Pefhany said:
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
http://www.speff.com
 
S

Spehro Pefhany

Jan 1, 1970
0
You also get 2 wire & 4 wire PT100's

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
 
V

Vincent

Jan 1, 1970
0
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


Spehro Pefhany said:
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
http://www.speff.com
 
S

Spehro Pefhany

Jan 1, 1970
0
What is the different between 3-wires and 4-wires PT100 in term of its
advantages and disadvantages.

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).
Also,when must use 3 wires and not 4 wires and vice vessus.

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
 
Top