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why the max cable length depends on logo?

Created by: miltos at: 10/27/2010 8:28 PM (3 Replies)

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10/27/2010 8:28 PM	Rate (0)
miltos Bronze Member Joined: 3/4/2009 Last visit: 10/28/2020 Posts: 57 Rating: (5)	Hi! I was wondering if AM2 PT100 would have any problem at 50m distance. Solar element made me feel better. I cant undestand why the max cable length depends on logo. It should depend on the enviroment of the cable. How is logo involved on max cable length? Splitted from pt100 cable length.
	Last edited by: O_Moderator at: 10/28/2010 9:15 AM new subject after splitting
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11/4/2010 12:38 PM	Rate (0)
Helmut_13 Posts: 2930 Rating: (432)	Hi, LOGO! only allowes Pt100 in 3 wire. To eliminate the wire resistance a4 wire connection is requested. Longer cable lenght will effect the meassure value. Try and extend and see what happens.
Helmut_13 Posts: 2930 Rating: (432)
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11/4/2010 6:40 PM	Rate (0)
miltos Bronze Member Joined: 3/4/2009 Last visit: 10/28/2020 Posts: 57 Rating: (5)	Hi! Cable resistant compensation of 3 wire connection isn't good enough? The effect of cable resistance can be eliminated by programming an offset.

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11/16/2010 9:00 AM	Rate (0)
LOGO!Barbarian Posts: 173 Rating: (95)	Hi miltos! Usually a 3 wire Pt100 connection is sufficient to eliminate 'lead resistance', but there are limits. Inside the AM2 PT100 or AM2 RTD modules, there is a resistance 'Wheatstone Bridge' configuration (per sensor channel) and also a small micro controller that does the calculations and makes the necessary compensation. The sensor Pt100 or Pt1000 is a resistor (100 ohms at 0 deg C or 1000 ohms at 0 deg C) that forms one part of this bridge. The small internal controller compensates for changes in this resistance to balance the bridge. The resulting small internal voltages and currents are then interpreted as the 0..1000 signal that gets passed to the LOGO!. If your leads go through some harsh environments where there are large temperature gradients etc, then the resistance in the leads will be too high for the small range over which the internal controller can 'reverse-compensate' and you will simply get abnormal readings. Since laed resistance is related to temperature gradient, conductor cross section and length, you could try compensating for longer leads, with thicker gauge wire to the sensor (all 3 of them), since you cannot do anything about the temperature gradients the leads are going through. Like Helmut_13 said - try it and see what happens! Cheers
LOGO!Barbarian Posts: 173 Rating: (95)
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