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pt100 cable length

Created by: john_smith at: 7/21/2009 11:09 AM (4 Replies)

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7/21/2009 11:09 AM	Rate (0)
john_smith Regular Member Joined: 5/5/2009 Last visit: 4/20/2015 Posts: 8 Rating: (0)	Hello, I would appreciate your help ... I want to measure temperature with 4 Pt100 sensors using two AM2 PT100 modules, but the place, where I can install LOGO! is about 15 meters from sensors . What do you think about influence of this distance to measured values ? I know, LOGO! manual says ... cable length 10m . Thanks for any idea ...

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7/21/2009 11:59 AM	Rate (3)
LOGO!Barbarian Posts: 173 Rating: (95)	Hi john_smith (Cool name!) Great thing about our PT100 module is that it can be wired to do a 3 wire measurement. What this means, is that if you are using a sensor with 3 wires from it (2 in parallel), then the module can actually determine the 'lead resistance' of the wires and therefore calculate a value of the temperature at the actual measuring point. Unlike 0/4..20 mA signals, the temeprature measurement is done with a resistive platinum 'filament' in the sensor tip. The resistance of this filament alone is theoretically 100 ohms at 0.01 °C (the 'triple point of water' where liquid, solid and vapour phases of water can all co-exist stably). The length of the leads to this sensor introduce more resistance and can also be influenced by thier length, conductor thickness and the temperature that these wires are exposed too, even at a local point if they are going past a hot process or motor or light bulb! (llok up the formula for resistance based on length, thickness and temperature) With 3 wires, the resistance in one leg of the loop is half that in the other lead and the PT100 module (which 'balances' an internal 'wheatstone resistance bridge' circuit) can therefore determine the resistance contributed by the leads and correct the calculated value by this before passing the result to the LOGO! program. If using only a 2 wire sensor, then it is up to the length, quality and thickness of the leads, as well as the ambient temperature and any localised hot-spots. To get the accuracies shown in the manual on page 263, it states to use 10m of shielded wire. As a rough guess, I would say that for the additional 5m to get to 15m, that the accuracy would deteriorate by 50% from ±1.0% (between 0°C ... +200°C) and ±1.5% (between -50°C ... +200°C) to ±1.5% in the positive range and ±2.25% for the full range including the negative portion. A 3-wire sensor/leads combination would theoretically eliminate any worsening of the accuracy. Hope this helps. Look up any of the temeprature terms mentioned above on google for more explanation. (P.S. my background is in temperature and humidity measurements, accuracy, calibration and international standards certifications of these parameters - always fun to see them being applied with LOGO!) Aaaaaarrrggh! ("Have a nice day!" in barbarian speak!)
LOGO!Barbarian Posts: 173 Rating: (95)
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This contribution was helpful to 5 thankful Users dchartier Betel Skip ring24h AutoGen_5172896

7/21/2009 12:26 PM	Rate (0)
john_smith Regular Member Joined: 5/5/2009 Last visit: 4/20/2015 Posts: 8 Rating: (0)	Hi Barbarian, and thanks for your answer, really helped me a lot . My sensors are 4-wire, so as far as I know, one wire will not be connected . Btw, while browsing this forum, I enjoyed your answers ... Have a nice day !

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This contribution was helpful to 1 thankful Users LOGO!Barbarian

7/21/2009 1:45 PM	Rate (0)
Solar Element Posts: 40 Rating: (1)	Hi john_smith, I use every day 3-wire PT100 sensors with LOGO! on my solar thermal systems. I've reached 40 mt max of lenght without any problems. I could bet that we can reach 100 mt without any great problems. Accuracy is proved by lots of thermal systems installed. PT100 3-wire shielded accuracy was proven on one home thermal system with wood burning fireplace and water inside boiling at 99,75-100,00 °C at 30 mt of distance from LOGO! The critical point is the braid shielding that must be reallycontinuous and connected to PE LOGO!connector. Hope this helps.. Paolo
Solar Element Posts: 40 Rating: (1)
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This contribution was helpful to 4 thankful Users Betel miltos LOGO!Barbarian john_smith

10/22/2015 11:01 AM	Rate (0)
Min_Moderator Platinum Member Joined: 9/3/2014 Last visit: 3/13/2019 Posts: 4766 Rating: (122)	New question published by saeedplc is split to a separate thread with the subject S7-200, pt100 cable length. Best regards Min_Moderator

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