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How to measure a current loop. crack meter. looping 4-20mA as wire continues to extend


Shanks.J Apr 7, 2021 08:30 PM

Hi Peter

I am attempting to create a program that can effectively communicate with this crack meter to measure displacement of a crack. https://www.sisgeo.com/products/crackmeters-and-jointmeters/item/wire-crackmeters.html .
Manual here : https://www.sisgeo.com/uploads/manuali/Surface_wire_crackmeters_EN_00_12.pdf

The instrument outputs 4-20mA signal, depending on the relative extension of the wire in the crack meter. Initial discussions with supplier led me to believe that the extension would be proportional to current change
i.e. 0m =4mA  while 2m (max extension) =20mA.
As I have now discovered the instrument functions on a current loop, so as the steel wire extends it has several cycles of 240mm moving from 20-4mA then a dead zone then as it extends further it begins its cycle again from 20-4ma.

Is it possible to program a cr6 to measure these variable segments? If for example I am looking at a  2m extension over a long period, how would I program the logger to recognised the new segment as it occurs.
ideally I would be using multiple sensors and shunting the signal with an external resistor and reading the relative mV across the analogue inputs.

I’m open to any ideas or advice. Sisgeo communicated with this device with a dedicated datalogger, but we are more interested in having this as pat of our system with a cr6.

thanks for your help

 

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