Oakton Conductivity meter

[Steve Bailey]

I'm trying to get the measured conductivity from an Oakton CON2700 conductivity meter into a PLC. The unit has an RS232 output for a printer. Instead of connecting to a printer I plan write a script in the machine's HMI to read the stream of ASCII characters coming out of the RS232 port, decode the conductivity value from that, and pass the value to the PLC.

I purchased a cable from Cole-Parmer that was listed in the Oakton manual, but as soon as I connect it to the serial port on my laptop the measured conductivity value jumps by a significant amount and becomes unstable.

Is there anyone familiar with the conductivity meter who can offer any suggestions?

 

[danw]

Ground Loop.

It can be between the measured fluid, the analyzer or the RS-232 ground on the laptop.

If the measurement floats, like in a glass beaker, and the offset from the ground loop disappears, you're probably stuck, because the process is not at the same ground potential as the final data destination, the Laptop COM port.

Many times, the ground loop occurs when the laptop is charged by AC, but disappears when the laptop goes to battery power.

If the serial port is through a USB converter, there isolated USB/232 converters that will solve the problem, if that's where the ground loop is.

Industrial conductivity analyzers are probably isolated, have Modbus, panel mount and are not much more than the bench top lab analyzer, but maybe you need that lab unit.

 

[BryanG]

I didn't know if one existed but I websearched for 'RS232 Isolator' and got lots of hits ranging from £4.00 on ebay to $200.00 from http://www.bb-elec.com. Can't advise which would be best for the job but as long as it is self powered and opto-isolated I think it would be OK.