The flow rate in Flow meter display is fluctuating at zero flow

Since your post was split from a thread with RS-232, I can only assume that you are using the RS-232 interface, but the catalog sheet for the 6ES7241-1CH32-0XB0 comm module says that the module is an RS-422 interface, not RS-232.

The Mag8000 catalog sheet says this about the RS-232 interface:
"RS 232 serial interface with Modbus RTU (Rx/Tx/GND), point to point with max. 15 m cable"

I'm too lazy to look up the signal differences between RS-422 and RS-232, but apparently they are not significant enough to prevent communications.

However, neither the Mag8000 nor the comm module mentions "isolation".

RS-422 needs a ground connection as shown by the GND notation in the Mag8000 catalog description.

The symptoms you are experiencing are typical of a ground loop between the comm module and the flow meter.   The ground potential is affecting the flowmeter's sensing electronics and producing the 'noise.'

There are two potential solutions.

1.  The magmeter has a low flow cut-off parameter where flow rates below a cut-off value are represented as zero flow rate.   See the attachment for details.  

2.  The other solution is to use an isolator so eliminate the ground loop.  However, with RS-232 on one side and RS-422 on the other side, it's an issue to find an isolator.

I see that there are RS-232/RS-422 isolators, like this one here:
https://tinyurl.com/sayg2en
but it is for RS-422 4 wire and your application is most likely RS-422 2-wire with a ground, but you don't say and I can't find a wiring diagram for the comm module.

If it were me, I would avoid any isolator that uses "port power".    I had real difficulties in the era when PC's RS-232 era with port powered converter isolators.

It might take some experimentation.  Maybe a 4-wire 422 isolator will work.  Maybe an RS-232 isolator will work, because you are now connecting 422 to 232.   But you need to find an isolator to make your problem go away.

All in all, if your application can tolerate low flow cut-off, use it.
 

Ground loops are caused by differences in ground potential at two different locations.

I presume the flow meter is in a pipe somewhere, at some ground potential and the panel is at some other remote location at a different ground potential. 

The run of cable picks up electrical noise that flows in the ground loop leg. 

An RS-485 repeater with isolation should eliminate the ground loop in the serial circuit.