The best option would be to have a glandplate for some reasons:
- The thickness can be higher if this is needed.
- A glandplate can be drilled at site more easily. If it is also for hazardous area, you can loose the certification if you drilled directly a box. I should not be a problem when...
I agree that the first approach would be to have two flowmeters one for water and one for oil. With the signal from the flowmeters it will be easy to adjust the speed of the pumps. Probably your VFDs already have a PID controller built-in.
For a better accuracy you would need a oil in water...
RTDs are resistors, with a value that changes usually with the temperature. The more common type is PT100. However I think it is a very high temperature for a PT100. For that temperature I believe the correct choice would be a Thermocouple. Thermocouples provide a very week voltage signal, that...
The reason of using current instead of voltage is that with current you avoid errors due to voltage drop in the cable. If the transmitter sends for instance 10mA, the controller will receive 10mA, independently of the impedance of the wires.
You have a short explanation in the following link...
