I need to find a way of measuring the differential pressure between the inlet and outlet of a heat exchanger used to heat pickling acid (10% hydrofluoric acid and 25% nitric acid). Inlet pressure is approx 2-2.5 Barg (30-35 PSI) and pressure loss is approx 200-250 mBarg (0.3-0.35 PSI).

I've looked at various diaphragm seals but all I've found are suitable for gauge pressure only, not diff pressure. As the pressure drop is quite small compared to the swing of the sensors, I'm worried that measuring errors might affect the result too much if I go for gauge pressure sensors.

Pipes are in PVDF plastic but PTFE also works.

Anyone have an idea?

 

A valve that sequentially connects a single gauge sensor to the two gauge guards and you calculate the differential mathematically in the (a) PLC? That would at least eliminate linearity differences between two transmitters. The single one will also have imperfect linearity, but you're working in the same region on the sensor for both measurements, so it should cancel itself out. Make sure you have a pretty precise sensor and high resolution input channel.