A New DP Flow Meter

Apparently the DP flow transmitter is working, but the results aren't satisfactory. No surprise, you have at least one issue, probably two.

Issue 1: If your readings typically max out at 300kg/hr, why do you think you can use a 0-1400 Kg/hr span?

DP technology has a very low turndown, typically no more than 4:1, which means that the useable accuracy of a 0-1400 span is between 360 kg/hr and 1400 kg/hr (360 = 1400/4). Accuracy drops off and noise increases dramatically at flow rates lower than the lower end of the turndown, that is, lower than 360kg/hr

What is your observed maximum flow rate?

If your observed flow rate is near or below 50% of the 1400 flow rate, you need to get a new orifice plate sized properly to your observed max flow rate. Orifice plates are relatively cheap, DP transmitters are relatively expensive. Then your re-range your existing DP flow meter to the new orifice plate.

Issue 2: You mention "measuring supersteam". If you mean 'measuring super heated steam', then your DP transmitter is incapable of providing accurate readings because plain DP cannot take into account the added heat in superheated steam. DP is fine for measuring saturated steam because temperature and pressure are dependent variables for saturated steam. But with superheated steam temperature and pressure are independent variables and need to measured independently and applied to the calculation for a mass flow rate value.

That is done with either a 'multivariable transmitter' or a flow computer because either requires separate DP, absolute static pressure and temperature measurements, all of which are used in a calculation to derive a compensated mass flow value.

Multiple xmtrs, flow computer on the left
Multivariable on the right

DP technology infers a volumetric flow from a differential pressure and line size, but it does not infer mass flow. DP is traditionally used for saturated steam flow. The units of mass flow, kg/hr are assumed by the host system's scaling. A multivariable measurement is an inferred mass flow measurement, not an assumed mass flow measurement with just a mass flow label.

As an aside, 'ASAP' is probably what produced your current results. Thorough and methodical engineering will give better results than haste.