flow transmitter with conditional orifice plate as element

Hello Everyone,

I am seeking your support and attention towards frequent wrong reading for flow transmitter. i have conditional orifice plate connect with 4 flow transmitter 1 for controlling and 3 for trip all the 4 transmitter give wrong reading, while both pump in stop condition the flow reading keeps showing even if the pump start reading not matching with operation parameter. by the way all basic maintenance been conducted but still reading not match. kindly look to the existing parameter transmitter calibration range 0 to 6 mbar. full scale 0 to 623 mbar. pressure drop 283 mbar flow rang 3100 KG/H.

please what could the possible causes?