Wrong indication

In the DP type FT, despite of temperature and pressure is stable, indication of flow changing approximately between 0- 40 kg/h, flow is not stable. What can else influence the flow?

Hopefully you have resolved the issue by now but if not. Have you blown the meter down? If so when you do does the reading stabilize for a period of time? Is your shield landed? Does the meter read a stable 0 on the PV when blocked and vented to atmosphere? Have you tried driving in a signal to ensure it is stable coming into the CS ( without the meter just simulate on the wires)? If all of these things check out look at your impulse lines. If the meter is in a gas process ensure it is located above the taps with no low points. If it is in a liquid service ensure you are getting good fill legs.