Differential pressure transmitters with an orifice are widely used as field flow meters. The typical trouble of the differential pressure transmitters under operation is an impulse line blockage. Thus, the authors develop a detecting method of the impulse line blockages using the R.M.S. value of fluctuations of measured pressures. This method always monitors not only the fluctuation of differential pressure but also those of two static pressures, which means the high side pressure and low side pressure. The experimental investigations show that this monitoring makes it possible to detect a both side blockage and either side blockage before the blockages have an serious influence on measuring the differential pressure.
