English Tech/Engineering Engineering (general) flowmeter manual

4.30 Pressure Effects In The Impulse Lines
The pipe or tubing between the primary element and the secondary device is a complex and imperfect dynamic pressure transmission line. At a constant pressure or with slow changes, the difference between the primary and the secondary devices will be due only to elevation effects. Compressible fluids inside impulse lines will have acoustic resonant frequencies with standing waves and pressure maximums ***¼ wavelengths apart***.
Depending on the properties of the flowing fluid, the geometry of the pressure tap and the tube connecting the pressure transmitter, certain frequencies can be amplified in the lead line. Amplified pressure pulses may affect the secondary device.
The magnitude of this effect varies with the type of secondary device, the geometry of the meter, flowing conditions, frequency response of the pressure transmitter, etc. Significant errors are reported with meters in reciprocating gas compressor discharges when pressure pulsation is in excess of 10% of the static pressure.
The problems are minimized with the use of short and direct pressure transmitting lines of constant inside diameter and with a minimum of extra fittings.