All flowmeters have some degree of flow impedance. Connecting a flowmeter to a flow control system (including a GC) will present this impedance to the system as a load. Loading the system changes the system dynamics for which it was factory tuned. This will cause the flow control system to react differently and can result in a flow output that differs from the intended setpoint.
Additionally, some GCs reference flow rates to normal temperature and pressure (NTP) or 25 °C and 1 atmosphere. This means that the flows displayed on the front of the GCs are only valid for the aforementioned reference conditions. The actual flow (which volumetric flowmeters display correctly) will differ based upon how much the present conditions vary from those reference conditions.
