of sonic nozzles systems is to measure real mass airflow.
This system is particularly useful in the measurement
of the mass flow rate delivered by air compressors
The system demands to expel
to the atmosphere the gas that is being measured.
From this point of view, air has two advantages: it
is non-contaminant and cheap.
The possibility of expelling to the atmosphere
the gas and using the instrumentation of the compressor(manometer
and thermometer) allows that the system be extremely
simple and accurate. In addition, it doesn't have
moving parts and the calculations are based directly
on physical formulae and not on empirical approaches.
The system of sonic nozzles
allows the building of the real compressor's operation
curve, this is, a curve that relates the mass flow
delivered versus the pressure in the tank. If this
real curve is known, and the load curve of the system
to be driven by the compressor is known too (for
example, a DTH hammer), it is possible
to predict very accurately the point of equilibrium
that will be obtained in actual operation.
Although the manufacturer delivers
the characteristics of the compressor at the moment
of its purchase(maximum pressure -maximum mass flow
rate), these values are only referential because they
are measured in standard conditions, that aren't necessarily
the real conditions in which the compressor will operate.
Actual conditions can reduce(or in exceptional cases
increase) its mass flow rate for different reasons
above sea level in which the compressor is operating:
Density of atmospheric air in the suction is extremely
important. A less dense air will reduce the delivered
mass flow in a percentage that can be important with
regards to the nominal flow.
Mechanical conditions of the equipment: a deficient
maintenance of the engine or of the compressor or
the wear of its parts can notably reduce the mass
flow delivered by it.
Leaks in the feed system in the case of air
Pressure losses due to changes in the flow
path and flow area.
The effects of the local atmospheric
density are easily accounted for, due to the fact
that any notorious discrepancy between this estimate
and measurements obtained using the system of sonic
nozzles can only be explained by the four factors
in the previous list.
The measuring in different points of the air circuit
can help detect the anomalies in the system.