Flow and Volume
Flow and Volume
The SI unit for volume is m³. Other units, like Liter, Gallon or ft³ are also often used.
Flow is simply volume per time, and can be measured in m³/s or any other volume and time unit that has a reasonable scale for the application.
Flow measurements have many applications;
- HVAC – monitor airflow in ducts.
- Natural gas – monitor usage over time and record.
- Water supply – use volume readings for billing and flow to monitor instantaneous use.
- Wastewater and drainage – monitor volume and flow to catch performance or flooding issues.
- District heating – combine flow measurements with temperature differential to calculate energy useage.
Flow and volume meters are often similar and sometimes performs both functions in the same device. The simplest form of flow meter is a paddle wheel with a lap-counter, just like a bicycle speedometer. For every rotation the meter will output a pulse, so the speed of the pulses correspond to the rate of flow. The number of pulses would then correspond to the volume.
There are many other methods to measure flow, such as using ultrasonics, loaded vanes, pressure differentials, magnetic sensors or vortex detectors. Your choice of meter will depend on the specifics of the application.
The signal from the flow meter can be pulse, analog or digital.
The most common type of volume meter has a pulse output. Each pulse will correspond to a certain volume passing through the meter. The meter will be marked with either the volume per pulse (for example 1G/p), or pulses per some volume (100p/m³).
This type of meter is usually designed to measure the volume, although in many cases we can assume that the flow is relative to the rate of pulses.
Note that some electronic flow meters count pulses internally and will send out bursts of pulses every few minutes to save battery. This type of meter can’t be used to monitor flow.
Meters with analog outputs are designed to measure flow. Output is typically a voltage (0-10V is common) or a current (4-20mA). The markings on the meter should tell us what the output scaling is (for example 20L/s@10V).
By monitoring flow over time we will get an estimate of the volume, but please note that this method is not as accurate as using a volume meter as small measurement errors accumulates over time. Therefore flow meters with analog outputs are not recommended for billing purposes.
More advanced flow/volume meters may have digital interfaces, such as Modbus. Modbus is an very well established industrial communications standard, and is used by many manufacturers to make equipment interoperable.
Meters with mobus interfaces can accurately communicate both instantaneous flow and volume over time, and multiple meters can be connected together on the same communication wire.