Before going into the different types of flow monitoring methods and the devices used to measure flow,
let’s look at the units-of-measure commonly used to indicate flow rates and flow volumes.
Flow rate is simply the speed that a liquid, gas or vapor is moving in combination with how much of
it flows past a predetermined point. The rate can be measured in various units. The one preferred in
your application would be one that you are familiar with and is most likely used in other parts of your
process. Some of the most common units-of-measurement:
• Gallons per Minute (GPM – normally used in liquid flows)
• Cubic Feet per Minute (CFM – normally used in gas flows, including air)
• Gallons per Second (GPS)
• Cubic Feet per Second (CFS)
• Million Gallons per Day (MGD – normally used in large facilities that process large amounts of
liquids, for example, a water treatment plant)
Volume measurements use simple mathematics to translate flow rates into a calculated total amount.
For example, if there is a flow rate of 100 GPM for a liquid (liquids are normally measured in gallons in
the U.S.) over a period of 10 minutes, the total flow volume is simply:
Volume = 100 GPM X 10 Minutes
Volume = 1000 Gallons during the 10-minute time period
This value can be converted to the total number of gallons in a day (provided the flow rate of 100 GPM
doesn’t change) by multiplying the rate of 100 GPM times the number of minutes in a day:
Volume = 100 GPM X 60 Minutes in an hour X 24 Hours in a day
Volume = 100 X 60 X 24
Volume = 144,000 Gallons in a 24-hour time period
Now that you know the numbers involved in the above flow rates and volumes, you can make the
following statement: It will take 24 hours to fill a 144,000-gallon tank, if the liquid going into it flows at a
constant rate of 100 GPM and, of course, the tank was empty before you started filling it.
Engineers get big bucks to use these very same calculations every day. Their next step would probably
be to figure out how big the tank has to be to hold 144,000 gallons of some liquid. (That calculation is
easy too but we’ll save that for another day.)
The good news is you don’t have to memorize or even do all these calculations yourself. Flow
monitoring systems make these calculations for you. All you have to do is read the values from a dial
or computer screen.