What Flow Units Do You Want To Use?
When it comes to gas measurement, what’s the difference between Standard and Normal? It’s basically the reference conditions. Standard and Normal are different depending on which country you’re in and what industry it’s applied to. However, if you know the reference conditions, e.g. 0°C and 1 bar, then everything can be calculated. Mass flow of gas is often expressed as standard liters per minute or normal cubic feet per hour. This simply means that the number is corrected for the pressure and temperature as if it were at standard or normal conditions. Here we provide some considerations that will assist with deciding on what flow units you should use.
WHEN CHOOSING, ASK YOURSELF THESE QUESTIONS:
What Are the Units of Viscosity?
For most liquids, this is a clear cut centiStokes or centiPoise (cSt or cP), but odd units do crop up. When measuring sugar solutions, the most common terminology is Brix. cSt can be expressed in SI units as mm²/s (strictly, the SI unit should be m²/s). The most common error is substituting density or specific gravity for viscosity, as they are two different physical properties of a liquid.
How Do You Measure Pressure?
Normal units are bar or psi, but particular attention should be paid to an ‘a’ or ‘g’ after the unit. This indicates whether the pressure is absolute or relative (gauge). In gas measurement, this can make an enormous difference to the potential reading, but not quite as much for a liquid flow.
Isn’t Temperature Simpler?
Practically, yes, degrees Celsius or Fahrenheit can only be complicated when translated to Kelvin or Réaumur. Thankfully, the latter is almost unheard of, since, as mentioned, in standard and normal terms, getting the reference temperature wrong can be a contributor to errors.
Mass or Volume?
Although gasoline is sold by the gallon, its energy content is proportional to its mass. Most process reactions are mass-based, so you should assess which is going to give you the figures you need, such as kg or liters and pounds or gallons. Coriolis meters provide a mass output, but density is a by-product of the math’s complex calculations, therefore, they can produce a decent volume flow rate.
Imperial, Metric or American?
Don’t get confused by mixing Imperial Gallons with US Gallons. If it says microns – is that thousandths of an inch (no, really) or micrometers?
What Are the Time Units?
The time units are usually seconds, minutes or hours, but can also be days for some low flow applications.
Why Doesn’t It Look Like a Flow Unit?
Just like how kW/h doesn’t look like a unit of energy, there are some units such as BTu (British Thermal Units) that can be converted to a flow rate, as long as you know where the gas was extracted from (the calorific value changes depending on the source). That will be a mass flow rate since it is energy.
What about Velocity?
In gas measurement, it’s quite common to specify the ‘flow rate’ in speed terms and then calculate the flow rate by knowledge of the pipe area – circular, square or octagonal. So, cubic feet per second are completely different from feet per second. Insertion meters are usually calibrated in a random size duct for velocity, then back-tracked to flow rate.
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