What flow units do you want to use?

Our CEO Charles Wemyss answers the top 11 questions that will help you better understand flow measurement and flow units.


Gas measurement: what’s the difference between Standard and Normal?

It’s simply the reference conditions. Standard and Normal are different though depending which country you’re in and what industry it’s applied to. However, if you know the reference conditions eg 0°C and 1 bar then everything can be calculated. Mass flow of gas is often expressed as standard litres per minute or normal cubic feet per hour. All that means is that the number is corrected for the pressure and temperature as if it were at standard or normal conditions.

What are the units of viscosity?

For most liquids this 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, as given by some academic customers). The most common error is substitution of sg (specific gravity) for viscosity or saying: it’s like water, when they really mean like washing-up 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, not so important for liquid flow.

Surely temperature is simpler?

Practically, yes, degrees Celsius or Fahrenheit can only be complicated when translated to Kelvin or Réaumur; the latter thankfully is almost unheard of. In standard and normal terms (see 1 above) getting the reference temperature wrong can be a contributor to 7% errors.

Hang on, is that mass or volume?

Although petrol is sold by the litre it’s energy content is proportional to its mass. Most process reactions are mass based so: assess which is going to give you the figures you need, kg or litres, pounds or gallons. Coriolis meters provide a mass output but density is a by-product of the complex maths calculations so they can produce a decent volume flow rate.

Not got a closed conduit?

What? That’s just a fancy way of saying it’s a pipe not an open channels. The units often switch to velocity rather than mass or volume flow. If that’s the case, and there are plenty of flowmeter technologies like ultrasonic and electromagnetic that measure the velocity then infer the flow rate, you have to know the area of the pipe and also, naturally, ensure the pipe is full.

>>Read our top ten tips for flow meter selection

Imperial or metric or American?

Don’t get caught out by mixing Imperial Gallons with US Gallons. If it says microns – is that thousandths of an inch (no, really) or micrometres.

Watch out for odd multipliers!

Americans have been known to use MM to indicate a ‘million’ as in MMBPD, million barrels per day. Not that well known in Europe. Also M meaning 1000 not million.

flow unitsWhat are the time units?

This is usually seconds, minutes or hours but can be days for some low flow applications. Or hogshead/fortnights.

But it doesn’t look like a flow unit?

Just as 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 as it is energy.


In gas measurement it’s quite common to specify the ‘flow rate’ in speed terms then calculate the flow rate by knowledge of the pipe area – circular, square or octagonal. so metres cubed per second is completely different from metres per second. Insertion meters are calibrated, usually, in a random size duct for velocity then back-tracked to flow rate.

Contact us

For more information on flow meters or for a quote on your specification, you can contact our team at [email protected]. Alternatively you can speak to us directly on 01296 670200.

Originally posted 24/10/2013
Updated 13/05/2021