It was over 35 years ago when I began what was to become a career in pumps. The company was Byron Jackson, an American firm headquartered in Los Angeles. US Customary System (USCS) units remained prevalent within the company despite its international manufacturing presence and a growing global adoption of metric.

Years ago there were relatively few occasions when I found it necessary to work in metric units. But when I did, my practice was to convert the client’s data and perform calculations in USCS units. After obtaining the final results, I converted the USCS values back into metric for transmittal to the client. This always seemed easier than learning to work in both systems. Eventually, the company evolved into a major global player, and metric (International System or SI) units became dominant. Even so, I continued to convert, work in the USCS units, and then convert back to metric.

Working with USCS units I had learned to think in gallons per minute, inches, mils, feet, pounds per square inch, rpm, horsepower and so on. I was comfortable and familiar in the world of pumps in terms of US customary units. While I was conversant with metric conversions, I seldom practiced working through problems within the metric system itself.

A few years back after I had gone into independent consulting practice, my first client was overseas; all engineering work was carried out in metric. To work and ‘speak’ fluently in metric, I found conversions used up too much time. It would be like trying to converse in Spanish by translating English thoughts first, listening to the response in Spanish and then translating back to English. That’s too slow for language fluency. So I began working the problems directly within metric, without conversions. While it was not difficult, at first it did require extra time. But in the course of several days I actually felt about as familiar with SI metric as I did with the USCS units. It is perhaps similar to a musician beginning with one instrument and eventually learning to play two or more.

Do I have a preference for metric or USCS? Metric is easier to work with because most of its units scale up or down by factors of 10 and there’s less fuss over the definitions of mass and force. Except, the non‑SI practice of using a kilogram for both weight and mass is not likely to go away. It goes without mention that both metric and USCS users are tasked with converting time into seconds or degrees into radians, using factors that are not multiples of ten.

Both systems of units are founded on primary defined, but arbitrary, units. Actually, the USCS units of length and mass are legally defined in the US by metric values. The primary metric unit for length is the distance travelled by light in a vacuum during a time interval of 1/299 793 458 of a second. The meter was originally intended to be equal to 1/10 000 000 of the length of the meridian through Paris from North Pole to the equator, but in any case, it is an arbitrarily defined unit. The primary metric unit of mass is defined by precision cylinder of dense platinum-iridium which is nearly identical to the mass of one liter of water. The kilogram is also an arbitrary unit. The modern definition of a second supersedes a definition based on the earth’s orbit around the sun and is now based on 9 192 631 770 oscillations of a caesium-133 atom. Thus the definition of time is arbitrary. Even our standard base-ten system of counting is arbitrary. Base sixteen, for instance, might be more appropriate in the age of digital electronics.

One cannot argue that there is anything fundamentally superior with arbitrarily defined units in either SI or USCS. SI does, however, have the distinct advantage of being a coherent system, requiring no special factors, other than decimal multiples, to relate forces, energy and power.

One finds in practice that a particular field of technology or commerce will evolve towards the most practical units of measure. For instance, the typical dimensional units for pump components are in millimeters or inches which are subdivisions of meters and feet respectively. Both millimeters and inches, or decimal divisions of these, work well for a wide range of machinery and equipment.

‘Gallons per minute’ has worked well across a broad range of industries for many years and is fairly well entrenched in the US as a workable unit for rate of flow. Same can be said for head in ‘feet’ and pressure in ‘pounds per square inch’. ‘Horsepower’ is usually used for mechanical power while metric ‘kilowatts’ is used for electrical power.

Metric users express volumetric rate of flow in ‘cubic meters per hour,’ ‘cubic meters per minute,’ ‘cubic meters per second,’ ‘liters per minute’ and ‘liters per second’ – depending on industry conventions. Head is normally expressed in ‘meters’ which works well for most industry applications. Pressure may be expressed as ‘bar’ or ‘kilopascals’ depending mainly on industry convention. In metric, ‘kilowatts’ applies to both mechanical and electrical power thanks to the genius of 19th century scientists, physicists and engineers, notably James Clerk Maxwell and Giovanni Giorgi.

Is either system of units better suited for pumps? Both are workable but some units are perhaps more intuitive or convenient than others. For instance, the US oil industry uses ‘barrels per day.’ A barrel is easy to visualize, but often the quantities are staggeringly large and the numbers have meaning only for those intimately involved in the business. A ‘cubic meter’ is a large volumetric quantity and is more suitable for the likes of power plant cooling water pumps or storm drainage water pumps. A ‘liter’ and a ‘gallon’ are familiar to most people – these are sizes found in everyday commodities. For many applications the units of ‘meters’ or ‘feet’ relate well to the scale of objects or dimensions in our immediate environment. A ‘bar’ relates to approximately one atmosphere and works conveniently for elevated pressures. ‘Millimeters’ is useful over the range sizes found in typical pump machinery components and manufactured products in general. ‘Kilowatts’ and ‘horsepower’ are each relate-able to machines and vehicles we use every day. I have to say I’m sort of attached to the term ‘horsepower’ – a term that obviously defines itself and is closely associated with classic 1960’s American cars.

The US Customary System is still firmly entrenched in certain US industries, including industrial pumps. Increasingly publications and standards are providing quantities in both SI metric and USCS. Internationally, SI metric is well established and is the default system of units for most industry standards. Most would agree that SI will ultimately supplant the USCS units, a process which is already well underway in the spheres of technology and business trade.

For an independent evaluation of a pump or a pumping system, in any system of units, contact an experienced consulting engineer who can help with your specific application.

Ahmed Raza says

Thank you for the insights. It is always a pleasure to read your articles!

Here locally we use all sorts of conversions…many a customers would prefer using head in meters and flow in USgpm and similar cross units. We are familiar with both SI & US unit (almost all) formulas so we quickly make conversion of either unit in US or SI and perform the calculation.

Jaap Zwart says

Hi Randal,

I appreciate what your problems were that time, to think in US unit and converting in SI. Same was with me working 35 years ago for Caterpillar

in Netherlands. All application handbooks were in US units which I needed to convert in SI. In the ealy 80-ies Cat started to manufacture all in metric. I now BJ as they were represented in NL, I did business with them for my Caterpillar special firepump packages, some with vertical duplex SS pumps and Amarillo right angle gear box, mostly for my sales to Aramco.

Suzanne Swint says

Hello Randal,

I am an American working in Holland. Everyday, I appreciate my education that forced me to convert back & and forth constantly between the systems. The Europeans cannot convert in their heads and have no feeling for English units, for example they don’t get impressed when I tell them I walk thirty miles per week, which is more than a 42km marathon :-). Here we use cubes/hr, meters of head, bar and °C for pump calcs. Sometimes we get wacky units for viscosity and sometimes they throw in a kPa. I find SI to be easier and more convenient in general. Watch out for tonnes. Thanks for the article!

Randal Ferman says

Dear Suzanne, Interesting observations you made. In my decades association with Flowserve I visited Holland a number of times and the Dutch engineers at the company were ‘bi-lingual’ with respect to units. I’m sure their experiences visiting the U.S. at that time were probably more like yours now. By the way, the easiest way for me to think with kPa is in terms of 100 kPa which is equal to 1 bar. Anyway, thank you for your comments! Randal