Conversion from US Gallons per second to Cubic inches per second

Information about the US Gallon per second (gal US/s)

The US gallon per second is a unit of volumetric flow rate equal to one United States gallon passing a point every second. Its symbol is gal US/s. The US gallon is exactly 3.785411784 litres, descended from the English wine gallon of 231 cubic inches, so one US gallon per second is a little under four litres every second.

Fire protection in the United States works at this scale. A fire hydrant flows several gallons per second, a pumper truck delivers 20 or more, and a large sprinkler system's design demand runs to tens. Fire codes state these figures in gallons per minute, but hydraulic calculations of friction loss in the mains use the per-second value, and the two are the same number divided by sixty.

Municipal water and wastewater engineering also reaches it. A neighbourhood water main carries a few gallons per second at peak hour, a lift station handles tens, and a treatment plant's influent hundreds. American practice usually states plant capacity in million gallons per day, and one gallon per second is 0.0864 million gallons per day — a conversion any water engineer performs from memory.

Industrial cooling and process water are described here as well. Condenser cooling water, quench flows and washdown systems all reach gallons per second, and the pump power required follows from the flow multiplied by the head, so an increase in flow of ten per cent shows up directly on the electricity bill.

The difference from the imperial gallon deserves a clear statement. The US gallon is about 17 per cent smaller than the imperial one, so the same numerical rate is a materially different flow on either side of the Atlantic. Equipment documentation should say which gallon it means, and prudent engineers convert to litres before comparing anything.

For scale, one US gallon per second is about 3.79 litres per second, and 0.264 US gallons per second is one litre per second. A flow of ten US gallons per second is nearly 38 litres per second, enough to fill a domestic bath in three seconds and comparable to a small stream.

One US gallon per second equals about 3.785 litres per second, about 0.003785 cubic metres per second, or about 0.8327 imperial gallons per second.

Information about the Cubic inch per second (in³/s)

The cubic inch per second is a unit of volumetric flow rate equal to one cubic inch passing a point every second. Its symbol is in³/s. A cubic inch is 16.387 cubic centimetres, a little over a tablespoon, so this unit describes the modest but visible flows of imperial engineering practice.

Hydraulics is where it survives most strongly. American hydraulic pumps and cylinders are still catalogued in inches, and a cylinder's speed follows directly from the flow divided by the piston area: a 2-inch bore cylinder has an area of 3.14 square inches, so 31.4 cubic inches per second extends it at 10 inches per second. Working in inches throughout avoids converting the bore, the stroke and the flow separately.

Machining uses the same idea for material removal. A milling cutter taking a 0.1-inch depth at 0.5 inch width and 20 inches per minute of feed removes one cubic inch per minute, and a modern machining centre roughing aluminium can exceed twenty. Tool manufacturers publish removal rates in these units because their customers' machines are dimensioned that way.

Small engines and pumps carry the unit too. An engine's displacement in cubic inches, multiplied by its speed, gives the air it swallows: a 350 cubic inch V8 at 3,000 revolutions per minute ingests 525,000 cubic inches a minute, or about 8,750 per second, ideal efficiency assumed. The old American habit of naming engines by cubic inches is the same measurement standing still rather than flowing.

Metering and dispensing in imperial industries follow. Adhesive dispensers, lubricators and grease systems on American plant are specified in cubic inches per second or per minute, and a shot of grease from an automatic lubricator is quoted in cubic inches per cycle, which the cycle time converts into a rate.

For a sense of scale, 61 cubic inches per second is about a litre per second, and one cubic inch per second fills a US gallon in a little under four minutes. That is roughly the rate of a garden tap opened a quarter turn, which makes the unit easy to picture despite its unfamiliarity outside the imperial trades.

One cubic inch per second equals about 16.39 cubic centimetres per second, 0.01639 litres per second, or about 0.2597 US gallons per minute.