# How do I convert pressure and flow from Imperial to metric units?

I'm not a physicist or have much experience in engineering. However, for a programming task, I need to change code that calculates "pressure, head, feet of fluid, gauge" into "meters of head, absolute". I also need to change a flow rate calculated as volume flow into mass flow.

I believe this requires several changes all in one:

Foot to meter could be a conversion scale of about 3.28 (3.3 feet per meter.) Or, it could be pressure in PSI (?) converted to Pascals. My reading of various sites talks wildly in terms of gallons and pounds, units I'm moderately unfamiliar with, and while I can convert one gallon to one liter easily that doesn't seem to be what's required here. Also, it is pressure in fluid and I believe, from my little engineering knowledge, that's specified and calculated and converted in a different way to pressure in gas. There may be temperature and mass values depending on what the liquid actually is. For example, this site's conversion methods specify temperature and water. I also have to convert volume flow to mass flow - again something I'm not sure about - and I'm certain that the specific liquid involved matters there.

How do I convert these units correctly?

• have you tried this thecalculatorsite.com/conversions or these instructions this mathsisfun.com/imperial-metric-conversion.html – anna v Sep 6 '15 at 16:36
• @annav The first site you list doesn't mention head, gauge or feet of fluid. The second - sure - I know feet to meters of course, as I wrote - but is that all that's involved, a scale of 3.28? I mentioned that in my question, and I don't know. – David Sep 6 '15 at 16:45
• sure it is feet to meters, when seen in units. If you had feet*pounds as units you would need the conversion factor for both. Same with temperature if it is in the units that accompany a number – anna v Sep 6 '15 at 17:48

I have answered a similar question here: https://stackoverflow.com/questions/5424543/java-convert-any-unit-to-any-unit/40695294#40695294

Consider to use UCUM (Unified Code for Units of Measure). This can help to develop a converter in any programming language. What UCUM is, is explained by UCUM itself with:

The Unified Code for Units of Measure (UCUM) is a code system intended to include all units of measures being contemporarily used in international science, engineering, and business.

I have used UCUM for my project. Here an example: calculate.plus

Guage pressure is pressure above the atmosphere.

To change feet of head to metres of head, the conversion is (exactly $ft = m /0.3048$).

To get the absolute head, you need to add the height of fluid, which is 10.3333 metres or 33.898 feet, if it's water, or 76 cm, or 30 inches, if it's mercury.

If the unit's in feet of head, it is most likely the pressure head is of water, and you would use $m = ft/0.3048 + 10.333$, which converts feet of guage into standard metres absolute.

But the alternative is to convert through atmospheres, where

1 atm = 33.898 ft = 10.333 m = 14.696 psi = 101325 pa = 76 cmHg. absolute = guage + 1 atm.