How do you calculate pressure using english units?

[ENGR_student]

Homework Statement

The reading on a mercury manometer at 25 degrees Celsius (open to the atmosphere at one end) is 25.62 inches. The local acceleration of gravity is 32.243(ft)(s^2) Atmospheric pressure is 29.86 (in Hg). What is the absolute pressure in psia being measured? The density of mercury at 70 degrees Fahrenheit is 13.543 (g/cm^3).

Homework Equations

Hydrostatic Pressure = pgh
psia = atm pressure + pgh

The Attempt at a Solution

I know how to do this question in metric units but, I am having a hard time figuring out the answer in english units.

So far I have converted all units for hydrostatic pressure to english units.
p = 844.65 (lb/ft^3)
h = 2.135 (ft)
g = 32.243 (ft/s^2)

I get 57706.508 psi (Is that the correct units?...)

The answer is suppose to be 27.22 psia. I didn't bother adding (inHg) because i already know its too much...


I'm really confused on how to the english system works.
How do you properly convert/interpret problem?...

 

[SteamKing]

The first thing to remember is that a pound is a unit of force, not mass.

To find abs pressure, you must add atmospheric pressure to the reading from the manometer.

The density of mercury (13.543 g/cc) is also numerically the same as the specific gravity. Fresh water has a specific gravity of 1.0 (by definition), and the weight density of fresh water (without looking at a table) is approx. 62.4 lbs / cu. ft.

The weight density of mercury is the specific gravity of mercury times the weight density of water. (the gravitational acceleration is already included, since we are dealing with weight rather than mass)

Once you know the absolute pressure in inches of mercury, you can finish the problem in two ways.

1. Convert inches of mercury to feet, and use the weight density of mercury in lbs / cu. ft.
2. Convert the weight density of mercury from lbs/cu. ft. to lbs / cu. in. 1 foot = 12 inches

3. If you use method 1, remember to check your units. The answer comes out in lbs / sq. ft. In order to convert to psia, you must divide pounds per square foot by 144 sq. in./ sq. ft.

 

[Borek]

Not enough information to say what you did wrong, at first sight everything you wrote so far looks OK.

English units work the same way SI units do. You deal with length, mass and time, regardless of what symbols and units are, the idea behind is identical.

 

[Borek]

The first thing to remember is that a pound is a unit of force, not mass.

I thought it can be both, depending on the context (lb_m and lb_f).

 

[mfb]

I know how to do this question in metric units but, I am having a hard time figuring out the answer in english units.

If it does not work with imperial units, you can convert everything to metric units, calculate, and convert back for the answer. Not a very handy way, but it is possible.

I get 57706.508 psi (Is that the correct units?...)

Did you consider the ft<->inch conversion?