How do you calculate pressure using english units?

In summary, the mercury manometer reads 25.62 inches at 25 degrees Celsius, the local acceleration of gravity is 32.243 ft/s^2, and atmospheric pressure is 29.86 in Hg.
  • #1
ENGR_student
6
0

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?...
 
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  • #2
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.
 
  • #3
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.
 
  • #4
SteamKing said:
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 (lbm and lbf).
 
  • #5
ENGR_student said:
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?
 

FAQ: How do you calculate pressure using english units?

1. What is the formula for calculating pressure using English units?

The formula for calculating pressure in English units is: pressure = force / area, where force is measured in pounds (lbs) and area is measured in square inches (in2).

2. How do you convert pressure from pounds per square inch (psi) to other English units?

To convert pressure from psi to other English units, you can use the following conversion factors:

  • 1 psi = 0.068046 atmospheres (atm)
  • 1 psi = 6.894757 kilopascals (kPa)
  • 1 psi = 0.0689476 bar

3. Can you calculate pressure using other English units besides pounds and square inches?

Yes, pressure can also be calculated using other English units such as pounds per square foot (psf) or pounds per square yard (psy). However, these units are not commonly used in scientific calculations.

4. What is the standard unit for pressure in the English system?

The standard unit for pressure in the English system is pounds per square inch (psi). However, some industries may use other units such as pounds per square foot (psf) or pounds per square yard (psy).

5. How do you calculate pressure if the force and area are given in different units?

If the force and area are given in different units, you will need to convert them to the same unit before calculating pressure. For example, if the force is given in pounds (lbs) and the area is given in square feet (ft2), you will need to convert the force to pounds-force (lbf) and the area to square inches (in2) before using the formula: pressure = force / area.

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