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Wednesday, March 29, 2006
HW #1 - P#6 - Manometers and Gage Pressure
Please post any questions about this problem as comments on this posting.
12 comments:
Anonymous
said...
I'm having issues with the specific gravity (SG)...do I need to convert that into density in kg/m^3 or can I just use it in the barometer equation verbatim? I've done it both ways and get two completely different answers. The correct one using SG, why is that?
I'm not sure if this is right, but from class, SG = (density of fluid)/(density of water)...is this the definition of SG? Anyway from here, we know SG (given) and the density of water (1000kg/m^3), and I think we can solve for the density of fluid, which is the density of mercury or oil.
Yes, because the density of the air in the tank is considered to be negligible, the pressure everywhere in the air (including right at the top surface of the water) is the same. But it is 80 kPa GAGE. You probably want to convert that to absolute pressure.
Yes, the pressure above the oil the right-hand leg of the manometer is atmospheric 101.325 kPa.
I am not sure how you managed to get the right answer using SG in the barometer eqn instead of density...unless you used density = 13.6 g/cm^3. That would work ! But it is important that you understand what you are doing and why (not just trying to match the answers :)
Now im confused... I took the three eq given in class, converted the heights to ft, converted the gaugue pressures and pressures to psia, converted the specific gravitys to densities to (lbf/ft^3), and just plugged in, I know that g/gc is one because im in AE. but my pressure 2 is larger than my pressure one!! is this the right idea?? I have no idea whats going wrong
ok, now i think units are the big problem, just to get this straight... AE units such that g/gc = 1 are pressure: Kpa, height: feet, density : lbf/ft^3
1- Since the all of the values in the problem are given in SI/metric units, I suggest you solve the problem in the SI system of units.
2- The units of density in AE are lbm/ft^3.
3- g/gc = 1 lbf/lbm in the AE system. g/gc = 9.8066 N/kg in SI.
I am not sure what is going wrong with what you have done either. I would guess it is a units problem. The units issue will be less complicated in this problem if you do not convert to AE. Stay in SI.
If you are still stuck go to the TA's office hours today for help or join the online chat at 8 PM tonight.
2- In the AE system of units... g/gc = 1 lbf/lbm density [=] lbm/ft^3 h [=] ft P [=] lbf/in^2 (psia)
BUT you still have to be very careful with the units in the barometer eqn ! You probably need to use the fact that 1 ft^2 = 144 in^2 to convert the pressure to units of lbf/ft^2 in order to make the units in the barometer eqn work out.
Bottom Line: on't switch systems of units. If the values in the problem statement are SI, use SI. If they are AE in the problem statement, use AE.
12 comments:
I'm having issues with the specific gravity (SG)...do I need to convert that into density in kg/m^3 or can I just use it in the barometer equation verbatim? I've done it both ways and get two completely different answers. The correct one using SG, why is that?
I'm not sure if this is right, but from class, SG = (density of fluid)/(density of water)...is this the definition of SG? Anyway from here, we know SG (given) and the density of water (1000kg/m^3), and I think we can solve for the density of fluid, which is the density of mercury or oil.
Does pressure above the 30cm high water in the tube equal 80,000Pa, and does the presure right above the oil equal 1 atm...101325Pa?
SG = fluid density / reference fluid density
Unless otherwise stated, the reference fluid is water at 4 degC and the desity of the reference fluid is usually approximated as 1000 kg/m^3.
So, if SG = 13.6, then fluid density = 13.6 * 1000 kg/m^3 = 13,600 kg/m^3
I hope this is helpful.
Yes, because the density of the air in the tank is considered to be negligible, the pressure everywhere in the air (including right at the top surface of the water) is the same. But it is 80 kPa GAGE. You probably want to convert that to absolute pressure.
Yes, the pressure above the oil the right-hand leg of the manometer is atmospheric 101.325 kPa.
I am not sure how you managed to get the right answer using SG in the barometer eqn instead of density...unless you used density = 13.6 g/cm^3. That would work ! But it is important that you understand what you are doing and why (not just trying to match the answers :)
What is the standard measurement of pressure in SI units?
Now im confused... I took the three eq given in class, converted the heights to ft, converted the gaugue pressures and pressures to psia, converted the specific gravitys to densities to (lbf/ft^3), and just plugged in, I know that g/gc is one because im in AE. but my pressure 2 is larger than my pressure one!! is this the right idea?? I have no idea whats going wrong
ok, now i think units are the big problem, just to get this straight... AE units such that g/gc = 1 are pressure: Kpa, height: feet, density : lbf/ft^3
is this right???
The standard unit for pressure in SI is the Pascal, Pa, which a N/m^2.
1- Since the all of the values in the problem are given in SI/metric units, I suggest you solve the problem in the SI system of units.
2- The units of density in AE are lbm/ft^3.
3- g/gc = 1 lbf/lbm in the AE system. g/gc = 9.8066 N/kg in SI.
I am not sure what is going wrong with what you have done either. I would guess it is a units problem. The units issue will be less complicated in this problem if you do not convert to AE. Stay in SI.
If you are still stuck go to the TA's office hours today for help or join the online chat at 8 PM tonight.
1- Don't use the AE system in this problem !
2- In the AE system of units...
g/gc = 1 lbf/lbm
density [=] lbm/ft^3
h [=] ft
P [=] lbf/in^2 (psia)
BUT you still have to be very careful with the units in the barometer eqn ! You probably need to use the fact that 1 ft^2 = 144 in^2 to convert the pressure to units of lbf/ft^2 in order to make the units in the barometer eqn work out.
Bottom Line: on't switch systems of units. If the values in the problem statement are SI, use SI. If they are AE in the problem statement, use AE.
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