This is so basic, I never thought of it before. Suppose you perform a pressure drop calculation and the answer is in inches of water (iwg). How do you know it is NOT inches of Hg? In my specific case, I am performing a pressure drop calculation of air across an orifice. My co-worker argues how do I know it is not inches of air???
I don’t really have an answer. In fact, the only way I know it ACTUALLY is iwg is because we’re following an example in a text. This example just happens to gives results very close to another method which we certain HAD to be in units of iwg!
They are different units. This is like asking the question, I drove from NY city to podunk and it was 100 away. Was it kilometers or miles? The answer is it depends on the units you used in your calculations.
As in, if you used hours times miles/hour, your answer is in miles. Look at the units you used and see what happens when you combine them all together as you did in your calculations.
Flight, yes, I clearly understand that…up to the very last part. I can only assume iwg is the convention unless otherwise stated. Most problems involve the conversion of psi to ft of head, understood to be water. But, perhaps for only this one type of problem, it simply is not obvious.
All numbers should have units–simple as that. However, if you have even a rough idea about the scale of the answer, it shouldn’t matter. One inch of Hg equals 13.6 inches water. If your measurement of ground barometric pressure gives 22.4, then most likely you’re talking about the pressure at 2500 meters in in-Hg, and not the pressure at 20,000 meters in iwg.
There is no assumption. Either the units you have come to iwg or inHg, or more likely they come to something like psi and you include a conversion factor to bring it to iwg. One conversion factor is for iwg, one for inHg. You get the answer for whichever one you use.
Try taking your formula and ignore values. Just plug in units for all the terms. See what units you get in the end. That is the units of your answer. Very likely you have a factor in there that amounts to iwg/psi or something similar which is how you end up getting iwg.
The constants aren’t any different for the different units. In fact, most of the constants are probably 1, and most of the ones that aren’t 1 are familiar things like pi or 2. They only look different if you do sloppy things like chop the units off of them, which is yet another reason why you should never do that.
I skimmed the responses, plus I have given this more thought. I know what some have said… the problem should guide me with clues that units should be in inches Hg instead of H20. Ultimately, I should always go back to basics recalling that feet of head is not an absolute measure of pressure. For example 10 ft of a column of H20 would not result in the same psi as a 10 ft column of Hg. It all boils down to dP = (p)(g)(h) where p is rho, for density. Also, I guess i’ve used the realtion of psi to ft (employing 2.31) that I’ve forgotten it is inherently for feet of head (for water). I dunno…this one fluids analysis generated a brain fart, I guess! Thanks, all!
No! You did not employ 2.31! You employed 2.31 somethings. In this case, I believe that what you mean is “2.31 feet of water per psi”. But the number “2.31”, by itself, is meaningless.
Now I think I understand where the confusion lay. You probably learned to use a conversion factor that gave you an answer in “feet of head”. Then you and your friend wonder how you know it is water and not air or mercury. The answer to that one is that, to my knowledge, feet of head is always talking about water. You could have a different conversion factor to give you feet of head (mercury), but since people only use that term to talk about water, no one mentions the water anymore.
Thanks, all!