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Old 14-10-2015, 23:23   #211
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Re: Amps v Amp hours v Amps/Hr

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Originally Posted by sailorchic34 View Post
GPM is not just a number. It implies a volume, weight and mass if you know the type of fluid. Every engineer knows this.
No it implies volume, weight and mass PER UNIT OF Time if you know the fluid.

I don't know when you took the PE but 20yrs ago they would have deducted points for using the wrong units even if the answer was right. If you had inserted this line, they likely would have given you a zero for a lack of basic understanding of units.
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Old 14-10-2015, 23:27   #212
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Re: Amps v Amp hours v Amps/Hr

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Would you folks consider eventually returning to the electrical units discussion? Treating electricity as a flowing liquid in a pipe is a barely-adequate analogy at best.

And A/h (Amperes per hour) means rate of change of current, and in most cases is not a particularly useful measurement. It is most certainly not equivalent to Ah (Ampere hours).
The flowing pipe analogy works fine, if you can grasp the difference between gal and gph.

What would you like to know? Stu's original post covered it pretty well.
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Old 14-10-2015, 23:35   #213
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Re: Amps v Amp hours v Amps/Hr

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Originally Posted by sailorchic34 View Post
I actually understand the difference between quantity and rate. You can have 100 gallons and 100 GPM for 5 minutes. To different things, I get that. Yet just saying 100 gpm implies a rate duration per hour and a volume of flow in gallons. If I have 100GPM flowing I can calculate weight, mass, Btu's in minutes or hours, etc. Most of what I do engineering wise just uses GPM with no extra time units, such as GPM/minute or GPM/Hour. As most of what I do is GPM for 24/7.

I expect my knowledge has a few gaps. yet I've read pretty much every engineering text I could lay my hands on. That includes the ASHRAE books, ASPE and NFPA manuals and codes. Mind you I've not read all the chapters in the NFPA codes, probable only about half. Mind you I found high school boring and would have read all the books by the end of the first quarter.
100gpm does not imply a duration or a volume. It implies a flow rate.

Not until you add more information can you say anything about the duration or volume.
- Add that a 50 gal tank was filled and it implies the flow lasted for 1/2 min
- Add it flowed for 3 minutes into a large tank, it implies you added 300gal to the tank.
- Don't give us either a duration or a volume to work to go with the flow rate and you only know the flow rate.

Stu's example where he found an effort in your math that was off by an order of magnitude is a perfect example of using short cuts with units. If you carry your units thru, it's much harder to make those mistakes. That's why engineers are known to be persnickety about such things.
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Old 15-10-2015, 10:01   #214
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Re: Amps v Amp hours v Amps/Hr

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What I think you were trying to explain but don't grasp is you can convert gal/min to lb/min factor in the temperature delta and get BTU/min. But until you know how long the flow lasted, you can't calculate the BTU requirement.
So if the flow lasts 24/7 how many BTUH do I need per hour.

I actually understand what your saying. Really I do.

From a physical world standpoint when you say there is 100 GPM of water flowing in a pipe that means there is 100 gallons per minute passing a point in space in one minute. I don't need to know how many minutes that flow rate lasts to calculate a BTU per minute requirement.

It's a rate of volume (gallons being a volume measurement), passing a point in space per minute. If the 100 gpm flow lasts 10 minutes it's 1000 gallons. But I already know what a 100 gallons weight. that fluid has mass, weight and volume. GPM implies a rate of gallons flowing per minute. It has a base duration and it has a base volume

I know ya'll are smart fella's and I barely have the brains to tie my shoes. I keep trying to tie my flipflops, Doh. So I'll say ya'll are right and I'm just another dumb blonde. But I'll continue to use the formula that the aeronautical engineer who first explained it to me ages ago in the manner in which he showed me.

I would like to thank fryewe for his lovely write up of a turbine steam condenser. Took a second to understand it was a steam condenser. It's a nice way to solve cavitation in the feed pump.

I'm so done with this thread.
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Old 15-10-2015, 10:18   #215
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Re: Amps v Amp hours v Amps/Hr

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Originally Posted by sailorchic34 View Post
So if the flow lasts 24/7 how many BTUH do I need per hour.
Again, the units are not making sense. BTU is a unit of measure for energy. BTUs per hour (BTU/H) is a unit of measure for energy flow (similar to GPM). You may want to know the total BTUs (which could be used to tell how much total fuel will be needed). Or you may want to know BTUs per hour (which can tell how much fuel is required per hour). But BTU Hours (BTUH) is not a unit that is useful for much of anything.

The formulas you are using are correct.

And please stop it with the "dumb blond" stuff. Nobody here cares what the color of somone's hair is. We are trying to help explain engineering units and no one should take offense. Certainly I have not seen any intentional name calling so let's not go there. It is just as offensive to be accused of name calling as it is to be the one on the receiving end.
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Old 15-10-2015, 11:11   #216
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Re: Amps v Amp hours v Amps/Hr

I never had the faintest idea that PE's could be so sloppy with dimensional analysis. This is truly scary. This is making me think twice about entering large buildings

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Old 15-10-2015, 11:35   #217
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Re: Amps v Amp hours v Amps/Hr

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So if the flow lasts 24/7 how many BTUH do I need per hour. What is a BTUH? If you meant BTU/hr, that is a different question from your example. In your original example, you were determining the BTU.

I actually understand what your saying. Really I do. Given your repeated mistakes, it doesn't appear that you do.

From a physical world standpoint when you say there is 100 GPM of water flowing in a pipe that means there is 100 gallons per minute passing a point in space in one minute. I don't need to know how many minutes that flow rate lasts to calculate a BTU per minute requirement. You changed the example. Originally, you asked for how many BTU not how many BTU/min. Again it seems

It's a rate of volume (gallons being a volume measurement), passing a point in space per minute. If the 100 gpm flow lasts 10 minutes it's 1000 gallons. But I already know what a 100 gallons weight. that fluid has mass, weight and volume. GPM implies a rate of gallons flowing per minute. It has a base duration and it has a base volume No it is a rate. Until you know either the volume or the duration, you can't determine the other.

I know ya'll are smart fella's and I barely have the brains to tie my shoes. I keep trying to tie my flipflops, Doh. So I'll say ya'll are right and I'm just another dumb blonde. But I'll continue to use the formula that the aeronautical engineer who first explained it to me ages ago in the manner in which he showed me. Not sure about your flip flop tying ability but assuming he was a competent engineer, I'm certain he would be horrified at your misunderstanding of the use of units.

.
You are still showing a bad tendency to mix up units and has been shown in your examples, it has directly lead to mistakes.
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Old 15-10-2015, 16:49   #218
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Re: Amps v Amp hours v Amps/Hr

Just a minor correction to TD and V360:

BTUH is in fact a standard engineering abbreviation for BTU per hour. Not BTU hours.

I know it goes against the rules, but that's what happens at times with historical imperial units. Same with the MBH used earlier in the thread - 1 MBH = 1000 BTU/hr.

Hey isn't there a thread about imperial/metric that has just been revived? Yippee
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Old 15-10-2015, 16:52   #219
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Re: Amps v Amp hours v Amps/Hr

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Originally Posted by sailorchic34 View Post
So if the flow lasts 24/7 how many BTUH do I need per hour.

I actually understand what your saying. Really I do.
Clearly you don't.

"how many BTUH do I need per hour" means " how many BTU per hour do I need per hour".

See, you've still got one too many "per hour"s in there. The correct question is just "how many BTU do I need per hour" or "how many BTUH do I need".
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Old 15-10-2015, 17:34   #220
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Re: Amps v Amp hours v Amps/Hr

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SC..

I agree with previous poster who recommended a detailed understanding of units. All engineering equations must balance the units so the net units on the right match the left. If you know the units of every number or variable in the equation you can easily tell if the equation makes sense or not by ignoring the numbers and just check if the units balance (same on both sides). If the units don't match between left and right then the numbers don't matter because the equation is just gibberish. If the units balance then all you have to do is check whether the numbers are correct for the units they represent. If they are then you know the equation is correct. Balancing the units does not require any memorization of formulas. You can do it just by looking at the equation.
Eveything you say makes absolute sense to many of us outside the US who are used to learn engineering starting from math and physics, which makes engineering school a 6-year deal in some places. Unfortunately that takes longer than the time people spend in engineering school in the US.

I will not forget my experience of my first job in the US teaching at a top-three engineering program; I was dismayed by the studentīs reliance on formulas that only worked for a specific set of units and had constants in them that were a mix of "real" constants (such as pi=3.1416 or the 8 in the calculation of max flex moment in a beam with uniform load) and fudge factors to compensate for the facts that the units did not "add up". Unfortunately since then this approach has spread out of the US in some fields, such as structural engineering.

Later I learned that this approach is very effective at getting people to learn how to "produce" calculations without the hard work to understand where things come from (math and physics) and helps pass the PE exams, but leads to horrible mistakes when the problem is "out of the box".
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Old 15-10-2015, 18:02   #221
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Re: Amps v Amp hours v Amps/Hr

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The flowing pipe analogy works fine, if you can grasp the difference between gal and gph.

What would you like to know? Stu's original post covered it pretty well.
Thanks, but I already know it all!
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Old 15-10-2015, 18:50   #222
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Re: Amps v Amp hours v Amps/Hr

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Thanks, but I already know it all!
Heh...well played...
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Old 15-10-2015, 19:02   #223
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Re: Amps v Amp hours v Amps/Hr

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I was dismayed by the studentīs reliance on formulas that only worked for a specific set of units and had constants in them that were a mix of "real" constants (such as pi=3.1416 or the 8 in the calculation of max flex moment in a beam with uniform load) and fudge factors to compensate for the facts that the units did not "add up". Unfortunately since then this approach has spread out of the US in some fields, such as structural engineering.

Later I learned that this approach is very effective at getting people to learn how to "produce" calculations without the hard work to understand where things come from (math and physics) and helps pass the PE exams, but leads to horrible mistakes when the problem is "out of the box".
Which is exactly what we are seeing here. In spades.
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Old 15-10-2015, 19:04   #224
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Re: Amps v Amp hours v Amps/Hr

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............I was dismayed by the studentīs reliance on formulas that only worked for a specific set of units and had constants in them that were a mix of "real" constants (such as pi=3.1416 or the 8 in the calculation of max flex moment in a beam with uniform load) and fudge factors to compensate for the facts that the units did not "add up". Unfortunately since then this approach has spread out of the US in some fields, such as structural engineering.

Later I learned that this approach is very effective at getting people to learn how to "produce" calculations without the hard work to understand where things come from (math and physics) and helps pass the PE exams, but leads to horrible mistakes when the problem is "out of the box".
I couldn't disagree with you more.

The "fudge factors" you mention are actually conversion factors that are completely in line with the dimensional analysis that, I completely agree, are absolutely necessary.

They were included in a couple of the examples sailorchic included earlier.
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Old 15-10-2015, 23:42   #225
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Re: Amps v Amp hours v Amps/Hr

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The "fudge factors" you mention are actually conversion factors that are completely in line with the dimensional analysis that, I completely agree, are absolutely necessary.
Doesn't matter what you call them, either the factors are "fudged" or the basic quantities are "fudged" but that's the reason behind this "fudged" thread
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