# Basic questions while studying merm



## prichmon (Mar 1, 2013)

I have noticed in the us equations the terms g/gc keeps re occurring. are we to assume differing gravitational acceleration?

When reading through fluid measurement devices it appears but the pitot tube would have a significant negative impact on flow. Nearly all have a reduction in area which will reduce flow.

i'm sure i will have more in the future.

Thanks

Rich


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## Jonhnny123 (Mar 2, 2013)

1 - You should be aware of the gravitational constant and when it's needed (primarily when using imperial units). But it should be straight forward since it's included in the equations in the MERM. But I don't remember using anything other than standard gravity.

2 - Not exactly sure what you're asking. But if you're suggesting that a reading from a pitot tube should be de-rated because of its small cross sectional area... it doesn't.


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## pbrme (Mar 6, 2013)

The g/gc term is just the English system's way of explaining lbf. Like most of engineering, it's all in the units:

g (ft / s2)

______

gc (lbm*ft / lbf*s2)

which reduces to:

g/gc (lbf / lbm)

So for example:

F = m*a

F(lbf) = m(lbm) * g/gc (lbf/lbm) &lt;-- _True_


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## Rockettt (Mar 13, 2013)

boy a picture of a PBR makes me thursty in the morning. especially on Hump day


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## prichmon (Mar 13, 2013)

Thanks guys. I was thinking numerically and each term would cancel to 1. By units however the difference is for ib-m and ib-f. weight compared to acceleration.

For example... If we have a 3" pipe system and it reduces to 2.75" for a venturi flow device. The overall flow would be reduced by the change in area. Do we oversize the system or is the frictional and c.s. loss negligible? Am I over thinking this issue?


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## Jonhnny123 (Mar 13, 2013)

The flow would be the same. Mass flow rate in = mass flow rate out. The pressure and velocity would change. Every inch (length) of pipe, elbows, fittings, valves, etc will contribute to a head loss. You add that all up to determine the size of pipe you need. But to oversize a pipe simply for one venturi would never be done.


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## ongreystreet (Mar 13, 2013)

Meters are usually spec'd at smaller than pipe, not sure off the top of my head, but it increases the accuracy. I don't know why, I just follow the specs, and move on to the next issue.


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## kvarshne (Mar 19, 2013)

Can anybody tell me whether or not i can write with pencil in MERM book while preparing. I know i cannot write anything during the exam in the books.. I am taking it in NY


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## CRNewsom (Mar 19, 2013)

It is not recommended to write in pencil in any book you plan to take into the exam room since a pencil is all you will have in the room. You may be accused of trying to remove exam materials and your exam will not be scored.

If you must write in pencil, be sure to highlight the area as well to minimize the chance that someone will question you.


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## DavidPE (Mar 19, 2013)

I am taking the environmental exam in NY this April, and I emailed Castle Worldwide (the testing company) about handwritten/pencil notes. i received a response stating that handwritten/pencil are allowed. According to Castle, they follow NCEES rules without any further restrictions.


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## kvarshne (Mar 24, 2013)

Great!! I also called Castle and they said it is fine to write by pencil in the book however one cannot write anything anywhere but in the exam book during the test.


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