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Author Topic: Vacuum reading  (Read 670 times)

Offline hvactech83

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Vacuum reading
« on: November 01, 2016, 12:05:12 PM »
Wich indicates driest reading ?
A - 100 microns
B - 450 Microns
C - 29 hg
D - 0 hg




Offline Admin

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Re: Vacuum reading
« Reply #1 on: November 01, 2016, 01:15:36 PM »
This may be a trick question another user has asked about before,

http://www.hvactechgroup.com/hvacforum/index.php?topic=1834.msg5277#msg5277

If you look at the chart attached to that post, a 100% vacuum is equal to 0 microns or 0" Hg absolute.

I'm thinking whether it's possible or not, 0" Hg absolute or 0 microns is technically the driest possible system.

Offline hvactech83

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Re: Vacuum reading
« Reply #2 on: November 01, 2016, 08:59:51 PM »
 Appearently
This Is a on the exam 100%
I even googled got me more confused but

So what is the correct answer Final calll :/

Offline Porcupinepuffer

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Re: Vacuum reading
« Reply #3 on: November 02, 2016, 12:38:07 PM »
So looking back at the question, it seems D would be the most correct answer given the limited detail in the options. Throughout all the training material I've dealt with for air conditioning and refrigeration, I've never come across anywhere that refers to vacuum in terms of absolute. It's always expressed in gauge.

But since 0 hg absolute is a complete vacuum and the rest of the options are not, I would say it must be D.

I'm scheduled to write the exam next week, I'll see if it pops up.

Offline hvactech83

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Re: Vacuum reading
« Reply #4 on: November 02, 2016, 08:41:45 PM »
Sounds good

thnak you !and good luck with the exam

Offline rmuntz

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Re: Vacuum reading
« Reply #5 on: November 03, 2016, 02:03:26 PM »
My guess would be (A) - 100 microns

(C) 29"hg = 24,300 microns
(D) 0"hg = 730,000 microns

Offline Admin

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Re: Vacuum reading
« Reply #6 on: November 03, 2016, 04:55:54 PM »
It looks like we should be using the gauge pressure for their answers.

You can download the Yellow Jacket Tips, Tools for Pulling an Effective Vacuum - Here

Quote
When vacuum is measured in inches of mercury it must be remembered that atmospheric pressure (gauge pressure) is the starting point and that the gauge readings are negative numbers. Thus, -29.921 inches Hg is a perfect vacuum. It is not practical to measure a deep vacuum (low absolute pressure) in inches of mercury because the units are so large; it is measured in microns. There are 1,000 microns in a millimeter (Torr) and 25,400 microns in 1 inch of mercury, so the use of this unit makes the measurement of deep vacuum much more precise.

The Modern Refrigeration and Air Conditioning textbook also uses gauge pressure.

Quote
High-Vacuum Gauges
To measure deep ("high") vacuum, an electronic or a solid-state thermistor vacuum gauge is used. A regular compound gauge cannot read accurately to micron levels.  A high vacuum gauge is shown in Figure 12-100.  A vacuum from 29.25" Hg (about 17,000 microns) to 29.9" Hg (about 540 microns) is necessary to allow the moisture inside the system to evaporate at room temperature.

I suspect the exam is using gauge pressure with their answers as well.  I agree with rmuntz,  the correct answer is A, 100 microns.  Porcupinepuffer you had it right the 1st time  :angel:

In the previous thread with this question the poster had me convinced the answers were Inch Hg absolute.

The answers seem to be arranged from the lowest micron reading to highest.

Offline Porcupinepuffer

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Re: Vacuum reading
« Reply #7 on: November 04, 2016, 04:41:43 AM »
The big dilemma is trying to get into the head of the person that put this question down. Whether they meant gauge pressure, or were clever enough to make it a trick question?
I'm split down the middle now :-\

Offline rmuntz

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Re: Vacuum reading
« Reply #8 on: November 04, 2016, 08:23:32 AM »
Looks like my numbers were a little bit off

29"hg = 25,400 microns
0"hg = 760,000 microns

Of all the things I've lost, I miss my mind the most !!

Offline Admin

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Re: Vacuum reading
« Reply #9 on: November 04, 2016, 11:02:09 AM »
Anyone writing the exam should be able to ask the proctor if the answers are in absolute or gauge pressure, if that information is not given.

Offline Porcupinepuffer

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Re: Vacuum reading
« Reply #10 on: November 09, 2016, 10:21:30 AM »
Wrote the exam this morning and this question was word for word on it.

Offline Admin

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Re: Vacuum reading
« Reply #11 on: November 09, 2016, 10:39:47 AM »
Did we find out if it's gauge or absolute?

Are you going to pass?

Offline Porcupinepuffer

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Re: Vacuum reading
« Reply #12 on: November 09, 2016, 01:29:22 PM »
A huge portion of the questions from the exam were on this forum, so I wasn't overly worried about this one question. I should have asked tho. The invigilator was a smokin hot babe.

Offline Admin

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Re: Vacuum reading
« Reply #13 on: November 09, 2016, 01:30:46 PM »
Nice, good to hear!

Good luck :)

Offline hvactech83

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Re: Vacuum reading
« Reply #14 on: November 09, 2016, 08:40:28 PM »
whaaaaat this was on the examm?  so what now we all agree 100 micron ? yea?

Offline hvactech83

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Re: Vacuum reading
« Reply #15 on: November 09, 2016, 08:42:24 PM »
Bpuffer .>>? so what was on the exam more ,hope ur memory stillll sharp  ;D

Offline Porcupinepuffer

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Re: Vacuum reading
« Reply #16 on: November 10, 2016, 04:04:45 AM »
Bpuffer .>>? so what was on the exam more ,hope ur memory stillll sharp  ;D

I'll be posting additional questions. I started writing them down yesterday  ;)