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robi
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Aug 12, 2014, 8:06 PM
Post #1 of 9
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AC Vacuum Question
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just curious what you AC pro's are getting on vacuum (in mercury)? is a nose hair over 28" acceptable? to put this in context, i have a YJ Titan 4 valve manifold with a harbor freight 3cfm 2-stage vacuum (i know, its not the bestest of vac's but the price was good). the vac gets down to 28" in about 1min, then i let it run for about 20min and the needle is just a hair past 28". its probably a tad more than that because the needle at 1ATM is a hair over zero (i used the YJ right out of the box, etc).
edit: forgot to say, i'm at 2600 ft above sea level, approx 27inHg. i am pulling down to about ~0.9" of ambient pressure (3.3%), is that good enough for 134a AC system?
edit: the pressure now in my area is right at 30inHg, so i am not sure the vac i have is good enough. 2" of Hg is not a very good vacuum.
any advice appreciated.
(This post was edited by robi on Aug 12, 2014, 10:29 PM)
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Aug 12, 2014, 10:45 PM
Post #2 of 9
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Re: AC Vacuum Question
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I'll try without use of charts and techno searches if you can handle it. Been done with this crap for some time so bear with me and may miss exact #s but give you the idea if possible.
Mercury - the metal commonly used (back when) for thermometers also referred to as it's chemical name Hg. We live in pressure of about 14.7 PSI not zero but don't think that way but just consider that as the zero mark. Wrong, it's pressure and has mass. Think - an airplane flies in it - weather and so on.
Pre historic (kinda) if you filled a glass tube with that liquid metal Hg (it's heavy too) and placed it upright it would drop down and the space above it is vacuum then you can measure that in inches how far it goes so enter stage left "inches of mercury" which is about 29.92 inches at sea level. The pressure we live in decreases with altitude so that for each 1,000 ft. of elevation a perfect vacuum (absence of air, moisture or vapors of any kind) such that you can expect ONE point drop in that # per that 1,000 ft.
Converse to pressure vacuum if you think plain water either boils or freezes at known temps but those are also pressure related. Water boils as low as -90F in a vacuum! Other is under pressure will not freeze. Chemistry and properties 101.
As it applies to air conditioning you see to have a pure product/gas that will be a vapor that will become a liquid at a curve of pressures and temps. Air we breathe doesn't condense at any practical pressure so we need another gas. Any air left messes up the show and air even in the driest dessert contains moisture that can unseen make frost messing the guts of A/C workings such that observations of how it's performing aren't very valid. Must know it's not contaminated or all bets off.
Understand at least a little what energy it takes to change state (meaning from solid to liquid and liquid to gaseous) for water H2O takes 1 calorie to change from a liquid at 212F to a gas at 212F and it's the same temp? Magic.
You capitalize on that by condensing a gas and let it evaporate in A/C in a controlled area appropriately named an evaporator. You are capturing the magic of superheat which is a book to fathom alone but it absorbs heat without changing temp consuming that hidden calorie of change of state.
It is a twist to understand but we live this all the time. Your skin sweats and the water evaporates making your skin cooler such that the heat lost is less than your skin was or the ambient temp! How can that be - in school we let some crap go as "flaming magic" and left it at that.
You are capturing change of state. I'll let you look that up.
Another real life thing you may know is a pressure cooker. That pot with water will not boil at our famous 212F but much higher and by chance about 3F higher before boiling per ONE PSI of pressure. When you release that pressure on plain water all at once you burn the snots out of yourself or the lid hits the ceiling!
Hard to be concise but it took quite some work and testing to create a condensable gas that would work at real life reasonable temps and pressures to do this for A/C as most commonly done as compressing a gas be it home (still) or moving things like cars and all.
You can cool with plain water for buildings with a non pressure concept called evaporative cooling not practical for MVAC (motor vehicle air conditioning) -- the HVAC of things that move.
OK - Most surely confused you enough but just know it takes a lot of perfect conditions to make A/C work at all never mind in a moving vehicle at assorted RPMs with a belt driven compressor exposed to elements, vibrations and deliver a climate control for you that lasts a minute never mind years.
#1 initial problem with vehicles is still leaks which unfortunately lead to more troubles. Systems really need the exact known charge now more than much older set ups and a gas called Freon or R-12 much better for range, larger molecules that leak less easily and commonly had sight glasses to show when it was filled properly taking a lot of guessing out of this game plus it was tolerant of not being perfect better. It also has the most caloric value - water being the staple as a ONE and all other things are less than that except one but not worth another two page discussion on.
The problem is when service of any kind is needed you need to keep things pure and in exact amounts. When not just a problem with the correct charge you open the book to all kinds of assorted controls that really just make it do its thing in the right spot at the right time.
As long as I'm writing this book try something fun which will show you just what change of state can do. Take a glass of ice full as you can and add water and a thermometer out of household refrigerator and tap water. See that temp close to the temp of the ice. Now add salt, and stir and watch the temp go lower than the ice was to begin with! Yup, it will as you are forcing it to change state faster than it would on its own.
For automotives there's a lot you can tell by touch and feel what is happening where or IR thermometer when things don't work properly. Add a couple different ways to achieve the same goal it still helps to know what it's doing or should be doing first then lose hair finding out why not when those problems come along.
This site has said in many threads that DIY A/C work is not friendly or cheap and the tuition of learning the hard way is most expensive,
Tom
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robi
User
Aug 12, 2014, 11:05 PM
Post #3 of 9
(2113 views)
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Re: AC Vacuum Question
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Tom G, all i can say to that reply is Yikes!!
i know all about the physics of it all, and you can boil water at 70F. just really wondering what the industry acceptable vac(min) is being used. i understand that pulling down to say 20microns would be great, but likely not practical.
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Hammer Time
Ultimate Carjunky
/ Moderator
Aug 13, 2014, 2:10 AM
Post #4 of 9
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Re: AC Vacuum Question
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The system must be pulled down to 29" of vacuum at sea level for a minimum of 30 minutes, longer if the system could have more moisture. Your higher elevation may make it impossible to pull the full 29".
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
(This post was edited by Hammer Time on Aug 13, 2014, 2:12 AM)
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Tom Greenleaf
Ultimate Carjunky
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Aug 13, 2014, 3:20 AM
Post #5 of 9
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Re: AC Vacuum Question
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Sorry about that robi: Hammer pointed out the industry standard. Waiting is acceptable proof of no leaks. The micron scale not going to show will break down boiling points to wild split hairs colder than needed even for A/C.
OK - Can't help it -- it helps to vacuum when engine is fully warm, Tom
PS: From NE part of US so Winter and A/C work almost ceases...........
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Hammer Time
Ultimate Carjunky
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Aug 13, 2014, 6:36 AM
Post #7 of 9
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Re: AC Vacuum Question
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30" is not possible. 29" is the max possible at sea level, less at higher elevations.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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robi
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Aug 13, 2014, 6:56 AM
Post #8 of 9
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Re: AC Vacuum Question
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30" is not possible. 29" is the max possible at sea level, less at higher elevations.
sea level is 29.92, so i suspect with good tools you can get much better than 29.
@29 you are at 3.07% of baro(sea level)
you can be higher than baro, like a storm, they call them "high" and "low" pressure when above or below elevation baro, this is what creates the wind, etc.
(This post was edited by robi on Aug 13, 2014, 6:58 AM)
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Hammer Time
Ultimate Carjunky
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Aug 13, 2014, 7:33 AM
Post #9 of 9
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Re: AC Vacuum Question
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What? are you trying to turn our help into some kind of game of wits? You're starting to turn into an idiot here.
I'm fully aware of what max is but you will never see that in an AC system. If you happen to have equipment capable of measuring those increments, they are used to determine just how dry a system is. Anything beyond 29.3 is getting into hard to reach places, even at sea level.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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