Esty's

 

Heating, Ventilation, and Air Conditioning Notes

This page is reserved for HVAC notes, tips, and tricks that I have found useful in my HVAC and MVAC (Motor Vehicle Air Conditioning) journey. It focuses on the more complex aspects of HVAC work, such as vacuum and leak testing, plus EPA exam key notes that we have to remember for the certification exams.

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HVAC Pressure Gauge
  • Understanding a vacuum pressure gauge (aka "compound gauge")--it measures both positive & negative pressures: see referenced web-sites listed below for general knowledge, as EPA exams reference BOTH inches of mercury (Hg") as well as microns (greek letter "mu" or "μ"), but articles I had found discuss millibars only; however, I found that 1 millibar (atmospheric) or 1 mbar(a)=750.06μ.

    Did you know "microns" is short for a "thousandth of a millimeter", specifically "microns of Hg" to replace inches -- and that "vacuum" is never expressed as a negative number? Vacuum implies negative pressure as shown on the image, right: see the "green" area where Hg" is from zero (atmospheric pressure) to ~30Hg (total vacuum). Important: everytime you see a tilda (~) in front of a number, it means "approximately". In fact, you cannot go past 29.92 Hg" (which is total absense of pressure) and once past the 28" mark, it becomes really "impractical" with a compound gauge. From 28" Hg, you must actually use a special device called a "micrometer" to get the total actual number of required vacuum via. EPA requirements! To demonstrate, I placed this table below with further explanation. Oh, lastly, "Torr" was devised by a guy named Torricelli, which created the shorthand for mmHg (millimeters of mercury). So, instead of saying "millimeters of mercury", you can just say "Torr" and it's the same thing.

    Sources: https://fluidpowerjournal.com/vacuum-measurement-a-basic-guide/
    and https://www.kodytools.com/units/pressure/from/millibar/to/micronhg

    Atmospheric pressure at top & "total vacuum"=29.92 Hg" at bottom


    Hg"    		 kPa
    (%)    		 microns
    (μ)    		 Torr
    (mmHg)    		 mbar
    (a)    		 Water
    Boils
    0" ~0~760k7601014 212*F
    + 3" -10675k684900 207*F
    + 6" -20600k608800 200*F
    + 9" -30525k532700 194*F
    + 12" -40450k456600 187*F
    + 15" -50380k380500 178*F
    + 18" -60300k300400 168*F
    + 21" -70225k225300 156*F
    + 24" -80150k150200 141*F
    + 27" -90 75k 75100 114*F
    + 28" -93 51k 51~68 100*F
    + 29" -97 21k 21~28 73*F
    29.3 -98 11k11~15 56*F
    29.6 -994500μ5~6 33*F
    29.92 -100
    kPa (%)    		 0 μ 0 Torr
    (mmHg)    		0 mbar
    (a)    		 <-2*F
     
  • Why I need to know microns vs. other units: EPA 609 MVAC certification requires a vacuum of 500 microns or less when repairing systems and refilling them after a leak repair, and EPA 608 HVACR certification requires a vacuum of 200 microns or less, (dependent on the type of system & type of refrigerant used).

    Details are found in this table:
    HVAC Type Vacuum Requirement Notes
    MVAC/HVAC Default <= 500 microns
    or > 29.8" Hg    		 Done right before a recharge after repair. Note that numbers below are needed for 'leak testing procedures'.
    Very hi-pressure appliance 0" Hg or more (atmospheric pressure) e.g. Nitrogen systems
    Hi-pressure
    < 200lb    		 0" Hg or more (atmospheric pressure) Domestic Type II Appliances
    Hi-pressure
    >= 200lb    		 10" Hg+ or
    < 500k microns    		 Large Type II Appliances
    Med-pressure
    < 200lb    		 0" Hg or more (atmospheric pressure) Domestic Type II Appliances; does NOT apply to Class I small appliances.
    Med-pressure
    >= 200lb    		 15" Hg+ or
    < 380k microns    		 Large Type II Appliances

Contact Me

Feel free to contact me at patrickesty@hotmail.com with any suggestions or questions. Give me a day or even a week, depending if you end up in the junk-mail folder :-)

Orlando, FL   32812
USA

My cell: 407-956-7581