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  1. #40
    Join Date
    Oct 2004
    Location
    Central WA
    Posts
    1,554
    It seems pretty common across multiple brands for small 134a cap tube systems to run cold evaps and low back pressures - almost like a freezer. Some even use freezer coils: wide fin spacing with channels cut for heaters.

  2. #41
    Join Date
    Aug 2010
    Posts
    3,264

    Errors in low psi measurements

    One thing about systems that run in these pressures is that its very hard to measure accurately. Errors that are insignificant for HT (water coolers, auto A/C, chillers) become significant for LT 134a systems.
    Two techs doing everything as they're supposed to using gauges perfectly in specs, the same coil can be reported as 15F by a tech in Denver and -2F by another tech in Key West.


    Saturation of R134a is 23.77psi(a)bsolute at 5F.
    That's 11.5psig in Denver.
    9.1 psig at sea level.
    Inside the system is still 23.77 psi(a) regardless of barometric pressure.

    If you're inflating a 10 psi ATV tire, you adjust it to psi(g), because it is about balancing it against the atmosphere, but refrigerant doesn't care about atmospheric pressure.

    A typical ACR service gauge is 120psig FS and the sensing is referenced to the atmosphere. Common gauge is rated 3% (OF FULL SCALE, not what it reads!!) accuracy on the lower/upper 1/3rd!

    So, the gauge is only good to +/- 3.6 psi. So, a 5F saturation evaporator can read 15 psi at Denver (11.5psig + 3.6 psig error = 15 psig. This is what happens after atmospheric adjustment + error on a perfectly fine meter reading on the high end.
    23.77psi absolute - 12.5 psi abs in Denver + 3.6 psi error = 14.9 psig reading

    You take this fridge to sea level. A tech checks it out and it can read 5.5 psig. The fridge runs at 9.1psig relative to sea level. This tech's gauge is fine, but it is on the lower end of tolerance.

    23.77 psi absolute -14.7 psi abs on your boat - 3.6 psi error = 5.5psig reading.

    This all assumes the tech followed gauge manufacturers recommendation which is to set the "zero" with nothing hooked up, at their locality.

  3. #42
    Join Date
    Sep 2002
    Location
    Virginia
    Posts
    3,230
    Quote Originally Posted by kklobas View Post
    . When I see 8 suction on medium temp though, i worry. Could just call it good, but dosent seem right.maybe i worry to much, but i shoot for no call backs....
    you have good reason to worry , 8 psi is low fo sho ... it may work, but how long .. theres definately a partial restriction , or low airflow

  4. #43
    Join Date
    Mar 2013
    Location
    Port St. Lucie, Fl
    Posts
    1,389
    Quote Originally Posted by ICanHas View Post
    This all assumes the tech followed gauge manufacturers recommendation which is to set the "zero" with nothing hooked up, at their locality.
    Who recommends that? that's a pretty dumb way to calibrate your gauges. Not that it would make a huge difference for most situations, but you can't say you calibrated your gauges by zeroing them out.

    Sent from my DROID BIONIC using Tapatalk

  5. #44
    Join Date
    Aug 2010
    Posts
    3,264
    Quote Originally Posted by shellkamp View Post
    Who recommends that? that's a pretty dumb way to calibrate your gauges.
    Some reputable manufacturers.
    http://www.jbind.com/pdf/4%20valve%2...structions.pdf For one.

  6. #45
    Join Date
    Mar 2013
    Location
    Port St. Lucie, Fl
    Posts
    1,389
    Quote Originally Posted by ICanHas View Post
    Some reputable manufacturers.
    http://www.jbind.com/pdf/4%20valve%2...structions.pdf For one.
    For that reason I find them not so reputable. Why don't we calibrate our thermometers based on what the weatherman says, then?

    Sent from my DROID BIONIC using Tapatalk

  7. #46
    Join Date
    Feb 2013
    Posts
    739
    Quote Originally Posted by pecker head View Post
    Sounds like a restricted drier or cap tube.. 134a is know for caking up cap tubes and cheap pencil driers.. I have blown cap tubes out and gotten a milky substance.. Anyone have this experience a lot with 134a?
    I've had this problem before. It seems after a high head pressure event; condenser fan motor outage, condenser fan blade outage, condenser coil dirty, etc.

  8. #47
    Join Date
    Jul 2012
    Posts
    1,194
    Quote Originally Posted by pecker head View Post
    Sounds like a restricted drier or cap tube.. 134a is know for caking up cap tubes and cheap pencil driers.. I have blown cap tubes out and gotten a milky substance.. Anyone have this experience a lot with 134a?
    Got a free upright holiday freezer from a yard sale. Had the milky refrigerant oil that looked like lithium grease. I blew it all out and put new POE in it. Installed a big 3/8 odf drier before the cap tube and weighed in 134a. After freezer got to -10 F, I had 14 degrees of superheat@ 0 psi R134A.

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