1. 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. ## 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. Originally Posted by kklobas
. 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

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Originally Posted by ICanHas
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.

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5. Originally Posted by shellkamp
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.

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Originally Posted by ICanHas
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?

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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.

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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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