I own a Mastercool gauge set marketed for charging and testing R12 and R134a. Up till now i've only used it on R134a systems.

I plan on working on my friends R12 equipped vehicle by leaving off the R134a couplers.

In reading Mastercools instruction sheet it says: "Do not use the gauge set on refrigerant systems containing mineral based oils". Does this mean i can't use them since R12 typically (?) uses mineral oil. Why would they market it for R12 and R134a?

Is the risk in damaging my gauges or is it in contaminating between systems? the gauges are used infrequently.

i remember reading somewhere that hoses will absorb the mineral oil but I don't plan on using them much on R12 systems and will never load mineral oil directly through the hoses.

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1972 450SL
1982 300D Turbo

It makes no sense to me that they would market a set of gauges designed for R-12 and R134A, then include a caveat about avoiding use in systems containing mineral based oils.

Then again, it sorta makes no sense that they would make "bi-gauges" when over time, different oils could conceivably be intermixed.

Here's their customer service URL. I'd ask why their marketing "bi-gauges" and telling you not to use them on systems containing mineral oil.

http://www.mastercool.com/contact_us.html

It is a VERY BAD idea to use the same gauges on both systems. You will cross contaminate both system types, not a good thing.

The good news is that a set of gauges for R12, R22 and R502 are very cheap. As low as about $30.

Have a set for 134 and a set for R12 and never mix the two.

Good luck,

The guages do not care what you measure. The real problem is cross contamination of the oil used in the two systems. Oil for R134a is not campatible with that of R-12/22

Unless you flush carefully, you WILL ruin a compressor

Thanks for the link to Mastercool's phone #'s.

I finally got to the bottom of this:

We know that R12 and R134 refrigerant will not damage the manifold hoses or the gauge set. The concern is cross contamination within a car's A/C systems and recovery tanks. Mastercool said that when R134a was being introduced as a replacement for R12 the rules established by SAE and several A/C groups was that any oil that remained in the manifold hoses could contaminate a system when moving between R12 and R134a. It was only later after extensive field experience and a sufficiently long passage of time that it was observed that a converted R12 (to R134a) system was not chemically contaminated by trace amounts of incompatible oils. I'm only talking about trace amounts of incompatible oils which is what reamains of the oil after a flush.

This being the case, a very small amount of incompatible oil (lets say a drop or so) that is carried from your hose set is not going to ruin your compressor or even come anywhere close to damaging the A/C system. As an example, when converting a car from R12 to R134a your system will have traces of mineral oil imbeded in the hi and lo pressure hoses and this will be the case even if countless flushes are done before converting. These R134a systems don't suddenly go south because of traces of mineral oil clinging to the hoses! If it makes you feel better just blow out the hoses to your manifold gauge set to eliminate the unlikely case that they are loaded with oil, this is what i plan on doing.

In conclusion, it may be handy for backyard mechanics who do systems regularly to invest in two sets of devoted manifolds or a 6 hose set up. But it is not a problem for the manifold set or A/C system to use one set of hoses (with removable couples) for both R12 and R134a. These facts will be rejected by some technicians because they were trained at a time (or with information from a time) that correctly observed incompatibility between oils and refrigerant but incorrectly stated that contamination between oil was a problem. Mastercool's old instructions state this assumption but no longer.

This has nothing to do with contaminating R12 or R134a. I'm not talking about that. You will not have refrigerant contamination (which is a real problem/concern) by using the same gauge set with different systems.

__________________
1972 450SL
1982 300D Turbo

If you blow out your hoses you will be violating the law. 134 hoses have the adapters that seal the hoses to prevent leakage to the atmosphere. To be legal you must have valves at the end of your 12 hoses for the same reason.

Blowing out the hoses is considered venting and is a violation of FEDERAL law.

I know, I know someone will ask how they will ever be caught doing this. I don't care if you do it or not, it's your business. I just want to make sure you know that you are violating FEDERAL law.

I will not do this because my gauge sets are used with my recovery machines. If I used the same gauge set on both machines I risk contaminating recovered refrigerant.

Have a great day,

I understand there is concern about releasing tiny amounts of refrigerant into the atmosphere. i think the amount that stays in the hose is a relatively small quantity to worry about for the FIY guy. Maybe it's illegal but so is going 5mph over the speed limit.

I took a quick scan of the MVAC 609 test. Here is what i found under:
"Recommended Service Procedures for Using a Manifold Gauge Set"

<<1. High-, Low- and Center-Service hoses must have shut-off valves within 12 inches (30 cm) of their service ends. These valves must be closed prior to hose removal from the air conditioning system. This will reduce the volume of refrigerant that would otherwise be vented to the atmosphere.>>

It says RECOMMENDED which may have some legal ramifications. Maybe I won't get into trouble with the law after all?

__________________
1972 450SL
1982 300D Turbo