Compressor sight glass ???

[m_l_ca]

Okay, so recently I did a new install of an R22 A/C system on a rail car with a rebuilt carrier 06D compressor. By the end of the break in period before the oil and dryer change, the compressor sight glass was black on the inside where the oil sits 1/2 way up the glass and the rest was clear. The evacuation was done properly. Oil has been changed and the acid test was clean.

So, why for the black on the glass?

[stufus]

A "NEW R22" installation that's erm odd.Was there any welding done during the installation?
If so the pipework should have been purged with OFN during the welding process,if it wasn't the discolouration is probably down to oxidisation (forming of soot inside the pipe)
Or alternatively since the install was on a rail car was hydraulic hose used instead of copper,it could be the hose lining breaking down.
You say the acid test was clear , but was the oil clean
Cheers
Stu

[Tesla]

I used to get a black coking in the capillary tubes on bottle coolers where the HP temp got high or from burnouts so changed them otherwise they would be restricted or block. But like Stu said it could be to do with hydraulic tube or welding oxidation/carbonation. In your case it must be in the oil or possibly an effect of the oil it's self. What type of oil is it m_l_ca?

[chemi-cool]

It can also be from the cast iron or aluminum of the compressor body.

One more thing that comes to my mind is mixing of mineral and synthetic oils.

[m_l_ca]

Thanks for the quick reply guys. The condenser unit is under the car body and the evap section is in the car ceiling. There was a lot of copper to copper welding done. Approximately 40 to 50 feet of hard copper "K" pipe was used and routed through a lot of angles. 5/8 high side and 1 1/4 suction line. The welding was done by another tech and I don't think he used a nitrogen flow while soldering. That would explain why the other system didn't have this problem. A nitrogen flow was used while welding the second system.
If you are wondering, there are two separate systems per car, one on each end. Passenger rail cars.

Also I did find it strange that the system was 22. I guess the engineers know more than me ;)

Thanks.