I had major issues with my Sintra plus ovens and I spent enough time to get a phd in sintering, struggling to figure this problem out myself. This is what worked for me:
I would do a few calibration cycles with the Orton ceramics temp tabs. In the Sintra plus you can go to options and increase or decrease the calibration (base) temp from there. Follow the instructions. I increase or decrease the temp by 1/2 of the discrepancy until I get it just right. Make sure you use a good micrometer preferably from the same temp tab manufacturer. Doing this type of change will increase/decrease the temp universally for all types of restorations you use so keep that in mind. You can pm me for the calibration chart and calibration program specs or contact Orton directly. I used to get greening, and milky look to the crowns as well as the dreaded milky “margins” but now I calibrate and purge regularly (at least every month depending on how often you sinter) and no more issues. Also remember that preshaded pucks and acid based stains will contaminate quicker so either you have to degass ( with white pieces of old zirconia pucks as it sucks up the contaminants) more often and/or use a degass (air compressor connection) period of time in the beginning of you sintering program as this will eliminate more of the contamination from each cycle. Remember to degass until your pieces of old Ziconia come out pure white. Any shade in them indicates contaminates are still present. Remember to change out your elements as directed by manufacturer. And don’t go cheap on them. Third party places aren’t reliable IMO. You know, the ones that ask you for the dimensions of you old elements so they can “custom” make them? In the Sintra plus, there are three elements. I got them for $200 a piece from some place that makes them (usa) and they didn’t work. I had to buy them from Zahn at $533 a piece and even though it was more expensive, it fixed the issues. I have two Sintra plus furnaces and haven’t had any more issues since I changed the elements out. Cost of doing business I guess. I change them out now every three yrs as recommended. Remember, you get what you pay for with tech stuff, Especially with elements. And remember your ROI. With high tech stuff like this it’s best to stick to manufacturers’ instructions and recommendations. They got it down to a very accurate science.
Sorry for the rant but I struggled for yrs trying to figure out the problem only to lose a ton in revenue and gain more gray hairs than I care to admit in the process. Not worth it IMO. Hope this helps! Pm me with any other questions I may help with. I know this is a very frustrating thing!