Hang in there, these skimmer rock once you get them nailed...
Less than 1 cup in a year and a half? That seems crazy, especially when mine is pulling that every couple days. Let's see if we can't troubleshoot.
Ok, I'm just going to go over everything because you never know which part you might have missed or where a light bulb will appear above your head.
I'm going to also assume that you have completely taken the skimmer apart, and cleaned the air valve, injector (make sure there is nothing stuck in the clover leaf shaped part), and actually cleaned the skimmer body itself. If you haven't recently, soak the entire thing in vinegar overnight. Take a look inside and where the injector sprays water in to make sure there are no obstructions. Also, to make sure you've covered all your bases, it might not be a bad idea to take out the Mag 7 and inspect and soak in vinegar overnight to make sure it's running at full throttle. Once everything is completely rinsed, put it all back together (good idea to use thread tape on the injector threads). Something to also point out here is that the exit flow from the skimmer cannot be restricted and must leave the skimmer above the water level in the sump. If there are any 90 bends or if the outlet is underwater, it will create back pressure and change how the skimmer will work. You've probably read these next steps, but read again just to make sure you didn't miss anything:
1. Open gate valve on output, close air valve completely, and turn skimmer pump on. (there should be no bubbles in the water, if there are, make sure you don't have any uncovered holes)
2. Use gate valve on output to set water level even with where the clear plastic riser meets the black mixing box. You should be able to see the top of the water kind of rippling at that level (still no bubbles, or very few coming from just the turbulence).
***PS-when making adjustments to the gate valve or air valve, allow an hour between adjustments for things to settle***
3. Open air valve to halfway. The water should immediately start filling with bubbles. It could take some time for the bubbles to actually form foam.
4. Let the skimmer run for a day or so to allow the foam to stabilize. Watch out for floods at this point until you get consistent foaming.
5. Close the air valve for dryer skimmate, and open for wetter. Very small adjustments here make a big difference, so just nudge the valve when adjusting. If you have a flood going on, and the water level is still set where it should be, close the airvalve some.
Couple of notes:
You may have to find a way to stabilize the water level where the pump is located. I know an inch doesn't seem like much variation, but an inch of head pressure difference will change the water level in the skimmer. Baffles are the easiest way to accomplish this. But, you can also put the pump in a bucket or container that receives water from the drain and stays at one level. I don't know what size sump you are working with, so I can't help too much there.
If you have the water level set correctly and the air valve all the way open and still can't get skimmate into the cup, there may be something going on with your water. I kid you not, I can pretty much tell when my alk or calc is out of line because my skimmer will not produce the same quantity of foam. That may just be the case with my individual setup, but it definitely makes a difference and is something to consider for you.
What are you using to measure your salinity? What are you using to calibrate this instrument?
What salt are you using? Mine runs better with Kent than it did with IO. However, this could also be attributed to my calc and alk levels being low at that time.
Are you using any additives or chemicals? Do you use carbon, ozone, or anything similar?
Sorry for the long post, I am just trying to throw out anything that has helped me with mine. Some pictures of your sump/skimmer setup would probably help. You never know what a second/third/tenth set of eyes will notice.
