I'm not an expert by any means, but thought I would share what I know. I have a AH 375LP.
1st- What meter to buy. You must buy a meter that reads Propylene Glycol. Not all meters will read this. This is a link to the one I bought on Amazon.
https://www.amazon.com/Refractometer...2&sr=8-2-spons
2nd-After opening your meter you need to calibrate it. Follow the instruction in the manual. Every time you use this meter after storing it you will need to recalibrate it.
3rd-Take the pipette that comes with the meter and take a sample of the fluid. You may have Century Green or Camo Pink. Place sample on window and look to see the reading. My reading was -15@46%
Sample picture: Blue white line would be at -15@46%
4th-Now you have the reading go to your chart in the AH manual. You see were I fall on the chart. Its really what it says in the column to the right that matters. It says that my freeze point should be above 35% mine is 46% and my burst point should be below -50 degrees F mine is -15. So my fluid is in good shape.
5th-This is a link to a thread on how to change the fluid in your AH.
http://www.irv2.com/forums/f278/aqua...sh-329845.html
The chart was not in my manual I had to get it from the manual on AH's website. It also talks about the meter you need.
If you take the cap off the filter tank and shine a flash light into the tank you can see if you have sludge. This also works for the line going to the tank.