I've had this combo pin for a month,and it works really well, but I have a question. Here is an example. I'm testing the storage liquid. When you cut it on you have 3 choices using the method button. ph pops up in top right corner and I get a reading of 6.34 simple enough. When you hit method button again you get a pop up in corner that says U S the storage liquid reads reads 46(I don't know what this reading means) When you hit method button again the ppm pops up at top and reads 19 again simple enough. So what is the U S reading I'm getting???