I was having the same issue a few years ago using the Hanna HI713 tester, can be confusing for sure. I'd definitely buy the Hanna HI736, Hanna does make a newer HI774. The HI774 gives you a phosphate reading in PPM, the HI736 gives you a reading in PPB. the Hanna HI736 Ultra Low Range Phosphorus Checker measures in parts per billion rather than PPM. The HI736 has a range of 0 to 200 PPB, to convert phosphate you will need to multiply the reading by 3.066 and then to translate that to parts per million, divide that number by 1,000. You get used to the HI736 readings pretty quick, example a reading of 52 with the HI736 = 0.159 PPM.
There's a chart online here somewhere that gives you the conversion without doing the math. Good luck
