To get from
ppb P to
ppm PO₄ you need to multiply with
0.00307 or divide by
326.2.
Phosphorus (P) has a molar mass of 30.974g/mol,
Phosphate (PO₄) has a molar mass of 94.97g/mol
94.97g/mol PO₄ / 30.974g/mol P ≈ 3.066
So the direct conversion between P and PO₄ works with the factor 3.066 (or divisor 0.3262).
But don't forget the conversion from ppb to ppm, so you need to scale with 1000:
3.066 / 1000 ≈ 0.003066 (multiply by this)
0.3262 ⋅ 1000 ≈ 326.2 (or divide by this)
Example:
If your ppb ULR Phosphorus checker reads 12ppb P you can get ppm PO₄ by:
12ppb P ⋅ 0.00307 ≈ 0.037ppm PO₄
12ppb P / 326.2 ≈ 0.037ppm PO₄