The Insta-Test phosphate pool & spa test strips provide fast and reliable results. Phosphate is an important nutrient for algae. Try to keep phosphate levels below 300ppb (0.3 mg/l) to avoid problems in your pool. Measuring range: 0, 100, 200, 300, 500, 1000 and 2500 ppb. Phosphate is always measured in PPB (parts per billion). To convert ppb to ppm or mg/l, divide by 1000. Example: if the test strip measures 300 ppb then this is 0.3 mg/l (ppm).