GIFSQ Tools · Module 2 — Food Chemistry & Physicochemical Testing · Tool 36 of 500
Convert a turbidity reading between NTU, FTU and mg/L as silicon dioxide, on the standard formazin equivalence.
Try sample values — fills the form with the worked example below.
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NTU
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FTU
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mg/L SiO₂
Equation: 1 NTU = 1 FTU = 1 mg/L SiO₂ (formazin equivalence). Value in any unit = the same number in the other two units.
Method: Nephelometric turbidity, EPA Method 180.1 / ISO 7027 — formazin is the calibration standard; NTU (nephelometric turbidity units) and FTU (formazin turbidity units) are the same scale, and 1 mg/L SiO₂ is the historical silica-scale equivalent of 1 NTU.
Assumptions: the reading was made on a nephelometric instrument calibrated with formazin; the sample is within the instrument range.
Limitations: turbidity is method-dependent — a nephelometric reading, a Jackson candle reading and an attenuation reading on the same sample are not the same number; this conversion is an equivalence between scales, not a new measurement; particle size, shape and colour change the reading for the same solids level.
Rounding: results to 2 decimal places; intermediate values are not rounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Nephelometric turbidity, EPA Method 180.1 / ISO 7027 — formazin is the calibration standard; NTU (nephelometric turbidity units) and FTU (formazin turbidity units) are the same scale, and 1 mg/L SiO₂ is the historical silica-scale equivalent of 1 NTU.
Assumptions: the reading was made on a nephelometric instrument calibrated with formazin; the sample is within the instrument range.
Limitations: turbidity is method-dependent — a nephelometric reading, a Jackson candle reading and an attenuation reading on the same sample are not the same number; this conversion is an equivalence between scales, not a new measurement; particle size, shape and colour change the reading for the same solids level.
Rounding: results to 2 decimal places; intermediate values are not rounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: A water sample reads 25.0 NTU. On the formazin equivalence that is 25.00 FTU and 25.00 mg/L SiO₂.
Enter values above to see results.
Typical values: drinking water below 1 NTU, clear river water 1–10 NTU, cloudy process water 25–100 NTU. The EPA drinking-water limit is 0.3 NTU in 95 % of samples. Tool 36 of 500 — GIFSQ
