Turbidity meter · Calibration procedure
Formazin standard calibration
Calibrates the turbidity meter with a series of formazin standards covering the working range, then verifies the calibration with an independent standard.
PURPOSE
Why this is checked
Frequency
Typically every 3 to 12 months as the manufacturer and your SOP require, after lamp replacement or repair, and whenever daily verification fails.
Learn how the Turbidity meter worksYOU WILL NEED
Standards & tools
- Formazin stock suspension, 4000 NTU (prepared from hydrazine sulphate and hexamethylenetetramine, or purchased stabilised formazin)
- Dilution water: purified water filtered through a 0.2 µm membrane
- Calibrated volumetric pipettes and flasks
- Independent verification standard (e.g. stabilised formazin or secondary gel standard)
BEFORE YOU START
Prerequisites
- Instrument warmed up
- Cuvettes clean, unscratched and indexed
- Standards freshly diluted on the day of use (dilute formazin is not stable)
PROCEDURE
Step by step
- 1Prepare the calibration standards by diluting the 4000 NTU stock, for example 20, 100 and 800 NTU (or the levels the manufacturer specifies).
- 2Measure the dilution water as the blank (it should read very low, typically below about 0.1 NTU).
- 3Mix each standard gently by inversion, fill a cuvette, wipe it and measure it in the calibration mode, from lowest to highest.
- 4Accept the calibration when the software confirms it.
- 5Measure the independent verification standard and record the result.
RECORD
Example observation table
| Standard | Preparation | Nominal (NTU) | Verification reading (NTU) | % difference | Result |
|---|---|---|---|---|---|
| Dilution water | 0.2 µm-filtered water | < 0.1 | 0.05 | — | Pass |
| Std 1 | 5 mL stock → 1000 mL | 20 | 19.6 | −2.0% | Pass |
| Std 2 | 25 mL stock → 1000 mL | 100 | 101.2 | +1.2% | Pass |
| Std 3 | 200 mL stock → 1000 mL | 800 | 792 | −1.0% | Pass |
CALCULATION
How to calculate the result
C2 = C1 × V1 ÷ V2; % difference = (Reading − Nominal) ÷ Nominal × 100
- 20 NTU standard: 4000 NTU × 5 mL ÷ 1000 mL = 20 NTU
- 800 NTU standard: 4000 × 200 ÷ 1000 = 800 NTU
- Verification reading 19.6 NTU: (19.6 − 20) ÷ 20 × 100 = −2.0%
ACCEPTANCE CRITERIA
Pass if
- Each verification reading within the limit set by the manufacturer or your SOP (commonly within ±5% to ±10% of the nominal value, with a tighter absolute limit at low NTU).
- Blank (dilution water) within the low limit in your SOP.
IF IT FAILS
What to do
- Clean or replace cuvettes, prepare fresh dilutions and repeat.
- Check the stock suspension's age and the dilution water.
- If it still fails, check or replace the lamp, call service, and assess results since the last pass.
In LimsPulse · Calibration Management
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.