Turbidity is a measure of the cloudiness of a fluid associated with the presence of particulate matter. The result is a permanent cloudiness associated with the scattering of light, which is reported in nephelometric turbidity units (NTU). The TN400 measures scattering at 90 degrees to a laser beam at 850 nm (which is the ISO 7027 method).

To maximize accuracy, the TN400 comes with four EPA certified reagecon polymer standards with NTU values of 0.02, 20, 100 and 800 NTU for the calibration purpose (see the attached photo). The calibration procedure is simple, nicely automated and outlined in detail in the manual. The accuracy of TN400 is dependent upon carrying out the calibration sequence properly, but the meter requests the standards in order so a mistake is unlikely. You can also use only a pair of standards if you know the approximate NTU value of the unknown samples. Because linear interpolation is used between the two closest standards, this approach does not diminish accuracy provided the measurement falls between the two calibration standards.

An important requirement of all high quality turbidity meters is to minimize scattering between the walls of the sample chamber and the sample vial, and this can be accomplished by putting a thin coating of silicon oil on the outside of the sample vials. Silicon oil is included along with the standards (see upper right of first figure). I found that using silicon oil was critical for accuracy below 20 NTU. At higher values, the silicon oil is much less critical. Because most work is done below 20 NTU so the extra effort is usually required. The silicon oil does not evaporate and will stay on the outside for an extended period so I found that it was not necessary to do this each time.

I used some formazin standards and confirmed that the meter is accurate to ±2 NTU units in the region 100 - 800. The manual reports a higher accuracy than that, but I was unable to confirm it independently because the standards have an uncertainty of ±1. The reproducibility is excellent with the silicon oil applied and calibration is done prior the measurements.

Turbidity is an excellent method of measuring the particulates in tap water, and checking the distilled water. Having a 0.02 NTU standard is critical for measuring my samples accurately. In our laboratory, we use turbidity to measure the growth process of archaea that we study for basic research studies. These one cell creatures scatter light like small particles and thus growth can be monitored very accurately by using turbidity. This meter is a welcome addition to our molecular biology laboratory.

The performance to price ratio for the TN400 is the best of all the available turbidity meters of which I am aware. The TN400 is easy to use and calibration is simple. I recommend the TN400 with enthusiasm for personal, commercial or basic research applications.