What is the TU5400 sc's % Relative Standard Deviation (RSD) Function?

Document ID

Document ID TE6768

Published Date

Published Date 25/01/2018
What is the TU5400 sc's % Relative Standard Deviation (RSD) Function?
Explanation of % Relative Standard Deviation available on the TU5400sc turbidimeter
The TU5400 sc offers a complementary parameter to its laser nephelometry measurement. This parameter is known as the relative standard deviation (RSD). RSD is a dimensionless parameter that provides a quantitative assessment of the variability (fluctuation) of the
laser turbidity measurement. Studies have shown that the baseline of a turbidity measurement will often increase in fluctuation before the actual laser turbidity measurement will begin to increase in response to a particle event. In addition, the RSD parameter has been shown to be more sensitive to a turbidity event in addition to serving as a precursor event to a turbidity spike. The Hach Technical Information bulletin, " Introduction to Laser Nephelometry: An Alternative to Conventional Particulate Analysis Methods", Appendix B provides more information on the application and use of the RSD parameter.

The sensitivity of the RSD parameter is directly related to the instrument design of the TU5400 sc nephelometer. This is due to the optical creation of a very small analysis volume within the turbidity sensor. This volume is small but well defined by the optical design. This volume also contains a high energy density from the incident light beam, which can easily be scattered by a single or low number of particles. When a particle passes through the view volume, there is a rapid increase in the scattered light signal while that particle is in the view volume. When the particle passes out of the view volume, the signal decreases rapidly. The change in signal is infrequent
and this instability of signal is quantified by the RSD parameter.

The RSD value is calculated as the standard deviation divided by the mean for a given set of measurements. The result is multiplied by 100 and is expressed as a percent. Equation 1 provides the calculation used to determine the RSD value:

RSD = (standard deviation n ÷ mean n) × 100

Where n = number of measurements used

The RSD calculation is accomplished through a process of evaluating the most recent seven displayed turbidity measurements. From these seven measurements, the standard deviation and the average are calculated. The value is then displayed on the secondary measurement line of the SC controller. When a new turbidity measurement is displayed, the value replaces the oldest of the seven measurements and the RSD is recalculated and displayed. This is referred to as the continuous RSD measurement and it is updated once every second.

The RSD parameter is treated as a separate and independent monitoring parameter relative to the laser turbidity measurement. The parameter is updated every second, which is the same rate the laser turbidity value is updated. The parameter is best used as an early warning parameter to an impending turbidity event and as a complementary parameter to the turbidity parameter. (A turbidity spike will also be complemented by a spike in the RSD parameter.) The parameter has been designed to be very responsive to particles in the
1-10 μm range at very low concentrations. While the laser turbidity parameter is currently approved for regulatory monitoring, the RSD parameter is not a regulatory approved monitoring parameter.

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