Eyesight to laser light: The evolution of turbidity measurement

Described simply, turbidity is a measure of the relative clarity - or cloudiness - of a liquid, compared to a predetermined standard.

By Nikolai Pitchforth, Research Analyst, Environmental Health and Safety, U.S., Frost & Sullivan

Turbidity measurement science has come a long way since 1865 when astrophysicist and Jesuit priest Angelo Secchi dipped a white disk into the Mediterranean Sea, in an effort to determine its transparency. Secchi disks are still used widely today, but it was an event in 1900 that led to modern turbidimetry; the development by Whipple and Jackson of the first turbidity standard, from distilled water and diatomaceous earth. Described simply, turbidity is a measure of the relative clarity - or cloudiness - of a liquid, compared to a predetermined standard. It is measured in a variety of ways, but each method relies on the light scattering, light reflecting or light absorbing qualities of suspended matter in a sample.

Turbidity measurements provide no information about the nature of particulate in

a sample, they simply allow us to evaluate the effect it has on light passed

through the sample and compare this to a known standard. In water treatment

applications this comparison provides a useful indication of overall water

quality and is used for such purposes as monitoring filter effectiveness.

Early last century, Jackson applied the standard he developed with Whipple to

create the Jackson Candle Turbidimeter, a glass tube that was positioned above a

candle. The user would add a turbid sample to the tube until the image of the

flame below diffused to a soft glow and the depth of the sample in the tube

would correspond to a standardized turbidity scale. The measurement was referred

to in terms of Jackson Turbidity Units (JTU). Jackson's standards were difficult

to formulate with consistency though and turbidity measurement remained

imprecise until 1926 when Kingsbury and Clark developed formazin.

Formazin, formed by dissolving precise quantities of hydrazine sulfate and

hexamethylenetetramine in distilled water, provided a standard so reliable that

it is still used today as the universally accepted standard for turbidity

measurement. Even with the advent of formazin however, the Jackson candle

turbidimeter remained problematic because it relied firstly upon an imprecise

light source and secondly on human judgment. Improved light sources were

introduced but these did not eliminate the possibility of erroneous human

judgment.

In response to this problem, photoelectric detectors were introduced as a more

accurate substitute for the human eye. While these were more reliable, they

produced inaccurate measurements from samples with very high or very low

turbidity since either too much light was scattered before reaching the

detector, or the change in light intensity across the sample was imperceptible.

The solution to this problem represented the next big advance in turbidity

science.

Rather than measuring the attenuation of light transmitted through a sample, the

detector could be situated at an angle to the incident light beam, permitting

measurement of light scattered by the particulate in the sample instead. This

measurement could then be used to calculate the sample's actual turbidity. A

detection angle of 90 was determined to provide the most accurate measurements

in most solutions and is the angle currently used for measurement of turbidity

in most water quality applications. Instruments that use this scattered light

technique are referred to as nephelometers or nephelometric turbidimeters.

Since the advent of nephelometry, most advances in water turbidity measurement

technology have been modifications of this technique for specific applications.

These have included variations on the scatter angle, an assortment of different

light detectors, and a variety of light sources as well as features to increase

ease of operation such as self-cleaning sensors, memory functions and networking

capabilities.

Despite the lack of incentives for technological innovation caused by the need

for end-users to comply with EPA or ISO prescribed methods for many

applications, nephelometry continues to advance. In the U.S., the EPA has recently

tightened turbidity limits for drinking water and has come under pressure to

reduce the limit further due to evidence pointing to a correlation between high

turbidity levels and the occurrence of harmful protozoa. This, along with

increasing interest in turbidity measurement from ultra-pure water users such as

semiconductor manufacturers, has resulted in increasing demand for instruments

capable of measuring low levels of turbidity. The demand has been met with

astounding effectiveness by instruments that utilize more consistent light

sources such as near infrared LED's, bubble traps to reduce error and multiple

light detectors to ensure measurement precision.

The most extreme manifestation of the growing demand for low-level instruments

is the recent introduction of an instrument with a laser light source by Hach

Company. Hach claims that this instrument is 150 times more sensitive than a

traditional turbidity meter, with a resolution of one-millionth of one NTU.

While some argue that such precision is unnecessary, the ability of this

instrument to detect solids that even a particle counter can miss may open up

entirely new applications for turbidity measurement in fields such as

electronics manufacturing and biotechnology.

The other trend evident in turbidity measurement technology is growing demand

for instruments capable of measuring over a very wide range. Such capabilities

are useful in applications where turbidity levels can be unpredictable and vary

greatly, such as some wastewater monitoring applications. While suspended solids

instruments are available to provide a quantitative measurement of high

particulate levels, they are time consuming and difficult to operate in

comparison to turbidity meters and readings from the two types of instruments

are not easily comparable. Demand therefore exists for an instrument capable of

measuring turbidity accurately over a range traditionally covered by suspended

solids meters, but this is technically difficult to achieve since large

particles or high levels of particulate disrupt measurement of the scattered

beam by absorbing a large proportion of the light.

One solution is ratiometric measurement, which utilizes detectors at several

different angles to the incident beam simultaneously. This eliminates much of

the error caused by the different light-scattering properties of different

particles and accounts for the absorption of light by including a transmitted

light detector to calculate light attenuation across the sample. Like laser

nephelometry, ratiometric measurement may encourage new applications for

turbidity measurement.

A hundred years ago Jackson would have been amazed to know what his candle

turbidimeter would become and how it would be used by the turn of the century.

Recent developments in turbidity science indicate that the heyday of its

measurement may not yet have arrived: modern instruments have created new

possibilities and will continue to improve the utility of turbidity measurement

throughout the new century.

For more information visit www.water.frost.com, or contact Kimberly Howard at 210.247.2488.

More in Environmental