List of 4 Commonly Used Turbidimeters | Instruments | Water Engineering

The commonly used turbidimeters are as indicated below:

1. Jackson Turbidimeter (or Jackson Candle Turbidimeter):

It consists of a graduated glass rube placed in a metal cylindrical container which is supported on a metal stand. At the bottom of the stand a standard candle of bees wax is fixed. The candle is so designed that it burns within the limits of 114 to 126 grains/hour.

The glass tube and the candle are supported in a vertical position so that the centre line of the tube passes through the centre line of the candle. Further the tip of the support of the candle is 76 mm below the bottom of the glass tube.

For measuring turbidity of water the candle is lighted and some quantity of water to be tested is poured in the glass tube and the image of the candle flame is observed from the top of the glass tube. The depth of water in the glass tube is gradually increased by adding more water till the image of the candle flame just ceases to be seen. The corresponding depth of water in the glass tube (which represents the length of the light path) is read from the graduations on the glass tube.

The Jackson Turbidity Units (JTU) is same as turbidity in ppm on silica scale. It is observed in the table that longer is the light path (i.e., more is the depth of water in the glass tube) lower is the turbidity of water and vice versa. For example for a length of light path (or depth of water in the glass tube) equal to 108 mm the turbidity is 200 JTU, and for a length of light path (or depth of water in the glass tube) equal to 29 mm the turbidity is 800 JTU.

With the help of Jackson turbidimeter turbidity from 25 JTU to more than 1000 JTU may be measured.

2. Baylis Turbidimeter:

It consists of a closed galvanised iron box, on one side of which there are two glass tubes and on the other side there is a 250 watt electric bulb with a reflector. The glass tubes are supported at their lower ends by a white opal glass plate and they are surrounded near their bottom portion by blue cobalt glass plates.

One of the glass tubes is filled with water whose turbidity is to be measured and the other glass tube is filled with a standard solution of known turbidity. The bulb is lighted and the colour of the blue light in both the tubes is observed from the top.

If the colours in both the tubes differ, the tube with standard solution is replaced by another tube with a different standard solution and process is repeated till the colour of the blue light in both the tubes match. The turbidity of the water sample tested will then be equal to the turbidity of the matching standard solution.

With the help of Baylis turbidimeter it is possible to measure accurately even very low turbidities in the range of 0 to 2 ppm.

3. Hellige Turbidimeter (or Aplab Turbidimeter):

It consists of a closed mild steel cabinet which houses a cylindrical glass sample holder (called cell) with a lid, a circular disc (called mirror unit) on which the cell is placed, a platform to support the mirror unit and the cell, an electric bulb, a reflector, a non-reflecting white plate mounted on a stand, and a slit with adjustable shutter.

An ocular tube with eye piece is provided at the top of the cabinet. The non-reflecting white plate is located below the cell. The circular disc supporting the cell is provided with a small hole in the centre and a mirror on its top surface.

The mirror has unsilvered portion in the centre equal to the size of the hole. On the inner side of the door of the cabinet a rectangular mirror with a cover is provided to have better illumination of the water samples having low turbidity, say less than 2 ppm.

Further for the moderation of the light passing through the cell from below, two filters, viz., dark and light, are provided which are inserted according to the ranges of turbidity as specified below:

The water sample whose turbidity is to be measured is filled in the cell and the lid is put on the cell. The cell is placed on the mirror unit and the appropriate filter is inserted below the mirror unit in the space provided for the same. The door of the cabinet is closed and the bulb is lighted. Now by observing through the ocular tube the scattered light from sidewise illumination of the sample is compared with the transmitted light passing through the cell from below.

The shutter of the slit is adjusted by turning the calibrated knob so that the light from both paths is equal in intensity. The dial reading of the turning knob at this instant is noted and the turbidity of the water sample in mg/l is determined from the turbidity v/s dial reading calibration curve provided with the turbidimeter.

With the help of Hellige turbidimeter it is possible to measure turbidity in the range of 0 to 50 mg/l.

4. Nephelometer:

It is a modern commercial turbidimeter which is commonly used these days particularly for measuring low turbidities as in the case of treated water or drinking water for which turbidities usually fall in the range of 0 to 1 ppm. The basic difference in the other turbidimeters and the nephelometer is that in the case of other turbidimeters the measurement of turbidity is based on the intensity of light scattered as it passes straight through the water sample, whereas in the case of nephelometer the measurement of turbidity is based on the intensity of light scattered at right angles to the incident light.

Thus a nephelometer consists of a source of light for illuminating the water sample and one or more photoelectric detectors with a read out device to indicate intensity of light scattered at 90° to the path of incident light.

The measurement of turbidity of a water sample by nephelometer is based on a comparison of the intensity of light scattered by the water Sample under defined conditions with the intensity of light scattered by a standard reference suspension under the same conditions.

The higher is the intensity of scattered light, the higher is the turbidity. Formazin polymer is used as the reference turbidity standard suspension, because it is easy to prepare and is more reproducible in its light-scattering properties than clay or any other material.

The turbidity of a specified concentration of formazin suspension is defined as 40 nephelometric turbidity units (NTU). This suspension has an approximate turbidity of 40 Jackson turbidity units (JTU) when measured on the Jackson candle turbidimeter; therefore nephelometric turbidity units based on the formazin suspension will approximate turbidity units derived from the Jackson candle turbidimeter but will not be identical to them.

The drinking water should not have turbidity more than 2.5 ppm. However, turbidity more than 2.5 ppm may be tolerated but it should not exceed 10 ppm which is the maximum permissible limit for the turbidity in water to be considered suitable for human consumption.