Turdibimetry Principes

Turbidimetry is a branch of light measurement in turbid media also call suspension. This suspension can be organic or mineral particles, inert or living, such as bacteria. Its simplicity and versatility in the turbid solutions studied make this technique one of the most widely used measurement methods.

The principle of turbidimetry is simple: the intensity of a light beam passing through a suspension of particles is reduced due to scattering by these colloidal particles. The loss of light intensity is related to the particle concentration and other physical factors such as reflection index, shape ...

Turbidity is measured using an optical system that measures the decrease in intensity of a light beam of known wavelength passing through the suspension. The turbidity of a solution, which we will call [τ], is therefore the absorption or attenuation of an electromagnetic wave for a scattering angle of 0°.

[ l] is the distance travelled by the electromagnetic wave in the medium being studied. [I°] and [I] are the intensities of the rays emitted ([I°] also called "incident") and received ([I] also called "transmitted"). It can be measured experimentally using a conventional UV-visible spectrophotometer that measures absorbance. If there is no molecular absorption at the wavelength used, the absorption is equal to the turbidity at the measurement wavelength.

Probe principles

In principle, the turbidimetry probe consists of a light source and a photoresistor, which are at least one centimetre away from the solution to be analysed. The light source can be either a fixed-length LED or an RJB LED, which allows us to choose the wavelength.

The probe is a 3D-printed body made from polylactic acid (PLA) that enables the components (diode, photoresistor and wires) to be integrated into a cylindrical structure. The cylindrical body is made from a recycled felt-tip pen barrel. Only the head, which incorporates the diode, photoresistor and wires, is 3D-printed.

1. A 5 mm diameter light detection resistor (LDR), GL5539.
2. A 220 ohm resistor required for LED protection (under 5 to 9V)
3. 4 to 8 wires from a Cat5 or Cat6 LAN cable for connections to the LED and LDR (diameter between 0.4-0.5 mm – AWG 24 – 26).
4. A fixed-wavelength LED, depending on the nature of the colloids to be studied; otherwise, a three-colour RGB LED.
5. Solvent-based glue – waterproof glue
6. A 3D-printed probe head is required to house the LED, photoresistor and connection wires. The design of the probe head depends on whether the LED is fixed or RGB. The probe head is printed in two symmetrical parts so that it can be easily assembled.

The two probe heads are printed in two symmetrical parts so that it can be easily assembled.

The design of this probe head must be adapted according to the type of LED used. In our case, we have designed two types of head :

1. The first one on the left is designed to hold a standard monochrome or white LED with a diameter of 5 mm. The cavity for housing the pen tube measures 8 mm. The entire device comes in the form of a 10 mm 'tube' that can easily be installed in any reactor or sample holder. The bottom of the probe houses the LDR. A window allows the liquid to be easily passed through for study. The LED is located opposite the LDR.
2. The second one from the right is for the larger RGB LED. Although it has the same components as the monochrome LED, such as the pen tube housing and window, the body of the head is wider to accommodate the LED. This allows us to create a wire path for the LDR.

To simplify assembly and increase versatility (i.e. the ability to change the probe), we have included the protection resistor in the probe itself rather than in the main circuit.

Consider a standard LED with a threshold voltage of 1.7 V, which cannot be exceeded by a current greater than 20 mA over an extended period. Our power supply is 5 volts. In order to protect it, we must therefore connect a minimum resistance of :

R = (5 - 1.7) Volt / 0.02 Ampere= 165 ohm

We soldered a 220-ohm resistor to the cathode of the LED (or the anode if it is an RGB LED). The connections were then sealed with solvent glue to ensure waterproofing and electrical shunting.

The LDR is connected to two wires by soldering and glued in a similar way to the LED. These elements and their connection wires are then assembled. The photoresistor wires are housed and placed either outside the probe head with a monochrome LED or inside the probe head with an RGB LED :

The head is then closed by its symetric part, and the pen tube, through which the electrical wires have previously been passed, is glued into the provided housing. The probe is now ready for use.

Its functionality is then verified using the turbidimeter that we have developed.

This 'Instructable' article is taken from the e-book The Turbidimeter, which is about making digital and analogue turbidimeters. You can find this e-book in the SciencExpert library.