Sensors & transducers


for many other parameters, including dissolved oxygen/ biological oxygen demand, nitrates, ammonia, and chlorine.


optics The optical path configuration for absorbance measurements is shown in Figure 3. With the CN0503, absorbance measurements can be made in any optical path (1 through 4). The incident beam is directed at the beam splitter where a reference photodiode samples the intensity of the beam. The remaining power is directed through the sample. Taking the ratio of sample to reference light removes variation and noise of the LED source, and synchronous pulse and receive windows provide ambient light rejection.


setup equipment CN0503 evaluation board EVAL-ADICUP3029 evaluation board API pH Test & Adjuster Kit pH standards


Figure 4. The CN0503 taking a pH measurement.


Figure 6. Absorbance calibration curve of pH at 615 nm.


be used for this. The firmware implements two fifth-order polynomials, INS1 and INS2. Once the polynomials are stored, the INS1 or INS2 mode can be selected so that measurement results are reported directly in the desired unit - in this case, pH. This makes it simple to quickly get a result for an unknown sample. To get the noise value, two different data


In this experiment, a colour indicator


(bromothymol blue) was added to prepared solutions with different pH values. The solutions were transferred into cuvettes and tested at two different wavelengths, 430 nm and 615 nm, where the indicator shows absorption changes vs. pH. The CN0503 makes this easy; the two different wavelength LED cards can be inserted into optical path 2 and optical path 3. Then the cuvette holder is simply moved into different paths for the different measurements.


results Using the CN0503 evaluation GUI, measurement results from both optical paths were easily exported into Excel. The resulting calibration curves for the two different wavelengths are shown in Figure 5 and Figure 6. In each case, the pH vs. absorbance was


plotted to create the calibration curve. The Add Trendline option is then used to get an equation for the curve. The equations are then used to determine concentrations of unknown samples. The sensor output is the x-variable, and the resulting y-value is the pH. This can be done by hand; however, the CN0503 can also


Instrumentation Monthly February 2022


points were chosen for each wavelength: one lower pH value and one higher pH value. Two points were used because the curve fit was not linear in this case. The standard deviation of a set of repeated measurements for each point, reported as the noise value in Table 1, describes the precision of the measurement, excluding variations in sample preparation. LOD is typically determined by measuring


noise at low concentration and multiplying by three to give a confidence interval of 99.7 per cent. Because pH is a logarithmic scale, pH 7


Figure 7. Optical path for turbidity measurement.


optics Figure 7 shows the optical path for turbidity measurement with a 90° or 180° detector. With the CN0503, turbidity measurements can be made in optical path 1 or 4 only, as a 90° detector is needed. There are various measurement configurations and standards for turbidity. This example demonstrates a modified version of EPA Method 180.1, calibrated and reported in nephelometric turbidity units (NTU).


setup equipment CN0503 evaluation board EVAL-ADICUP3029 evaluation board


Hanna Instruments turbidity standard calibration set


Table 1. pH MeasureMenT noise Values 6.1 pH Sample 430 nm RMS Noise Value (pH) 0.002098 615 nm 0.000183 7.5 pH Sample 430 nm 8.18994 × 10–7 Table 2. pH MeasureMenT liMiT of DeTecTion 7 pH Sample 430 nm Limit of Detection (pH) 0.001099 615 nm 0.001456


Continued on page 46... 45


615 nm 0.000165


was chosen as the number to check the LOD, as shown in Table 2.


Figure 5. Absorbance calibration curve of pH at 430 nm.


Measuring TurbiDiTy Turbidity background Turbidity measurements of a liquid sample use the light scattering property of particles suspended in the liquid. Ultimately, it is a measure of the relative clarity of a liquid. The amount of light scattered and the scattering angle differ based on particle size, concentration, and the wavelength of incident light. Measuring turbidity is conducted in many industries, including water quality and life sciences. Besides general turbidity, the CN0503 could be used to determine algae growth by measuring optical density.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74