Sensors for National Instruments DAQ

Colorimeter and Turbidity Sensor

The CTSN-100 Colorimeter and Turbidity Sensor performs two functions in one device. First the user can measure the absorption of the sample at the three different wavelengths provided by the Red, Green, and Blue emitters. Second, the user can measure the turbidity of the sample, using the infrared emitter to determine the turbidity of the sample.


Blue Peak Wavelength


Green Peak Wavelength


Red Peak Wavelength


Infrared Peak Wavelength





Input Range in Absorption

0 - 1.398

Input Range in Transmittance

100 - 4%



Output Range

2.5 VDC / 0.1 VDC




25mV/(% transmittance)

Default scaling Value

0V / 0 % transmittance

2.5V - 100% transmittance




Input Range in NTU

50 -  1000 NTU



Output Range

0.1 VDC  - 2.5 VDC




Required with known standards

Default scaling Value


Note: The cuvettes have an arrow marked on one side as a "key", and the CTSN-100 has a mark so that the cuvette may be inserted with the same alignment to ensure the alignment does not contribute to the measurement.

The Red, Green, and Blue LED emitters are factory calibrated to generate +2.5 Volts (+/-1%) at the output for a cuvette filled with distilled water. Absorption equals 0% for +2.5 Volts at the output.

The light-sensor exhibits a "Dark Voltage", which can be determined by blocking the light to the sensor through the cuvette cavity mechanically, and observing the output voltage when no light passes to the sensor. The device is specified to exhibit up to 100mV of "Dark Voltage".

Absorption is not linear with output voltage. To calculate the Absorption, divide the nominally +2.5 Volt output—measured with a cuvette filled with distilled water—by the output voltage measured through the sample, and then take the LOG10 of that value.

- Absorption = LOG10 (2.5 Volts / V_measured)

- For V_measured equal to V_distilled_water, this results in Absorption equal to zero.

- Assuming a worst case "Dark Voltage" of 100mV, the maximum Absorption that the device is guaranteed to be able to resolve is:

Absorption = LOG10 (2.5 Volts / 0.1 Volts) = 1.398

- Assuming a worst case "Dark Voltage" of 100mV, the maximum Absorption that the device is guaranteed to be able to resolve is:

Absorption = LOG10 (2.5 Volts / 0.1 Volts) = 1.398

- Assuming a best case "Dark Voltage" of 0mV, the maximum Absorption that the device is practically capable of resolving is limited by the resolution of the data-acquisition system, typically no better than 1mV.

- Alternately, if the Absorption is known to be 0.25, then Transmittance can be calculated as:
%Transmittance = 10(2-0.25) = 10(1.75) = 56.23%

The three LED emitters provide options for measuring the absorption of a sample. The “Absorption Versus Wavelength” of any sample has some characteristic, known or unknown. In a simple case, the sample will absorb light at all wavelengths other than in a narrow band—typically exhibiting a “Bell Curve” or Gaussian shape—where it does pass light.

It should be noted that some strategy is required in choosing which LED emitter to use to measure a given sample. For example, if the sample appears red, the blue or green LED emitters are better choices than the red for performing the measurement on that sample. This is because the red sample appears red because it absorbs very little of the red portion of the light spectrum.

An “idealized” red sample, centered at 640nM—the same wavelength as the red emitter, would have an “Absorption Versus Wavelength” graph that would have an Absorption equal to zero at 640nM, which would increase for the wavelength below and above 640nM to some larger value. Under this circumstance, regardless of the concentration of the sample, no significant amount of the light generated by the red-emitter will be absorbed, and so it is not useful to attempt to measure absorbance for this sample using this emitter. Assume however that the absorption increases to a value of 0.6 for wavelengths below and above 640nM—including at the 460nM and 520nM wavelengths of the blue or green emitters, and now the concentration of that color in the solution causes measurable absorption.

The Turbidity-Mode utilizes an infra-red LED emitter mounted at 90 degrees to the output-sensor, and the amount of light that couples through the sample is linearly proportional to the amount of particulate matter contained within the sample. The IR LED emitter is calibrated by measuring the voltage at the output-sensor for the 100 NTU and 400 NTU reference (National Turbidity Units) solutions provided, and performing a two-point calibration in software.

With no particulate matter within the solution, the output voltage is nominally zero—again, limited to the value of the “Dark Voltage” of the output-sensor, to within 100mV of zero. As the particulate matter increases, the amount of light that is reflected onto the output-sensor increases, and the output voltage increases.

Output-sensor values are in the range of 1.5 to 2.5 mV per NTU are not atypical.

Turbidity Mode:
1. Connect the iWorx Sensor Adapter to the myDAQ edge connector.
2. Connect the Colorimeter & Turbidity sensor to the iWorx Sensor Adapter
3. Connect myDAQ to your computer using a USB cable.
4. Set the sensor to Turbidity mode
5. Prepare a clean cuvette with the 100NTU standard. Label the cuvette. Click Set for SP1
6. Prepare a different cuvette with the 400NTU standard. Label the cuvette. Click Set for SP2.

Colorimeter Mode:
1. Use default calibrations for colorimeter to measure transmittance.

Measurement Instructions

1. Prepare a clean, dry cuvette with the solution to be analyzed. Place in the CTSN-100 and close the cover.
2. Record Data. Use the red, green, and blue settings to analyze the concentration of the sample.
3. When the experiment is complete empty the cuvette, rinse and leave to dry.
4. Convert the transmittance value into absorption if required for analysis.

1. Follow the calibration procedure above.
2. Prepare a clean, dry cuvette with the solution to be tested.
3. Record and determine the turbidity of the sample.
4. After the experiment is complete put the standard samples back into the vials. Ensure the standards are going into the correct vials.
5. Rinse out all cuvettes and leave to dry.
  • Model: CTSN-100

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