1220 - Wireless Colorimeter and Turbidity Sensor

The Colorimeter 

Colorimetry measures how a sample absorbs or transmits light. Accurate results depend on choosing the correct sample container and wavelength.

Sample Containers 

Three adaptors are supplied: 23 mm for vials, 16 mm for test tubes, 12 mm for 4 mL cuvettes.

Cuvettes have a directional arrow that must face the light source (Path I or II). Plastic cuvettes work well when new but may scratch with use. Because they have two clear and two ribbed sides, they are not suitable for turbidity measurements.

Glass vials and test tubes may have uneven wall thickness. Mark each container so it can be placed in the same orientation for calibration and measurement. If using several containers, check that they give similar transmission or mark each for its best alignment.

Wavelength Choice

Select the wavelength most sensitive to the expected change in absorbance. For example, 625 nm is appropriate for copper sulphate.

Compatibility and Stray Light

Ensure the container material is compatible with the sample (e.g., avoid plastic solvents in plastic cuvettes). Use the Light Seal Cap to reduce stray light.

Instrument Stability

This colorimeter requires no warm-up time. It is stable and ready for use immediately after switching on.

Spectral Selection

The Wireless Colorimeter supports standard wavelength selection (LED) at 470 nm, 520 nm, 573 nm, 625 nm and 850 nm. 

Wavelengths near the boundary of the 625 nm LED emission such 660 nm, can be accessed by restricting the LED’s effective bandpass.

The Wireless Colorimeter supports enhanced wavelength specificity through two complementary approaches:

Optical - using edge filters

Mathematical - using empirically derived correction factors

These methods allow the instrument to extract meaningful information from regions of the spectrum that lie beyond the nominal LED peak.

Access to the 660 nm extended measurements are through Path I using the 625 nm probe wavelength.

Optical   

For 660 nm measurements, the upper portion of the 625 nm emission is isolated using an a thin cut‑off filter. A practical configuration is a 650 nm plus window, which balances spectral selectivity with usable signal intensity.

Mathematical 

This enables the instrument to estimate a monochromatic absorbance from a broadband LED measurement. This approach is powerful because it allows two scientifically meaningful red‑region measurements to be obtained from a single LED:

620 nm region: high signal, broad pigment sensitivity

660 nm region: more selective, tied to well-defined biological absorption features

Most systems can only measure one of these regions, but by characterising both, the instrument can:

Distinguish overlapping absorbances

Validate results against recognised analytical standards

Provide a more complete picture of "red‑region" behaviour

This dual‑region capability turns a simple measurement into a more robust and interpretable analysis. The 660 nm channel is implemented through bandpass‑integrated sensing, and the same principle can be extended across the visible spectrum when appropriate filters or correction factors are available. This allows conversion from a bandpass‑averaged absorbance, Aeff , to an effective monochromatic estimate Aeff>650.                                                                            

Usage in Easysense 

The EasySense Calculate function applies the linear mapping to find the absorbance specific to 660 nm, Aeff>650 compared to the bandpass‑averaged absorbance from the LED (without physical filtering). 

Calculate uses the "ax + b" function to map the above relationship. Under standard conditions, b = 0. 

The "a" corresponds to the analyte‑specific correction factors listed in the table below - value for "a" in EasySense column. 

Empirical Factor R at 660 nm 

Under standardised conditions, the ratio of true 660 nm absorbance to the bandpass‑averaged absorbance has been estimated for several analytes:

Comments on the above.

We used a unified Gaussian‑band model to estimate R‑values for all dyes; it accurately predicts the qualitative behaviour and ranking of dyes in our optical setup, but the exact numerical values are only approximations because real dyes have more complex spectra.

You can incorporate more complex spectral information into the R‑value calculation simply by replacing the simplified Gaussian absorbance model with any wavelength‑resolved dataset, whether measured experimentally, taken from published sources, or constructed from multi‑peak analytical models. The computational method itself remains unchanged—R is still obtained by integrating the LED‑weighted transmission with and without the filter—but using richer spectra allows the calculation to capture real features such as multiple absorption bands, asymmetry, red‑tail behaviour, and scattering contributions. As long as the detailed spectral data are provided, the full calculation can be carried out in exactly the same framework.

Mathematical derivation for the above 

At any particular wavelength, let us consider the absorbance.

Assume the analyte obeys Beer–Lambert at each wavelength λ:

                                            A(λ) = ε(λ) c l

where: ε(λ) is the molar absorptivity spectrum (shape fixed, independent of c), c is concentration, l is path length. Spectral invariance with concentration means ε(λ) does not change with c; only the amplitude scales.

The intensity of the incident light varies with wavelength and we can assign this as  ILED(λ).

                                       ILED(λ) = I0 exp(-Δλ2/(2σ2))                

                                   

The term Δλ2 = (λ - λcentre)2 and σ is the characteristic spread (e.g. 15.4 nm for the red line).  I0 is the "peak" value at the centre band. All wavelength values are taken in nm. The λcentre value (the intensity peak) was set a 627 nm to allow for manufacturing tolerance, temperature, current drive and ageing.  

Other σ values are available for the instrument: 

470 nm, σ = 16 nm

520 nm, σ = 25 nm

573 nm, σ = 9 nm

This allows calculation of other spectral components to be made on other LEDs, should the user require. 

At any selected wavelength:                                                                                                                                 The light that reaches the detector with absorbance:

                                              Iabs(λ) = ILED(λ)10-A(λ)S(λ)

The light that reaches the detector without absorbance:

                                              Iinc(λ) =  ILED(λ)S(λ)

The term S(λ), accounts for the detector sensitivity with wavelength. In our case the variation is small but will be included for future work. 

                                               S(λ) = 0.6 + 0.0016(λ - 430)

The monochromatic absorbance is, A(λ) can be defined as:

                                           

                                                A(λ) = -log10​(Iabs /Iinc​)

 

Absorbance over the full LED spectral range 

Now let us consider the wavelength span available, not just an individual wavelength.

Define the integrated absorbed intensity:

                                

                                               Iabs,full = ∫ILED(λ)10-A(λ)S(λ)dλ

Define the integrated incident intensity

                                               Iinc,full = ∫ILED(λ)S(λ)dλ       

Use minus infinity to plus infinity as the integration limits for both equations.  

The effective absorbance,  Aeff, over the full band is then:                                                         

                                                  Aeff = -log10​(Iabs,full / Iinc,full​)

Absorbance for the filtered region (λ > 650 nm)

The filtered region 650-670 nm, defining the 660 nm at mid-point, has the following relationships: 

λ > 650: ILED(λ) unchanged  

λ ≤ 650; ILED(λ) = 0

Define the filtered absorbed intensity:   

                                              Iabs>650 = ∫ILED(λ) 10-A(λ)S(λ)dλ

Define the filtered incident intensity:

                                               Iinc>650 = ∫ILED(λ) S(λ)dλ

Use 650 nm to plus infinity as the integration limits for both equations. 

The effective absorbance, Aeff>650, for the filtered band is:

                                              Aeff>650 = -log10​(Iabs>650 /Iinc>650​)

Ratio of bandpass‑averaged absorbances

Finally we can define the ratio for the bandpass averaged absorbance:

                                                   R = Aeff / Aeff>650

The work above is from the following sources

1.  Lothian, G. F. and Lewis, P.C.; "The effect of slit width on the accuracy of spectrophotometric measurements, Transactions of the Faraday Society, 1923, 19, 234-239 

2. Randall, H.M.; "Correction for infinite slit width is spectrophotometry" Physical Review 1933, _43_(8), 667-   673. DOI: 10.1103/PhysRev.43.667

3. Gibson. K.S and Balcom, M.M. "Transmission measurements with a Beckman quartz spectrophotometer. J.   Opt. Soc. am. 1949, _39 _(12), 1014-1020. DOI:10.1364/JOSA.39.001014  

Additional Comments

In the above analysis, we have used the relationship:

                                      ILED(λ) = I0 exp(-Δλ2/(2σ2))       

to map out a Gaussian distribution for the LED. For most practical purposes, this is convenient and represents well the beam profile.

Further expansion of the analysis may be employed by some modification to the above to include Lorentzian elements. If purely Lorentzian  

                                     ILED(λ) = I0 [γ2 / (γ2 + Δλ2)]

where

ILED(λ) is the intensity of the incident light

I0 is the peak intensity

Δλ2 = (λ - λcentre)2

λcentre is wavelength that gives the peak intensity value

γ is the Half-Width at Half Maximum value 

So these components contribute (in accordance with Gaussian component) for any wavelength

                                   ILED(λ) = I0 [γ2 / (γ2 + Δλ2)]WL(λ)  +  I0 exp(-Δλ2/(2σ2))WG(λ)   

The full beam profile, if a mixture of the two will result from: 

                                   ILED(λ) = I0 {[γ2 / (γ2 + Δλ2)]WL(λ) + exp(-Δλ2/(2σ2))WG(λ)}

Where WG(λ) and WL(λ) are the respective weighting functions for the Gaussian and Lorentzian components.  

Taking the Lorentzian to be a small component, and that emission exists well above 660 nm with WG(λ) =1, we arrive back at:

                                   ILED(λ) = I0 exp(-Δλ2/(2σ2))  

System Configuration

The instrument software identifies a single physical red channel centred at 625 nm but exposes two logical paths (Path I and Path II). Both refer to the same hardware but allow future expansion, including additional LEDs or custom probe configurations.

The 660 nm configuration extends the system’s operational range toward the red edge of the visible spectrum, approaching the near‑infrared boundary.

Turbidity

Turbidity measures how strongly a sample scatters light. It depends on particle size, shape, and the wavelength used. Near‑infrared (NIR) light is used because it is influenced mainly by scattering rather than absorption. Turbidity is measured at right angles to the incident beam and expressed in Nephelometric Turbidity Units (NTU). Formazin is the standard reference material for calibration.

Sample Containment 

Choose the tube or vial you will use before calibration.

This instrument supports:

Medium adaptor — test tubes, large or medium adaptor — vials

The Data Harvest 100 NTU Formazin Reference Sample is supplied in a high‑quality glass vial and is intended for use with the Large adaptor.

Handling and Alignment 

Accurate turbidity readings require consistent optical conditions:

Use vials with clean, scratch‑free surfaces.

Always place each vial in the same rotational orientation.

Align all samples with the red arrow on the instrument label.

Even small rotations can change the reading. 

Before inserting a vial:

Ensure the optical surface is dry.

Apply a very thin layer of silicone oil (matching glass refractive index, n ≈ 1.5) to minimise the effect of minor surface imperfections. 

Standards and Calibration

Formazin standards slowly lose turbidity as they age and must be replaced periodically. The optional ACC‑26 Turbidity Pack (100 NTU) can be used to calibrate the working range. A quick visual comparison between your sample and the 100 NTU standard helps determine whether the sample is within the measurable range. Very cloudy samples may exceed the sensor’s limits. This instrument does not store turbidity calibrations, so EasySense must be available whenever turbidity measurements are taken to facilitate calibration.

Stray Light Control

Use the Light Seal Cap to minimise stray light and improve measurement accuracy.

Building An Experiment 

Colorimetry based

Data Harvest's EasySense software is used to gather all experimental findings. 

Start the EasySense software and select an experiment type from the "What type of experiment do you want to run?", if prompted. 

Using the Devices icon connect to the colorimeter. The Devices icon will turn green signifying that a connection has been successful.

Once connected, engage the "colorimeter slider", and then decide whether you will need Transmission or Absorbance. 

Activate the LED colour required for the study.  

Make sure that the correct sample insert has been chosen, inserted, and you are confident which way the sample is to be oriented within it. 

Place a control sample into the unit (usually water, note the orientation used for the sample's containment). 

Place the Light Seal Cap on the sample container. 

Use the Calibrate button (the "!" symbol means calibration required); run this procedure to achieve 100% transmission or zero absorbance.  

Close the Calibration dialogue.

Following calibration

Verify which type of experiment (Continuous or Snapshot) you wish to execute - using the Setup icon. Snapshot measurements are used for singular data points in a series. Should you need Continuous data acquisition mode (sample in situ but is changing with time), set an expected sample Interval that is appropriate (e.g., 1s). 

Insert the sample into the colorimeter, make note of the orientation. 

Place the Light Seal Cap on the sample container.

Click Start to initiate collection, Stop to complete.  

Analysis of the data is possible within the EasySense software. If deeper analysis is required, export the data as a CSV and import into a spreadsheet or specialist software. 

After use make sure no liquid remains in the sample chamber and wipe the surface to remove any traces of chemical that may have been spilt. 

Turbidity Based

Start EasySense software and select an experiment type from the "What type of experiment do you want to run?", if prompted. 

Select the Devices icon and connect to the colorimeter. The Devices icon will turn green signifying that a connection has been successful.

Once connected, engage the "Turbidity slider" (Colorimeter will disengage).  

Choose a measurement range to match your needs (200 or 500 NTU) most closely: an approximate gauge can be made by quickly comparing this to the Formazin Reference Sample. 

Prepare the Formazin Reference Sample control vial, by gently rocking the vial back and forth: the aim is to place in suspension the formazin but not to introduce any air or bubbles. 

Wipe the Formazin Reference Sample's surface clean with a soft cloth with a small amount of appropriate oil (n=1.5). 

Place the Formazin Reference Sample in the unit using an appropriate adaptor.

Make a note of the orientation.

Use the Light Seal Cap on the sample container. 

Click on the Calibrate button to generate this calibration point (Point 1).

The second control (Point 2, notionally 0.318 NTU for distilled water) is now put into the unit, and the vial surface prepared for measurement as above.

 

Make a note of the orientation.

Use the Light Seal Cap on the sample container. 

Click on Calibrate to generate this lower calibration point. 

Close the Calibration dialogue.

Following Calibration

Verify which type of experiment (Continuous or Snapshot) that you wish to execute - using the Setup icon. Snapshot measurements are used for singular data points in a series. Should you need Continuous data acquisition mode (sample in situ but is changing with time), set an expected sample Interval appropriate (e.g., 1s). 

Prepare the sample by gently rocking the sealed vial/ test tube back and forth: re-suspend the solution's particles but do not to introduce any air or bubbles. 

Wipe the vial/test tube with a soft cloth and appropriate light oil. 

Insert the sample.

Make a note of the orientation.

Use the Light Seal Cap.  

Click Start to initiate this collection, Stop to complete.

Analysis of the data is possible within the EasySense software. If deeper analysis is required, export the data as a CSV file and import this into a spreadsheet or other specialist software. 

After use, make sure no liquid remains in the sample chamber and wipe the surfaces to remove any traces of chemical that may have been spilt. 

Maintenance

The measurement chamber is not fully resistant to ingress of liquids. Care must be taken to wipe dry any cuvette, tube, or vial being used. 

Please do not use the instrument without the adaptors detailed in this manual.  

If material is being added to the reaction chamber (for example a catalyst to start the reaction being studied), care must be taken to not introduce any liquid to the sample chamber. 

When cleaning the inside of the chamber, please make sure not to damage the optical windows! Plastic solvents and harmful agents (e.g., acetone) should not be used.

---