Beer color estimation by use of a DIY spectrophotometer

Eureka, the following post is one I am really proud of. It took me some time to build the device and calibrate it. But it seems to work. I am talking about a self-made spectrophotometer to estimate the beer color. Isn’t that cool? I have to say that the measurements are more an estimation than a measurement. However, the precision is good enough to get an idea about the color. Of course there is the possibility to estimate the color of a beer with a kind of chart. I have one of my own. But I always wanted to build myself a spectrophotometer and why not build one for measuring the color of beer. I will shortly talk about the build up and then go into the calibration I did a while ago.

Preparation

I build myself a spectrophotometer as described here. Unfortunately, the descriptions are all in German. But the most important part, and I guess the only one I used from the cited page, is the circuit diagram. The cited page describes how to build a spectrophotometer for measuring nitrate in the water of an aquarium. The only difference to my spectrophotometer is the different LED. I used a 430 nm LED to measure the beer color. To determine the color of a beer, you measure the absorption of the sample in a 1 cm cell at a wavelength of 430 nm. You then multiply the absorption by 25 to get the color in EBC. In my case, I calibrate the spectrophotometer to assign the measured voltages to a EBC-value. Further information about the EBC can be found on Wikipedia.

Building the spectrophotometer

First of all, I do not fully understand the circuit diagram (although it is explained on the cited page). Please do not ask any questions about the circuit. I just ordered all the parts I needed and built the circuit.

Maybe some explanations about the circuit. I used a 12 V power supply unit for the power supply. The red LED (= LED3 rot) is for the “power on” sign. As already mentioned above, I substituted the second LED (LED3 grün) with another LED (emission wavelength of 430 nm) to measure the EBC’s. The light dependent resistor’s (LDR) task is to measure the light from the 430 nm LED after passing through the sample (marked as M). At the end, the resistance from the LDR is transformed into a voltage and measured with a voltage meter (indicated as V). The two linear potentiometers are there to adjust the voltage (for example to adjust the offset).

The next picture might be cruel for people in electronics… I am not very good in soldering and electronics. Luckily for me, I do not have to make money with my bad soldering skills. However, I managed to build this whole device.

Fig 1: Inside the spectrophotometer…

At the bottom of the box is the circuit board with all the connections to the parts such as the red LED (power) and the two linear potentiometers on the cover of the box (Fig 1). The measuring cell is implemented in the wooden block.

Fig 2: Measuring block with LED and LTR

The measuring block consists of two wooden parts, an upper and a lower one (Fig 2). Both parts have a hole with a diameter of 12 mm for the measuring cell (Fig 3). In the block itself are the LED (emitting light at 430 nm) and the LTR (collecting the light coming through the sample). The LED is on the left side of the block, the LTR on the right side (Fig 2). Unfortunately, I do not have any pictures of the inside of the block. However, if you stick the measuring cell in the whole, the LED shines light into the measuring cell and on the direct opposite side of the LED is the LTR collecting the light coming out of the measuring cell. Simple as that.

Fig 3: Cell to measure the sample (12 mm in diameter, total length 100 mm)

And now some pictures of my device how it looks like at the end and during a measurement.

Fig 4: Spectrophotometer ready to measure

You basically connect a voltage meter to the spectrophotometer, insert the measuring cell, adjust the voltage at the beginning (offset and blank) and off you go. In theory, the voltage you measure depends on the color of the beer. Lets see if the spectrophotometer really works.

Calibration

The first test of my spectrophotometer was a calibration with commercial beers. I found a list where someone measured the color of different commercial beers. I used: TsingTao (5.91 EBC), Pilsner Urquell (11.62 EBC), Anchor Liberty (22.26 EBC), Anchor Steam (31.52 EBC), New Castle Brown (56.54 EBC), Samuel Smith Nut Brown (61.46 EBC) and Guinness Draught (92.39 EBC).

I first filled a measuring cell with water and adjusted the voltage with the linear potentiometers to 1 V (blank and offset). Then filled the beers in the measuring cell and collected the voltages for each sample. I have to mention, the spectrophotometer only works with clear samples.

Results

The calibration with the 1 V offset showed a linear correlation between the measured voltage and the color of the sample (Fig 5).

Fig 5: Calibration curve with 1 V offset

I did two different calibration with different offsets. For the second calibration, I used an offset of 2 V. The slope of both calibration were very similar (0.071 V EBC-1). Further information from the calibration:

– Detection limit: 10.6 EBC

– Measure Anchor Steam (31.52 EBC) gives 35.6 EBC (SD = 2.23 EBC)

– Measure Guinness Draught (92.39 EBC) gives 95.1 EBC (SD = 4.04 EBC)

The measured values of the both beers are significantly different from the stated values. However, this is by far good enough for me. I do not need an exact EBC value anyway. On the other hand, my device can’t measure samples with an EBC value lower than 10.6 EBC. No problem here, I do not brew such light-colored beers…

Discussion

The calibration showed a linear correlation between the measured voltages and the color of the calibration beers. And this proves to me that my spectrophotometer can be used to measure (or estimate) the color of a beer in a rather easy way. I stick to estimate rather than measure.

I have to be honest, I never expected this to happen. Before doing this calibration I had to make an assumption: The color of the commercial beers I used were exactly the same as stated in the list. Differences in colors of the different batches should be very minor. In addition, the values stated in the list were true. I assume the differences from the linear fit curve to the measured values could be due to differences to the stated values. Nevertheless, it worked!

To test if this device really works, another calibration would be necessary. This time, measure the values of the different beers with a lab spectrophotometer at 430 nm and determine the voltages with my spectrophotometer. Maybe an experiment for the future.

I did this calibration a few years back and used my spectrophotometer just once. However, I build this device to understand the concept of a spectrophotometer in the first place. Now I know how such a device works and even have a tool available to estimate the beer colors.

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