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Sensors

There are a bunch of different sensors out there, based on price and sensitivity I have chosen to buy two different sensors the TGS822 from Figaro sensors and the MQ-3 from HANWEI ELETRONICS. If i wasn’t on a budget I would also have ordered a WSP2110 –  air polution detection since I believe that a combination with on of the TGS882 or MQ-3 with the WSP2110 could be a good match where the WSP2110 is has a sensitivity range between 1-50 ppm and the other two has a range between 10-1000 ppm. So the combined result of both could give us a more granular scale at lower concentration levels.

Figaro Sensors

TGS822 – Alcohol (ethanol) gas sensor, gas sensors is a tin dioxide (SnO2) semiconductor.

From datasheet for TGS 822

From datasheet for TGS 822

HANWEI ELECTRONICS CO.,LTD

MQ-3 – Alcohol Gas Sensor, is also a SnO2 sensor so it should have similar detection abilities as the TGS822 but acetone is not mentioned in the data sheet. According to one manufacturer this sensor has a detection range between 10-1000 ppm for alcohol and then about the same apply for acetone.

MQ-3 Sensor

From data sheet for MQ-3 sensor

MQ303A – Alcohol Gas Sensor, is essentially the same sensor as the MQ-3 however it works on a lower voltage. One serious disadvantage with this one is that a manufacturer states that the sensitivity of this sensor is 20-1000 ppm.

When you compare the MQ303A with the MQ-3 sensor’s range of 10-1000 ppm I think I will go with the MQ-3 sensor instead of the MQ303A since the concentrations we want to measure is between 0-200 ppm.

WSP2110 –  air polution detection, this sensor has a different ceramic substrate of subminiature Al2O instead of th SnO2 used in the other sensors and is more sensitive but also has a smaller detection range then the others, 1-50 ppm. it is also a lot more expensive as the other sensors.

ME3A – C2H5OH – This sensor works with a completely different type of chemical process.

”Detects gas concentration by measuring current based on the electrochemical principle, which utilizes the electrochemical oxidation process of target gas on the working electrode inside the electrolytic cell, the current produced in electrochemical reaction of the target gas are in direct proportion with its concentration while following Faraday law”

It seems to be very sensitive and detects between 0-1.000mg/L alcohol per liter, however it also have a price of /piece. Since this sensor works with a completely different technique I am not even sure if it detects acetone as well as ethanol and it is also way to expensive so just forget about this one.

Annonser

Expected aceton concentration

After doing some Google searching I found several different studies on the concentration of acetone in a persons breath and here I will look in to some of them.

Study 1

I found a Study published in the paper ”The American Journal of clinical nutrition” where the concentration of breath acetone was measured after every hour for persons while eating a ketogen diet for 12 hours which states. ”Changes in breath acetone, plasma acetoacetate, plasma β-hydroxybutyrate, and urinary acetoacetate over the 12-h dietary study period are illustrated in Figure 1. By the end of the study, breath acetone increased 3.5-fold (from 33 ± 13 nmol/L at 0 h to 116 ± 19 nmol/L at 12 h).”

Breath Acetone

Breath Acetone

This gives an indication for what concentration of acetone that can be expected, however the persons in the study had not been eating a ketogen diet before the study so I don’t know what levels a person that has been eating a ketogen diet for a longer while will have but I suspect it will be higher then the 116 ± 19 nmol/L the participants in the study showed. Since most sensors give the sensitivity to different gasses in their data sheets as ppm i need to convert the 116 ± 19 nmol/L to ppm.

Found concentration in breath = 116 ± 19 nmol/L, nmole = (10-9) of a mole.

To convert nmol/L to ppmv we need to know the volume of 116 ± 19 nmol/L of acetone. This can be done by using the Ideal gas law.

V = nRT/P

P = atm = 1

V = Liters

R = 0.08206 L·atm·mol−1·K−1

n = measured in moles

T  = in kelvin (273.15 Kelvin = 0 C)

At 23 degrees and at 1 atm that amounts to: ((116 * 10-9) * 0.08206 * 296) / 1 = 2.81761×10^-6 liter = 2.8176 µL (microliters) parts per million in a gas system is equal to µL/L So the concentraion for the subjects in the study was 2.8176 ppmv.

Study 2

In this study a prototype of a ”acetone breach detector has been built and the engineers tested it by fasting for 17 hours and then blowing in to it.

”Results indicate acetone concentrations of 2.5ppm and 0.7ppm. Notably, the author (‘Steve’) had fasted for 17 hours and recorded a slightly high breath acetone value. When the sensor is recently calibrated and has been optimized properly, acetone sensitivity for breath measurements is conservatively estimated at several tenths ppmv, and it is appropriate for breath acetone measurements of healthy, metabolically stressed, and diseased individuals.”

Study 4

”The daily average acetone concentration of the dieters during this period was 290 nmoI/L (SD 8.1, range 280-300 nmol/L). The control subjects showed a daily average breath acetone concentration of 15 nmoIJL (SD 11 nmol/L) .”

(((300 * (10^(-9))) * 0.08206 * 296) / 1) * liters = 7.286928 microliter = 7.286928 ppmv

Study 4

Some rats on a ketogenic diet reached 500 nmol/(L*kg)

Conclusions

So all studies I have looked at indicated that the breath acetone concentration should be around 2-7 ppm. When it comes to sensor sensitivity that i very low and it will be difficult to find a sensor that has that level of sensitivity.

However I think the concentration of a acetone in a persons breath that has been on a ketose diet is significantly higher. Since many people report that the smell of aceton is noticeable and the required concentration of acetone for it to be detected by smell is 200 ppm according to the CDC I have decided to go set as a hypothesis that the sensor range should be around 0-200 ppm.