I have been on a Low Carbs High Fat diet for about two months now, and I have been measuring my ketosis level using an expensive Ketonix device ($150 device) for about 6 weeks. I also have been comparing the results with readings obtained using a much cheaper Greenwon Digital Alcohol Tester ($15), and a Homasy Keychain Breathalyzer ($15).
The cheaper breathalyzers are sold to find the concentration of alcohol in the breath. Since they are are inexpensive, they tend to get confused by the acetone in the breath of individuals in ketosis. They report elevated alcohol levels even if there was absolutely no consumption of alcohol.
This is a well known problem of older or cheaper breathalyzers, and it could be an issue for people on LCHF diets. Thankfully, law enforcement should have new and updated breathalyzers that do not get confused by acetone, or at least you hope they do (otherwise being on a LCFH diet might get you in some trouble if you are pulled over and asked to take a breathalyzer test).
During 6 weeks I recorded a total of 31 readings with both the Ketonix and the Greenwon and plotted the results on a Graph. I plotted the BAC % value recorded by the Greenwon on the Y axis, and correlated it to the Ketonix Value recorded using the Ketonix device on X axis. The correlation is pretty clear and very close:
Note that the Greenwon doesn't record anything for Ketonix Values lower than 55 or so, which is at the lower limit of the nutritional ketosis range. This means that if you are not in nutritional ketosis, the Greenwon will give you a reading of zero. If you are in nutritional ketosis the Greenwon will give you a reading that is almost linearly correlated to the Ketonix value.
Here is the correlation of the Greenwon and the Ketonix Value, where the Ketonix Value was converted into a more standard PPM Acetone scale (using this method):
For example, a reading 0.042 BAC% on the Greenwon correlates to a 7.2 PPM Acetone, which indicates a strong ketosis state.
I did a similar correlation using the Homasy Keychain. The Homasy readings appear to be less precise and more discrete than the ones taken with the Greenwon, and they do not correlate as closely to the Ketonix Values. Here is the graph:
Note the distribution of readings around 0.050 BAC%, and how the same value correlates to a wide range of Ketonix Values. Also, note how some BAC% readings are really high (up to 0.160).
Here is the same correlation against the Ketonix Values converted to PPM Acetone value (using this method):
Below I plotted the readings taken with the three devices overtime:
|Blue Line = Ketonix Value ^ -3|
Black Line = BAC% measured by the Homasy Keychain
White Line = BAC% measured by the Greenwon
Note how the Greenwon (white line) and the Ketonix (blue line) follow a very similar trend, while the Homasy keychain (black line) generally follows the trend, but the readings are all over the place.
If you want to measure your ketosis state, I strongly recommend a Ketonix, which gives you good readings within a wide range of Acetone levels. With it you can find out if you are in ketosis, but if you are not it gives you a good idea of how far you are.
However, if you want an inexpensive solution, a Greenwon device is a good substitution. In general, if you get a reading > 0.000 BAC% then you are in ketosis. The higher the reading the higher and more intense is your ketosis.