That actually is a more interesting question. Thank you for correcting me on that point.
There is a minimal Gauss that would be expected to work, but this too can be tricky.
First (before I get too carried away): the relationship between the settings on the C5 (or B5) and the peak Gauss are linear with an offset (non-zero). So, by the technical definition of linearity as it relates to scientific instruments, they are not linear, only because the line that relates them does not have a zero intercept. Otherwise, the relationship between the intensity setting and the resulting Gauss (under standard test conditions) is linear. This is actually true for all of our devices: C5, B5, M1, and A9.
On the A9, HIGH corresponds to about 160 Gauss peak, whereas LOW = 70 Gauss peak. On the M1, C5, B5, the highest setting (15) corresponds to about 170 Gauss peak, but the lowest settings correspond to lower than 70 Gauss, about half of that, but it becomes increasingly non-linear as the power settings are reduced due to the peculiarities of DC-to-DC conversion electronics at low gain and low intermittent load. So, it would be safe to say that at the lowest settings, the C5/B5/M1 are generating only about 30 Gauss peak.
Now, be sure to understand that this is not a good number to use for, well, anything. It has been so thoroughly distorted, miscommunicated, and abused by PEMF marketers for the past two decades that you should not rely on it for any comparison. But when looking at minimum Gauss, it is possible to infer some meaning when you grind through the math.
Basically, the peak Gauss for a biophysically meaningful PEMF signal will be a product of two factors:
(1) The pulse has a minimum threshold dB/dt (slope of Gauss versus time). This must be determined experimentally.
(2) The pulse has a minimum pulse width, determined to be as a minimum ~ 80 microseconds. So, ICES-PEMF devices are all set to 100 micro-second pulse width.
Based on threshold experiments, where we observed the minimum magnetic slope required to elicit a biological response:
https://www.josam.org/josam/article/view/38#title-56
(see figure 22.4: dose-response)
and also see:
https://www.josam.org/josam/article/view/27
We found a minimum threshold around 700 KG/s, corresponding to a peak magnetic pulse of 70 Gauss for 100 microsecond pulses, but depending on the biological effect you are looking for, this threshold can vary quite a bit, from about 400 to about 800 KG/s (corresponding to 40 to 80 Gauss peak for 100 microsecond pulses).
But there did also seem to be species-specific differences in sensitivity. Horses, for example, seem to be much more responsive to PEMF than humans, dogs, cats, or rats. This was anecdotal, but was observed repeatedly by different individuals. Also, in the initial studies done at NASA:
https://www.josam.org/josam/article/view/5
https://ntrs.nasa.gov/citations/20030075722
In these, the peak Gauss level was very low, but the rate of the rise and fall of the magnetic field was very large, and the studies were looking at a very sensitive biomarker: gene expression in neuronal cells in culture.
So, under some circumstances you may be able to detect significant biological effects with very low peak Gauss levels. On the other hand, when looking at organismal-level responses in animals that respond similarly to humans, the lower limit does seem to be around 400 KG/s (40 Gauss peak), because the pulse does need to be active for about 100 microseconds, and in the final analysis: peak = rate X time.
The key take away message is this: it is the rate and the time that are important, but the peak (Gauss) is simply the product of the two. Thus, “Gauss” is not fundamental to the biological effectiveness of PEMF, it is a byproduct.
Taking all of this together, I would say the lower limit for a functioning PEMF device, if everything else is designed properly, might be about 40 Gauss peak field strength for each magnetic pulse. I made sure to set the minimum intensity values for the C5/B5/M1 to levels somewhat below this because some people and some animals are somewhat more sensitive than others. The maximum values for the C5/B5/M1 are set to slightly above the point where we see a leveling-off of biological effects.