Missing documentation for `read_all()` calculations

[benthorner]

Where does this calculation come from?

oxd = (oxd * 56000) / (vref - oxd)

I'm seeing an average value of 35KOhms for the OX sensor in normal air, which is much higher than the max of 20KOhms stated in the datasheet (assuming I'm reading it correctly). I'm wondering if the calculation may be wrong.

I understand that get_raw_oxd() returns a (scaled) voltage for the OX sensor - a 12 bit number scaled to 0-1 and then scaled by _vref to 0-3.3. I thought the issue might be with _vref, but that would make the values even higher.

I found a similar calculation in the CircuitPython library as well. The formula is the same, just expressed differently and with a constant of ~2**16 instead of Vref (and the values haven't been scaled to voltages). I still don't get it.

Could you help me understand the values that come out of read_all()?

[Gadgetoid]

Each channel is pulled up to 3.3v via a 56k Ohm resistor so the output value is formed by a voltage divider between 56k Ohm and the resistance of the gas sensor output channel. This formula converts from a voltage - read by the ADC - back to a resistance value, accounting for that.

Afaik the calculation is correct, but the datasheet isn't particularly helpful and the graphs suggest resistance increases exponentially outside of the given min/max ranges.

[benthorner]

Thanks @Gadgetoid ! I suspect you're right about the scaling - I noticed the scale on the graphs looks logarithmic, but it's not clear (or even clear what the numbers represent).

Do you know if there's any way to clarify this? All I can think of is emailing the manufacturer and hoping they have a sample algorithm they can share that's more detailed than the data sheet.

What do you think?

[simao]

@benthorner I'd also be interested if you get any more info from the manufacturer, I am also getting values in the 200KOhms range and cannot make sense of this.

[benthorner]

That's good to know @simao. Here's the email I just sent to SGX Sensortech:

I purchased a MICS6814 sensor from Pimoroni in a pre-soldered format 1. However, me and the folks at Pimoroni are struggling to interpret the voltage readings. In particular, the graphs in the datasheet 2 indicate there is some kind of exponential relationship between resistance and gas concentrations, but it doesn't specify what the relationship is.

Could you publish some more details or a sample implementation of how to convert data from this sensor into the units used in the datasheet graphs? I appreciate the readings are "relative", but without more specific instructions it's hard to make any sense of the data - the sensor may even be faulty, I just can't tell.

I've written up some more detail about this issue on this page 3. It looks like several people are having issues working with this sensor. Please could you help us out?