@hl68fx, the Grove sensor has an adjustable load resistor (RL) so you can adjust the maximum voltage output/sensitivity of the MQ2 sensor. You can power both the MQ2 heater and Vc (the sensor voltage) with 5V and measure the output of the Grove board using a voltmeter (NOT connected to your Core just yet). Adjust the trimpot to some safe value below 3.3V and then blow some smoke on the sensor to see how it reacts. Adjust the trimpot so the voltage never exceeds 3.3v. Typically, that would be around 4700 ohms or so. Once you are confident that the output voltage will not exceed 3.3v, you can connect it to your Core.
If possible, disconnect the MQ2 sensor entirely and measure the set resistance of the trimpot (RL). Then the formula
Vout = 5 * RL / (RL + RS) where RL is the trimpot resistance and RS is the sensor resistance.
You can solve for RS, giving (if I’m correct!):
Rs = RL * (5 - Vout)
---------------
Vout
You can then get the ppm from the datasheet chart. 