Liter per time: 24.8400

...and that is not what I had expected.

The engines I develop this flow meter to, uses between 3 and 50 liters per hour.

24 is between 3 and 50, so ?? Be aware that this is the consumption per hour!

The device gives 10.000 pulses per liter according to spec. So the formula seems OK

pulseCount = (pulseCountIN - pulseCountOUT) / 10000.0 * 3600;

better split the two (and give the vars good names) to debug them seperately.

float LPH_IN = pulseCountIN/ 10000.0 * 3600; // pulses to Liter/hour

float LPH_OUT = pulseCountOUT/ 10000.0 * 3600;

The fact that you get ~24 Liter means that the incounter get far more pulses than the outcounter. This makes sense as the device consumes fuel.

About AccuracyThe minimum consumption it can detect is one pulse. If you measure a delta of 1 pulse per second it equals a difference of 1/10000*3600 = 0.36L

Look at your measurements

Liter per time: 23.4000

Liter per time: 24.1200

Liter per time: 24.8400

Liter per time: 25.2000

Liter per time: 25.5600

Liter per time: 24.8400

and you see the delta's between the measurements are 0.36 or 0.72. That is one or two pulses difference.

The best way to have a better accuracy is to measure the fuel-consumption over the last minute * 60. A delta of 1 pulse will result in a delta of 0.006 L

The best way to do this is to make measurements per second and use them to fill a circular buffer of 60 elements.