So what I've got is a Noctua fan(product link: https://noctua.at/en/products/fan/nf-a4x20-5v/specification) connected to a Raspberry Pi 3. I want to know the RPM of the fan at every second interval. I have a python script that gives me a value for the RPM but the RPM does not seem to be accurate. The fan speed is 5000rpm(+/-10%) at full capacity whereas the script gives an RPM value which is half of what is expected. I need help with how to get an accurate reading using PWM(pulse width modulation) or any other technique.
def calculate_elapse(channel): # callback function
global pulse, start_timer, elapse
pulse+=1 # increase pulse by 1 whenever interrupt occurred
elapse = time.time() - start_timer # elapse for every 1 complete rotation made!
start_timer = time.time() # let current time equals to start_timer
def calculate_speed(r_cm):
global pulse,elapse,rpm,dist_km,dist_meas,km_per_sec,km_per_hour
if elapse !=0: # to avoid DivisionByZero error
rpm = 1/elapse * 60
circ_cm = (2*math.pi)*r_cm # calculate wheel circumference in CM
dist_km = circ_cm/100000 # convert cm to km
km_per_sec = dist_km / elapse # calculate KM/sec
km_per_hour = km_per_sec * 3600 # calculate KM/h
dist_meas = (dist_km*pulse)*1000 # measure distance traverse in meter
return km_per_hour
