August 17, 2016

Accurate temperature reading from Raspberry PI Sense HAT

When I first received the Sense HAT for my Raspberry PI 3 I decided to start building a nice weather station using the embedded environmental sensors and RGB LED matrix.
From a quick experiment it turns out that the temperature readings are not accurate. The problem is caused by thermal conduction from the Pi CPU to the humidity and pressure sensors on the Sense HAT.
I have experimented few algorithms and I have found the following formula to be the most reliable. It basically compensate the temperature reading (t) with the CPU temperature (tCpu).
tCorr = t - ((tCpu-t)/1.5)

Here is the full script to print each 5 seconds the environmental data including the corrected temperature.

import os
import time
from sense_hat import SenseHat

def get_cpu_temp():
  res = os.popen("vcgencmd measure_temp").readline()
  t = float(res.replace("temp=","").replace("'C\n",""))
  return(t)


sense = SenseHat()

while True:
  t = sense.get_temperature_from_humidity()
  t_cpu = get_cpu_temp()
  h = sense.get_humidity()
  p = sense.get_pressure()

  # calculates the real temperature compesating CPU heating
  t_corr = t - ((t_cpu-t)/1.5)
  
  print("t=%.1f  t_cpu=%.1f  t_corr=%.1f  h=%d  p=%d" % (t, t_cpu, t_corr, round(h), round(p)))
  
  time.sleep(5)

Running this script I have noticed that the CPU temperature reading is not very stable making the corrected temperature a little bit unstable. To fix this issue I have decided to apply a moving average to the temperature reading.
As a further improvement I'm also the temperature both from the humidity and pressure sensors and calculating the average.

import os
import time
from sense_hat import SenseHat

# get CPU temperature
def get_cpu_temp():
  res = os.popen("vcgencmd measure_temp").readline()
  t = float(res.replace("temp=","").replace("'C\n",""))
  return(t)

# use moving average to smooth readings
def get_smooth(x):
  if not hasattr(get_smooth, "t"):
    get_smooth.t = [x,x,x]
  get_smooth.t[2] = get_smooth.t[1]
  get_smooth.t[1] = get_smooth.t[0]
  get_smooth.t[0] = x
  xs = (get_smooth.t[0]+get_smooth.t[1]+get_smooth.t[2])/3
  return(xs)


sense = SenseHat()

while True:
  t1 = sense.get_temperature_from_humidity()
  t2 = sense.get_temperature_from_pressure()
  t_cpu = get_cpu_temp()
  h = sense.get_humidity()
  p = sense.get_pressure()

  # calculates the real temperature compesating CPU heating
  t = (t1+t2)/2
  t_corr = t - ((t_cpu-t)/1.5)
  t_corr = get_smooth(t_corr)
  
  print("t1=%.1f  t2=%.1f  t_cpu=%.1f  t_corr=%.1f  h=%d  p=%d" % (t1, t2, t_cpu, t_corr, round(h), round(p)))
  
  time.sleep(5)

2 comments:

  1. Interesting, I thought the problem was with the sensor being so close to the board. I will have to try your solution.

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  2. How hard would it be to add more sensors and cobble it all together to send to wunderground?

    ReplyDelete