Grove CO2 sensor example code returns bad values

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1

Hi,

Just tried the CO2 sensor example code committed yesterday and am getting bad values. I know the sensor is fine, I tested on an Arduino Uno. I’m using a Pi2, GrovePi+(1.2.5), sensor is plugged into the RPISER port

Here is a sample of the results:

CO2 Conc: 49086 ppm Temp: 173 C
CO2 Conc: 27037 ppm Temp: 154 C
CO2 Conc: 53242 ppm Temp: 187 C
CO2 Conc: 12457 ppm Temp: 70 C
CO2 Conc: 7152 ppm Temp: 79 C
CO2 Conc: 40726 ppm Temp: 58 C
CO2 Conc: 26828 ppm Temp: 145 C
CO2 Conc: 49962 ppm Temp: 5 C
CO2 Conc: 49470 ppm Temp: 99 C
CO2 Conc: 23616 ppm Temp: 10 C
CO2 Conc: 44813 ppm Temp: -15 C
CO2 Conc: 19914 ppm Temp: 3 C
CO2 Conc: 41800 ppm Temp: 103 C
CO2 Conc: 22261 ppm Temp: 96 C
CO2 Conc: 1999 ppm Temp: 150 C
CO2 Conc: 55712 ppm Temp: 9 C
CO2 Conc: 48392 ppm Temp: 203 C
CO2 Conc: 38661 ppm Temp: 14 C
CO2 Conc: 25733 ppm Temp: 51 C
CO2 Conc: 51262 ppm Temp: 112 C
CO2 Conc: 45235 ppm Temp: 93 C
CO2 Conc: 62213 ppm Temp: 43 C
CO2 Conc: 28262 ppm Temp: 58 C
CO2 Conc: 26769 ppm Temp: 96 C
CO2 Conc: 2979 ppm Temp: 67 C

Any ideas why this might be happening?

Thanks,
Keith

2

Hey Keith,
Not really sure about this since it looks like you are getting the correct values just bad ones. Can you try printing the self.inp here: https://github.com/DexterInd/GrovePi/blob/master/Software/Python/grove_co2_sensor/grove_co2_lib.py#L48 and posting the buffer output here so that we can check what the Pi is actually reading.

-Karan

3

He Karan,

I added a line to print:
print " ".join(hex(ord(n)) for n in self.inp)

Here is what I got:
x1 0x7f 0x2 0xd6 0x35 0x49 0x44 0xde 0x0
CO2 Conc: 726 ppm Temp: 13 C
0x0 0x24 0x14 0x0 0x0 0x0 0x0 0x45 0xe8
CO2 Conc: 5120 ppm Temp: -40 C
0x30 0xc2 0x46 0x44 0x4 0x0 0x13 0x24 0xa7
CO2 Conc: 17988 ppm Temp: -36 C
0xc0 0x81 0x92 0x84 0x84 0x50 0xd1 0xd7 0xe
CO2 Conc: 37508 ppm Temp: 92 C
0x1c 0x0 0x2 0xa1 0x2 0x0 0x0 0x9 0x3
CO2 Conc: 673 ppm Temp: -38 C
0x0 0x80 0x2 0x42 0x56 0x24 0x12 0x0 0x0
CO2 Conc: 578 ppm Temp: 46 C
0xe0 0x2 0x5 0x20 0x20 0x8a 0x20 0x0 0x0
CO2 Conc: 1312 ppm Temp: -8 C
0x81 0x0 0x40 0xa0 0x0 0x20 0x48 0x94 0x6
CO2 Conc: 16544 ppm Temp: -40 C
0x80 0x8 0x0 0x3 0x9 0x40 0x20 0x11 0x25
CO2 Conc: 3 ppm Temp: -31 C
0x0 0x80 0x80 0x88 0x42 0x0 0x88 0x40 0x2
CO2 Conc: 32904 ppm Temp: 26 C

FYI, If I use the same code with a FTDI cable and the sensor it gives me what I would expect as valid values.

Keith

4

And here is the output when using the FTDI cable.

0xff 0x86 0x0 0xbc 0x41 0x0 0x0 0x0 0x7d
CO2 Conc: 188 ppm Temp: 25 C
0xff 0x86 0x0 0xb8 0x41 0x0 0x0 0x0 0x81
CO2 Conc: 184 ppm Temp: 25 C
0xff 0x86 0x0 0xbd 0x41 0x0 0x0 0x0 0x7c
CO2 Conc: 189 ppm Temp: 25 C
0xff 0x86 0x0 0xbe 0x41 0x0 0x0 0x0 0x7b
CO2 Conc: 190 ppm Temp: 25 C
0xff 0x86 0x0 0xc3 0x41 0x0 0x0 0x0 0x76
CO2 Conc: 195 ppm Temp: 25 C
0xff 0x86 0x0 0xc6 0x41 0x0 0x0 0x0 0x73
CO2 Conc: 198 ppm Temp: 25 C
0xff 0x86 0x0 0xc6 0x41 0x0 0x0 0x0 0x73
CO2 Conc: 198 ppm Temp: 25 C
0xff 0x86 0x0 0xc8 0x41 0x0 0x0 0x0 0x71
CO2 Conc: 200 ppm Temp: 25 C
0xff 0x86 0x0 0xcb 0x41 0x0 0x0 0x0 0x6e
CO2 Conc: 203 ppm Temp: 25 C
0xff 0x86 0x0 0xcc 0x41 0x0 0x0 0x0 0x6d
CO2 Conc: 204 ppm Temp: 25 C

These messages are consistent with the datasheet.

Keith

5

Also I’ve added a checksum calculation and comparison in the library. Would you like a patch for that?

6

Hey Keith,
I am still looking into this. Can you post the checksum code here on the forums. I’ll try it out later today.

-Karan

7

Here ya go.