DS18B20 is throwing -127C from time to time

Getting Started
1

Hello,

Any ideas why my DS18B20 is giving -127C from time to time - this is completely messes with the temperature graph over time.

Thanks
Alex

2

Like the other thread…gotta catch CRC failures and throw those results out.

 if (OneWire::crc8(data, 9)) {
      Serial.println("CRC Failed");
    //do something here
    }
3

Sorry about that, I thought you were part of another thread about this. I have just published a lib that includes an easy to use CRC check that you can use to throw away bad results. Look at the example app and you will see what I mean.

DS18B20 0.1.00
DSB18XX Lib on Particle Photon
4

I have a compile error when I try to compile the code below:

sensors.requestTemperatures(); tempc= sensors.getTempCByIndex(0); if (OneWire::crc8(data, 9)) { { Serial.println("CRC Failed"); } else { Particle.publish("temperature_Hot_Water_Tank", String(tempc) + "°C"); }

as ‘data’ is not defined. I am using slightly different method than in the example.

5

Post your gettempbycbindex function. You need to compare the calculated CRC with the CRC in the 9th bit of the sensor returned data.

6

I found the lib you are using. A few things…
The -127 is a defined error value that is thrown when the device is considered disconnected. In DallasTemperature.h

    // Error Codes
#define DEVICE_DISCONNECTED -127

-127 is outside the range of the device so you can just toss that value like so…untested.

if (tempc == -127)
{
Serial.println("Bad temp read");
}
else
{
Particle.publish("temperature_Hot_Water_Tank", String(tempc) + "°C");
}

The getTemp() function in DallasTemperature.cpp is what the getTempCByIndex() leads to. There you can see in the comments that if sensor.isConnected(DeviceID); is false OR !sensor.isConnected(DeviceID); it will return the DEVICE_DISCONNECTED value that is defined in DallasTemperature.h.

7

Thanks @LukeUSMC

I have just implemented similar code before seeing your post given this is an internal temp sensor.
We are thinking in a similar way I guess

if (tempc < 0) { Serial.println("Temp read failed"); } else { Particle.publish("temperature_Hot_Water_Tank", String(tempc) + "°C"); }