Are they connected to the RJ45 I/O connector, or to the Pi’s GPIO?
What temperature are you reading? If you look at the software, an out-of-range temperature (< -55 °C or > 125 °C) is reported as -55. You then multiply that by 0.1 in emonhub.conf, to give the value -5.5 °C. If you are seeing exactly -5.5 °C, then your sensors are returning a temperature that is outside the permissible range.
Why? What else are you monitoring with the emonPi? The 9 V a.c. to the emonPi is NOT a power supply, it is a sample of the line voltage for measurement. You need it when measuring real power and voltage.
But they are correct for the remainder of the time? If that is the case, then I think there might be interference from something switching that is causing the temperature to be read wrongly.
Do the bad readings happen at the same time as something switches on or switches off?
Is it always the same sensors that give the bad readings? Where did you buy the sensors? (I ask because there are many not genuine sensors available cheaply, many do not work properly. See
You can remove the false -5.5 °C readings:
In emonCMS, on the Inputs page, you can add the process
If =, skip next with the value -5.5
The help says "If value from last process is equal to the specified value, process execution will skip execution of next process in list. Output: Does NOT modify value passed onto next process step.
The next process must be “Log to feed”, so that you do not log the value if it is exactly -5.5.
You will then have a NULL value instead of a wrong value in your data.
It is in Visualisation.
Step 1) Select Visualisation: Choose EditRealtime
Step 2) Set options: Feed: Choose the Feed you wish to correct.
Step 3) View.
You must then zoom in on the wrong value so that you can see every point in the data. If you saved a temperature every 5 s, then you must be able to see each one.
Click on the point that is wrong, This shows the time and the present value. Type a new value.