This article explains the different sources of noise that cause erratic temperature fluctuations.
Noise is a common issue in PC based temperature logging systems and can originate from different sources:
The temperature sensor itself can produce noise, e.g. when the probe is grounded or worn off.
Grounded probes are too sensitive for roast temperature monitoring, and prone to induce noise from ground loops, which results in a less accurate reading.
For more information check out Ungrounded vs. grounded probes.
It is not possible to determine whether the thermocouple is grounded or ungrounded just by looking at it. You will need a multimeter to perform this test as shown in this video by Omega.
We recommend replacing the grounded probe for an ungrounded.
Worn off probes
Over time your probe will become worn off due to constant vibrations caused by the coffee beans. On worn off probes, the sensor within the probe will come in contact with the sheath. Creating an electrical connection, which in turn can introduce noise from ground loops, similar to that of grounded thermocouples. Those can be noticed as irregular spikes in the temperature curve. In that case you will need to replace your probe.
Poorly isolated probe
When there is constant noise while the machine is running, it may refer to a poorly isolated probe. You can try moving the sensor a bit. If moving the sensor does not do the trick, you may need to unground the thermocouple from the machine by isolating the probe.
Signal wire noise
Signal wires, such as your thermocouple wires, are sensitive to electrical interferences and disturbances. The interference can come from many different sources (e.g. motors or power cables) in form of EMI (electro magnetic interferences) and RFI (radio frequency interference). The interferences distort the signal being carried by the cable and result into a noisy roast curve and RoR.
Signal wire noise can be eliminated by rerouting the sensor wires or using ferrite beads.
Ground loops, or earth loops, occur when two different points in an electrical circuit are at a different electrical potential.
This can happen when two electrical devices are interconnected that have a different path to the ground. Causing a closed conductive loop where small currents, also known as leaking currents, can flow. Ground loops are a major source of noise or interference that are noticeable as spikes in your roast graph. E.g. Your roast machine being connected to the earth ground and your computer interconnected by a USB cable or Cropster Connector (Phidget) to the power outlet. If you have a grounded or worn off probe that isn't isolated from the roaster it can be sufficient to close the loop between the two.
You can check if this is the case by disconnecting a laptop from the power and running it on battery mode. In that case only one ground remains and the noise should stop.
To eliminate a ground loop, without running your laptop in battery mode, you will most probably want to replace the thermocouple or isolate it against the roaster. If you are using a Cropster Connector (or Phidget), you may want to add a USB isolator in between the Phidget and the computer.