How Stuff Works
What We Did
 
   

 

Low Pass Filter (LPF) Smoothing

The ADR thermometer's accuracy is limited by white noise (normally distributed variations in the thermometer's electrical signal). In order to control the temperature more accurately, the control code needs to be able to separate the noise from the actual temperature. One way to filter out noise is to pay attention only to the long-term trend in the temperature and not the individual temperature readings themselves. To do this the control code keeps track of what it considers to be the approximate temperature of the system. To approximate the temperature of the system, each time the thermometer reports a temperature the control code adds a fraction of the difference between what it thinks the temperature is and what the thermometer says the temperature is. This way, if the readings from the thermometer oscillate above and below what the control code thinks the temperature is, the two tend to cancel each other out, removing the noise. On the other hand, if the thermometer consistently reports that the approximation is off, the control code will slowly adjust to reflect the new state of the ADR.