In the LaGarde system the card is read by a light source. As the card passes through the reader the light source activates limit switches. In the Stock system the card comes into contact with the limit switches as is passes through the card reader. The limit switches control the signals sent to the control valves which may be either electronically or pneumatically controlled. The profile of the cards are very important.
Care has to be taken to punch or cut the correct profile for each thermal process.
If the profile is not cut correctly for the process the correct process will not be provided. The cards also become worn or damaged during use and care must be taken to maintain the cards in good working condition. The logic controller may be set up as a single loop controller. A single loop controller would be responsible for controlling one function such as temperature in the retort. The controller loop would be programmed to control the retort temperature within set temperature parameters.