A datalogger is a device which collects and processes raw data from sensors such as light gates. This method involves dropping a double interrupt card through a single light gate connected to the logger, as you can see.
The logger is programmed with the length of each interrupt card ( I used 40mm), weighted a little bit to minimise air resistance and dropped vertically between the light gates.
The internal clock starts when the gate is interrupted and stops when it is not. It then calculates the average speed of each part of the double card as it falls. One speed is greater than the other because the card is accelerating. The internal clock in the logger records the time between the interrupt cards’ fall. Acceleration = change in speed / time.
The logger software can be used to display all speeds and times, or the calculated acceleration directly, which is simpler. Results were as shown.
As you see, all readings (should all be 9.8) are in error by approximately the same amount. This indicates a systematic error in the readings, perhaps because the weight of the card is unevenly distributed or the card lengths are not quite precise enough.
This is the calculation: