Vernier Software and Technology
Vernier Software & Technology
Tech Info Library

Why does my data table have holes (empty cells) in it?

Some data tables will have empty cells. Sparse data tables are not errors, but can happen for a variety of expected reasons. Here are several examples when this might happen:

1) If a motion detector fails to receive a return echo, so that the data received is known to be bad, Logger Pro and LabQuest App will display a blank cell for position, velocity, and acceleration. The point is then not graphed, but if connecting lines are enabled, there will not be a gap in the graph trace. Graphical Analysis does not do this; instead it will place the value 3.6 m into the data cell if no echo is detected.

2) Ultrasonic motion detectors cannot collect data faster than 50 Hz, and Go Direct Motion has a maximum rate of 30 Hz. When used with a sensor that can collect data faster than 50 Hz, Logger Pro and LabQuest will collect motion data at a sub-rate of the selected rate. For example, if a force sensor is used and the data rate is set to 100Hz, the force sensor will run at that rate, but motion data will be at 25Hz. This will leave gaps in the motion columns.

3) Some data-collection modes return values only every other point. For example, a photogate in gate timing will report a gate time (the time from blocking to unblocking the gate) only when the gate becomes unblocked. The gate time cell will be blank for the block time.

4) Data collection for certain combinations of sensors may generate many blank cells. For example, the Centripetal Force Apparatus, order code CFA, $449 uses a force sensor and a photogate simultaneously. When doing time-based data collection, the force sensor data will arrive at regular intervals, such as every 1/50th of a second. However, the photogate events happen whenever they happen; they are not confined to the regular 50th second rate. As a result, there will not be force measurements when there are photogate events, except by rare accident.

Sparse data tables can require an additional step for graphing. In the case of the CFA, one wants to plot force vs. angular speed. There are no speed measurements when there are force measurements, so a graph of force vs. speed will be blank. This is expected. To get the desired graph, one must interpolate between force readings to estimate the force at the time of the photogate events. The calculated column feature in Logger Pro, used with the interpolate() function, will do this for you. LabQuest App will do the needed interpolation automatically in most cases.

Logger Pro has a function to collapse blank data cells. One must be very careful when using the collapse function, as it may render the data table inaccurate. For more detail on this function, see Logger Pro's built-in help file.

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