My simple pendulum has a period about 1/3 what it should be

One of my mentors likes to say, “Never make a measurement unless you already know the answer.” By which he meant that, any time you are building a new measurement instrument, which is what most experimental apparatus is, you’re going to discover several times that what it’s measuring isn’t quite the same as what you thought it was measuring. Here you have a new data-acquisition-and-analysis pipeline that’s giving you unexpected results; you are reasonably assuming that some things in the pipeline are different than you believed when you built them.

Here you seem to have a pendulum of length $ell$, operated on Earth where the gravitational acceleration is $g$, which you predict should have period

$$ tau = 2pi sqrt{ell/g} $$

but you are observing $tau_text{calc} approx tau_text{measured} cdot 3$. You’ve also solved this for the gravitational acceleration,

begin{align} g &= left(frac{2pi}{tau}right)^2 ell, & text{but }g_text{measured} &approx g_text{reference} cdot 4. end{align}

Those two statements taken together are a little surprising: you would predict $(tau_text{calc} / tau_text{measured})^2 = (g_text{measured}/g_text{reference})$ with exactly the same data involved, but one doesn’t ordinarily get $3^2 approx 4$ with just one algebra error. I can imagine a few reasons why that might happen, but I don’t think it’s productive for me to speculate.

A common problem for intro-level students is “frequency doubling”: for some people it seems like the “period” of a signal should be the interval between zero-crossings, rather than the interval between peaks or between crests. In a comment, mmesser314 suggests

The link says the ioLab can measure force. If you are measuring the centrifugal force the pendulum exerts on the support, it reaches a max twice per pendulum period. Likewise, velocity reaches a max twice per period. Are you making a mistake like that?

to which you respond with interest. Note that halving your period would give $(tau_text{calc} / tau_text{measured}) = 2$ and $(g_text{measured}/g_text{reference}) = 4$.

You write in a comment that you are reluctant to do the experiment with a stopwatch because your digital timer is more sensitive than human reflexes and a stopwatch. This is a statement that you want to be able to quantify. I like to have my college students measure their own reaction times, by having a proper clock generate some noise at a known interval and letting them measure it. I find typical stopwatch reaction times of about a quarter-second, and jitter of about a tenth of a second. (The athletic kids tend to be better at stopwatches; apparently it, too, is a skill.). If you are getting timing discrepancies that are a factor of three, you don’t need a high-tech DAQ system: you should be able to tell whether the pendulum is really oscillating at 3 Hz, 1 Hz, or 0.3 Hz by looking at it and counting “mississippi-one, mississippi-two.”