Graphing Wavelength vs Inverse freq. or Period

[PSEYE]

According to our lab manual, the graph of Wavelength vs Period : the slope should be the speed of sound. Well, I have a negative slope of -32.2 which is ridiculous, but I can't find out what I'm doing wrong. We had 5 tuning forks , say for instance, 512Hz...the inverse
1/f = 1/512= 0.00195 , I am graphing this on the x-axis and Wavelength on the Y axis. This experiment was done with 2 tubes, one fitted into another, and you're supposed to pull the smaller tube out until the particular tuning for resonates. You are to take the distance of the pipe + the extended portion of the other pipe at 3 different places you hear resonance...then you are supposed to take the difference of the average and double it - that should be your wavelength...example

the tube is 90cm long
first trial @ 512 Hz - heard first resonance at 98.5cm or 8.5 depending...our lab instructor told us to use the entire distance so i did. Heard second resonance at 124cm, and 3rd resonance at 165. I took the difference between these three values and averaged it, then doubled it - i got 66.50cm or 0.665m I did this with 4 other tuning forks , got my info and graphed a negative slope that wasn't even close to being a straight line.

Can anyone give me a clue as to what I'm doing wrong?

Thanks.

oh btw, this is for lab : Resonance in Air Columns.

 

[Redbelly98]

If your textbook gives the actual speed of sound in air, you can use that to check whether the wavelength you get is close to what you expect for each frequency.