We have known for some time now that the number of points in a measured dynamometer card can affect the results of diagnostic analysis calculations. Also, it can affect the downhole pump card shape, which can result in incorrect interpretation of the downhole pump card shape.
To avoid these problems we recommend that measured dynamometer cards for most wells have over 100 points per card. This is a good recommendation for deeper wells but is not enough for shallow wells. Actually, we found out that the shallower the well is, the more points are required to accurately record the actual dynamometer card shape.
The reason you need more dynamometer card points for shallow wells has to do with the natural frequency of the rod string. Shallow wells have much higher natural frequencies than deep wells. This causes higher frequency vibrations that require a faster sampling rate when recording dynamometer cards. We recommend a minimum of 120 point per dynamometer card for shallow wells (less than 3500-ft depth).
We recently found out that a pump off controller manufacturer uses only 50 points per dynamometer card. This number is very low and in many cases it can cause the calculated downhole pump card to be distorted. This can cause an incorrect diagnostic analysis because the dynamometer card does not have enough points to accurately describe the shape of the dynamometer in many wells.