Checksums of 10 or 20 are actually within the spec of the probe as it leaves the factory. So long as the checksums are consistent, there will be no problem with the data. The main component of a checksum is the bias of the probe. (A check sum is 2 x the bias of the probe). The 0 and 180 surveys effectively cancel the bias, so data are not affected. However, there are two conditions that you should watch for:
(1) Check for large changes in the mean checksum from survey to survey. Use DigiPro’s checksum graph: If plots are close to each other, there is no error. If plots are separated widely, there may be bias-shift errors.
(2) Check for drifting checksums – checksums growing consistently larger or smaller from bottom to top. Use DigiPro’s checksum plot again. Each plot should be vertical. If the plot tilts to the right or left, there is drift. This is probably an indication that an electronics board is bad.