The gravimetric tests of the Qsoils are not so hot.
soil.b gravimetric readings have all been within 1% of each other (no change in soil moisture), whereas the EC5 data show a 4% change (still not much change). With such a tight clustering, we can't fit a line, but the data suggest an offset of ~-7%.
soil.a gravimetric readings are quite variable (38 - 51%), but the range in EC5 values are half as much. The middle data point doesn't agree at all, though a line with slope=1 and offset of -8% fits the remaining (2) samples.
Thus, the indication is that both EC5s are reading about 8% low. Clearly, we need a few more samples, preferably in a wider range of soil moisture conditions.