Sometimes the expert witness is in the department's own lab. It seems a tautology that they will favor conviction. Every officer bringing in evidence is begging "Give me something I can use to get this scumbag!" The pressure to fudge is enormous.

A fairly simple step is to certify equipment independently, and to do blind testing (one sample is provided from evidence; another by an independent unrelated source). Of course this costs more, but anything else is effectively an experiment without a control.