Re: Ground or chassis
Usually ground and chassis are refered to the same thing. But there are cases where the ground is the signal's ground (zero volts reference) and chassis is the equipments metal frame which might have nothing in common with the signal, low voltage ground. Although the chassis (metal frame) must be earthed in mains powered equipment, this does not means it is also connected to signal ground.
For voltage measurements always notice the zero volts reference-signal ground, in order to take correce measurements.