Active Instruments and Passive Instruments

Very vauge question.

Active transducers are those which require electric current (a source of energy) for working, while passive transducers are those which does not need an external source. Passive transducers directly produce electric signals without an external energy source.

Read more: http://wiki.answers.com/Q/What_is_the_difference_between_active_and_passive_transducers#ixzz1eNRICsFc

The threshold is the maximum flow the flow meter is capable of measuring, anything above threshold will most likely damage the meter.

Passive transducers - require an external power to operate, and the
output is a measure of some variation in passive components (e.g. resistance or
capacitance)
Examples of passive transducer :
-Slide-wire resistor
▫ Resistance strain gauge
▫ Differential transformer

Active Transducers - Self generating type - do not require an external power,
and produce an analog voltage or current when stimulated by some physical form of energy.
▫ Thermocouple
▫ Photovoltaic cell
▫ Moving coil generator
Read more: http://wiki.answers.com/Q/What_is_the_difference_between_a_passive_transducer_and_an_active_transducer#ixzz1ee7jUrZZ