What is the signal span of an instrument with an output range of 4-20 mA and an input range of 0-200 psi?

This question is using two instrumentation terms which are often incorrectly used interchangeably. The range of an instrument is the measurement of which it is capable and the span is the measurement for which it is calibrated. The span can equal the range or be less than the range. Whatever span you have calibrated the transmitter to will be represented by the 4-20ma output.

In this case, if the instrument has a measurement range of 200psi, you could adjust the span for 200 psi, but it would corrspond with the maximum range of 200psi, possibly introducing inaccuracies when operating near the end of the range. It would be preferable to select a device with a range of 250psi and then adjust the span so that 200 psi causes a full scale output of 20ma. This is a span of 80% of the range. (200/250)

The output of the transmitter is selected during design of a certain range which will be compatible with the other equipment in the loop. Standards are 4-20ma, 10-50ma, 0-10v and many others.

Standard instrument design procedures will normally try to select an input range that will cause the majority of the normal operation measurements to fall within 20-80% of the range.

Absolutely correct !

Note concerning range and span:

The range of any type of measuring instrument or device is determined by its physical limitations and so the design and construction of it determines the operational application limits and the aimed accuracy. For latter reason the selection of an instrument depends on both factors. The range is limited by the lowest and highest value that a device is adjusted to measure while the adjusted span is equal or smaller than the range. Note that the accuracy is only established by the range, not the span !