What is a transmitter?

A transmitter is an instrument (hence instrumentation) that gets input signals from an element and translates the values into a common communication signal such as 4 to 20 mA which a control system can then use for control or display purposes. 

Transmitters are analog devices that require a range in which to operate.  The range is made up of the zero and span values of the transmitter.  It is not enough to just set the range of an instrument, you also need to calibrate the instrument.  This means that you must apply known conditions to the instrument and see if it is giving back the proper value.  You should do this at the zero , mid-range and full range of the transmitter.  There are specific rules you should follow detailed by ISA.

Many newer transmitters no longer use the traditional 4-20 mA signal but rather they might use a BUS technology such as Foundation Fieldbus.  Many of these new devices are self calibrating and there is nothing the technician can do to calibrate them in the traditional sense.