What are the differences between sensors, transducers and transmitters ?

Sensor is device that senses a physical property such as temperature, pressure. A transducer is a device that converts one form of energy to another by relating the physical quantity to resistance. Transmitter is an electromechanical device measures the equivalent current relating to the resistance generated by the transducer, amplifying it and transmitting same to the control system.

Sensor is a devices that is used to sense something i.e temp or other ,that is directly proportional to the valves.

Transducer: is a device to used to change one energy form into another form. i.e microphone ,speaker.

Transmitter: is a device that is used to transmit energy from one device to other. 

Sensor is a device that measures or senses a physical quantity and converts it into another analogue quantity that can be measured electrically such as voltage, ohmic resistance, inductance or capacitance. The output needs to be manipulated, interfaced and calibrated by the system designer.

Transducer is a device that is connected to sensor(s) so as to convert the measured quantity(ies) into standard analog electrical signal such as0 to10 VDC, -10 to +10 VDC,4 to20 mA,0 to20 mA,0 to25 mA .... etc. The output can be used directly by the system designer.

Transmitters are current-output devices and may have two or three wires. Where two wires are used to both receive power and transmit an output signal, significant cost savings can be made where long cables are needed. They are frequently scaled to vary from4 mA to20 mA . transmitter normally contain a microprocessor which convert measured value into digital code which are transmitted to remote receiver  

A sensor is sensing element that sense the variation of any process and give us a signal corresponding to this variation.

A transducer is an element  catch the sensor output signal to convert it from its form to standard form that we can to measure it.

A transmitter is an input signal source which give us a signal which are received by the receiver to tack a certain action ( for example photocell consists of transmitter and receiver if the the transmitter signal varied or cut the receiver do an action )  

 A Sensor is used to detect a parameter in one form and report it in another form of energy, often an electrical signal.

A transducer is a device that converts a signal in one form of energy to another form of energy. Generally Transducers are voltage-output devices

A transmitter is a current-output transducer.

Formal definitions from http://www.electropedia.org/

Electropedia: The World's Online Electrotechnical Vocabulary  (IEC60050)

sensor (measuring element)

part of a measuring instrument, or measuring chain, which is directly affected by the measurand and which generates a signal related to the value of the measurand.

(signal) transducer

device for transforming a physical quantity representing information into a physical quantity of different kind representing the same information, one of the two quantities being electric

(measuring) transmitter

measuring transducer the output variable of which is a standardized signal