What is a 4-20 mA Sensor?

Travis Bottalico, CTO & VP Engineering for Avimesa, runs through what a 4-20mA industrial sensor is, and how a current loop is used to output data.

Avimesa offers hardware, device cloud, and IoT hosting for developing and deploying industrial IoT solutions. Our wireless boards connect with up to seven 4-20mA sensors, sending data to our device cloud which can be used for monitoring and remote actuation.

[johnnypartain]

What is a 4-20mA sensor? Still don't know. Maybe an op-amp?

[shakilahmad]

what about making of pcb or digital display of 3-wires 4-20mA output velocity sensor (0-1m/s), 24vdc power

[youtuuba]

Wow, what a mess this video is. Right off the bat, it is apparent that Travis Bottalico has not done enough preparation for what he wants to say and how to say it. Then the errors begin....just some of which I note here:
- He says it is a "2 wire sensor"; 2 wire 4-20mA devices like this are called "transmitters". A "sensor" is just the front end that converts the physical variable into an electronic signal, and when that is THEN adapted to control a 4-20mA current loop, it stops being called a "sensor" and becomes known as a "transmitter". These are widely accepted and used industry terms.
- He says that if you apply 24VDC to the "sensor", it will "output" 4-20mA. Incorrect and confusing. Power supplies are never applied to 2-wire 4-20mA transmitters....that would let the magic smoke out! Instead, the 2-wire transmitter is wired in series as part of a current loop, and one item in the loop happens to be a 24VDC (usually, but can be other voltages) power supply. By its connection, the supply is applied to the entire loop, not to the transmitter. And since the transmitter is not wired directly to the "receiver(s)", it cannot "output" 4-20mA....all it does is modulate the current in the loop to achieve a current that is proportional to the process variable. Now, there are devices known as "4-wire transmitters", where two of their wires ARE connected directly to a power supply (the type and voltage can vary)m and the other two wires DO "output" a 4-20mA current. But this video is addressing 2-wire transmitters at that point in time. When trying to teach others, it is important to use clear and correct and consistent terminology, and try to avoid misusing words such that the listener will become confused.
- He dives right into a chart to tell the rest of the story, which is well and good, but many beginners won't get the gist of how things work from that, and a better verbal explanation can help.
- The rest of the loop is not discussed, and the subject of the "receiver" is not addressed. A "transmitter" without a loop containing at least one "receiver" is worthless.