Updated: Nov 30, 2020

In this article, we will learn about electronic transmitter widely used in process industries which is the main part of an instrumentation and control engineering.

It can be an electronic level transmitter, electronic pressure transmitter, electronic temperature transmitter, electronic flow transmitter or electronic pH transmitter.

If a transmitter operates with an electrical supply, then it is called an electronic transmitter.

The input supply for an electronic transmitter is 24 v dc and standard output of an electronic transmitter is 4- 20 mA dc of the rated span of the instrument.

The sensing part is basically electrical sensors of the process variable. For example, for measuring level or flow capacitance differential pressure capsule are used and for temperature in RTD platinum is used.

The transmitting part is generally an electronic circuit which generates 4- 20 mA with respect to the sensing input.

Electronic transmitters are classified into two types. They are,

1. Analog transmitter (NON-SMART)

2. Digital transmitter (SMART)

Analog Transmitter :

Early Analog Transmitter was widely used before the arrival of the SMART transmitter but still, this type of transmitters exist. Analog Transmitter also provides 4-20 mA output.

On the electronic circuit of the transmitter, it contains zero and span screw. It doesn't contain a microprocessor so you can't connect it with HART.

For calibration of such type of transmitter, you can adjust zero and span. You can do a five-point calibration, too by calculating value at that point.

Below is a block diagram of Analog transmitter

The block diagram contains a few stages starting from a sensor to ends in receiving an output of 4-20 mA DC signal.

You can read the function of each stage on the block diagram itself.

SMART Transmitter :

Single module addressable ranging transmitter (SMART) is a SMART transmitter operates exactly same like Analog transmitter except it contains inbuilt microprocessor which opens an easy communication option.

All SMART transmitter can talk to HART. It also gives us 4-20 mA output same like Analog transmitter.

Calibration becomes easy as we can easily check different point using HART or using any calibrator we can check loop too.

Below is a block diagram of the SMART Transmitter.

You can trim a particular portion based on error. As you can see in the above-mentioned block diagram that on Input side you can trim the input portion.

If you find any fault on the input side like an error in process variable reading you can adjust it. ADC (Analog to digital) helps you in this process. This is called input trimming.

On the output side, you can trim an output. If your input side is okay then you can check corresponding output (4-20 mA) and trim it based on error. This is called output trimming.

So, next time when you calibrate any instrument remember this input and output trim term.

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