Calibration

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Thread Starter

Rajendra

I don't have any experience of working with smart type HART feild devices and this is becoming more problematic for me now.

I want to know the calibration procedure of the HART transmitters.
I did calibration of analog transmitters only.

I already did enough home work about this calibration thing of smart transmitter but where I found difficulties when I came across the words like zero trim, sensor trim and o/p trim.

As we have single output of the transmitter and from there we will get signal of 4-20 mA. How we can trim the current from same terminal for all and how to find which trim needed for transmitter.

last thing is how the trim function is different then zero and span adjustment.
 
basically trimming means to remove the offsets that are either due to sensors or signal conditioning ckts, in spite of zero adjustment, which we normally do manually still for zero input to system the tx always shows some fluctuations of reading, we basically reset the memory and makes the device understand that there is no input, and what we did manually now its done through software.
 
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Bruce Durdle

There are three parts to the signal train inside a smart transmitter.

First is the conversion of whatever the measured variable is into an internal electronic format - so for instance in a pressure transmitter the pressure sensing electronics will generate a digital value output which is related to the applied pressure. The characteristic of this conversion is defined in the sensor trim table. While you CAN alter this, it will generally not change for the life of the instrument and should not be touched.

The output from the sensor trim is used to generate the 0% - 100% output signal based on the calibrated zero and span settings. You can enter these digitally, or by applying an external pressure or other input and pressing the buttons on the side of the transmitter. This isn't a calibration - it's better referred to as configuration.

Finally, you can adjust the output circuit so that when the internal output representation is 100%, the actual current is 20 mA, and at 0% internal it is 4 mA.

The only adjustment you will normally need to make is the configuration where you tell the transmitter what the 0% and 100% inputs are. You may need to trim the current output under some circumstances. Before even thinking about touching the sensor trim adjustments, make sure that there is not some mechanical factor involved, and that you have access to test equipment of suitable accuracy - preferably better than 0.05 % for the usual range.
 
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