What is a HART transmitter?
The most common smart transmitter utilised in the process industry is a HART transmitter. When calibrating and trimming a HART transmitter, what factors should be considered? Let’s take a look at some of the most significant factors to consider while calibrating and trimming a HART transmitter. Also, let’s clear up some of the most common misunderstandings about HART communicators.
What is a HART communicator and How it works?
How are HART instruments calibrated?
The calibration of an analogue transmitter is rather simple. The zero and span adjustments can be utilised after an As-Fitted test to set the correct relationship between the input signal and the 4 – 20 mA output. The calibration is completed with an As-Left test.
How to use HART 475 and 375 communicators?
A HART instrument is more complicated, as it has three phases. The link between an input sensor and the PV, or primary variable, is established during the sensor input stage. PVs are measured in technical units, such as pressure or oF. Sensor Trim is used to altering the Sensor Input stage by digitally trimming it.
The relationship between PV (Primary Variable) and PVAO is established in the second stage, which is a computational stage (Primary Variable Analog Output). The PV Upper Range Limit and Lower Range Limit values are used to scaling the range. The PVAO is a 4-20 mA output signal’s digital value.
Output Trim is used to setting the last stage, Instrument Output, digitally. Traditionally, a HART configurator or communicator was used to do these trims and enter the URV and LRV. The precision analogue source and measurement functions required a separate calibrator.
The method for calibrating a HART device is determined by how the transmitter outputs are used. It can be used as an analogue transmitter if only the 4-20 mA analogue signal is employed. The right relationship between the input sensor and the 4-20 mA analogue output is established using the manual zero and span buttons on the transmitter, or by digitally configuring the PV LRV and PV URV.
The Sensor Input stage, however, has not been adequately adjusted in this circumstance. Even if the 4-20 mA output is correct, using a communicator to read the digital value PV will very certainly be erroneous.
If the control system will employ any of the digital signals, a more thorough methodology is required. If PV is to be employed, Sensor Trim must be used to accurately set the input stage. The PV LRV and PV URV should therefore be digitally allocated, and the manual zero and span buttons should never be used. Finally, the Output Trim is utilised to ensure that the relationship between the PVAO and the 4-20 mA analogue output is accurately configured.
Most common misconceptions about a HART communicator
There are so many misconceptions that are rumoured in the air about the HART communicator. Here I am trying to explain the misconceptions and the truth behind it. The most common misconceptions about a HART Communicators are
A HART communicator does not have the ability to calibrate a HART transmitter.
Trimming a HART transmitter is not possible with a HART communicator.
mA cannot be measured with a HART communicator.
A HART communicator does not have the ability to calibrate a HART transmitter.
A widespread misunderstanding appears to be that a HART communicator can be used to calibrate a transmitter - this is not the case. As previously stated, metrological calibration of a HART transmitter cannot be performed alone using a HART communicator; it requires the use of a reference standard (calibrator).
Trimming a HART transmitter is not possible with a HART communicator.
Another widespread misunderstanding is that a HART communicator can trim a HART transmitter by itself - this is not the case. Trimming (or modifying) cannot be done just using a HART communicator; measurements for trimming must always be taken with a reference standard (calibrator).
mA cannot be measured with a HART communicator.
People mistakenly believe that a HART communicator can measure the transmitter’s mA output signal. This is not the case. The AO value, which is the digital equivalent of the nominal mA value, can be displayed on a conventional HART communicator. As a result, it is not a true measured mA current signal, and it does not indicate how much current the transmitter is actually producing. Even though the HART communicator’s AO value is 4.000 mA, this does not imply that the output current is 4.000 mA. To measure the true mA output, you’ll need an mA metre or a calibrator.