How to Calibrate Smart Transmitters ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

### How to Calibrate Smart Transmitters

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In our last discussion: Introduction to Smart Transmitters, we have seen that a smart transmitter is remarkably different from that of a conventional analog transmitter. Consequently the calibration methods for both devices are also very different. Remember that calibration refers to the adjustment of an instrument so its output accurately corresponds to its input throughout a specified range. Therefore a true calibration requires a reference standard, usually in the form of one or more pieces of calibration equipment to provide an input and measure the resulting output. If you got here looking for information on analog pressure transmitter calibration, you may consult: How to Calibrate Your DP Transmitter

The procedure for calibrating a smart digital transmitter is known as Digital trimming. A digital trim is a calibration exercise that allows the user to correct the transmitter’s digital signal to match plant standard or compensate for installation effects. Digital trim in a smart transmitter can be done in two ways:
(a) A Sensor Trim: It consist of matching the process variable (be it pressure, level, flow or temperature) reading of the transmitter to a precision input. This process normally involves trimming the digital circuit of the input Analog-to-Digital converter in the smart transmitter.
(b) A 4 – 20mA or Current Loop Trim: This is done by trimming the output Digital-to-Analog converter in the transmitter.

Actions That Do Not Constitute Proper Calibration in Smart Transmitters
Before we discuss in detail what constitute a proper calibration, let us mention certain common practice that are not proper calibrations:
(a) Changing the range (LRV and URV) of a smart transmitter constitute a configuration change and not a calibration. This range change merely affects the mathematical computation done by the microprocessor. It has no effect on the digital process variable as read by a hand-held digital communicator.
(b) Using only the zero and span adjustments to calibrate a smart transmitter often
corrupts the internal digital readings. You may not notice this if you don’t use a hand-held digital communicator to read the range or digital process data.
(c) Using a hand-held digital communicator to adjust the current loop so that an accurate input to the transmitter agrees with some readout device on the loop does not constitute a proper calibration.

Procedure for Calibrating a Smart Transmitter:
To do a proper calibration on a smart transmitter will involve both a sensor trim and/or a 4 – 20m A trim depending on the application where the transmitter is being used. A smart transmitter typically has high and low trim functions which unlike the zero and span adjustments of an analog transmitter, are non-interactive. That is adjusting the high trim function has no effect on the low trim function and vice versa.

Before proceeding to the section below note that a smart transmitter has three outputs which must be clearly understood:
(a) Digital Process Variable (PV) usually read by a hand-held communicator
(b) Digital Value of the output current in mA (PVAO) which the communicator also reads.
(c) The analog 4 – 20mA signal output which can be read with a suitable milliammeter but cannot be read by the digital hand-held communicator. If they are not clearly understood please see: Introduction to Smart Transmitters for a clearer understanding.

For the smart transmitter to be properly calibrated, the error between the applied input to the transmitter and the digital output (PV) must be within the error specification of the manufacturer otherwise a sensor trim will be required to correct this. Similarly, the error between the digital milliamp value (PVAO) and the analog mA value must be within the error specification of the manufacturer otherwise a 4 – 20m A trim is required.

Performing a Sensor Trim:
Before performing a sensor trim, run a test, commonly referred to as the AS-FOUND TEST to confirm the consistency of the sensor and the input Analog-to-Digital converter. Connect the test setup as shown below:
 Performing a Sensor Trim on a Smart Transmitter

Use a precision calibrator to measure the applied input to the transmitter. Read the resulting output (PV) with a hand-held communicator. Calculate the resulting error between the applied input and the output (PV) since both are in the same engineering units. Note that the desired accuracy for this test will be the manufacturer’s accuracy specification. If this test does not pass, then follow the manufacturer’s recommended procedure for trimming the sensor. Below are general guidelines for performing a sensor trim:
(a) Apply the lower-range value stimulus to the transmitter, wait for it to stabilize
(b) Execute the “low” sensor trim function
(c) Apply the upper-range value stimulus to the transmitter, wait for it to stabilize
(d) Execute the “high” sensor trim function
Stimulus as used here should be understood to mean the process variable input to the transmitter.

Performing a 4 – 20mA Trim:
Before performing a 4 – 20mA trim, run a test, commonly referred to as the AS-FOUND TEST to confirm the consistency of the output Digital-to-Analog converter and the analog output of the transmitter. This procedure may also be called a 4-20 mA trim, a current loop trim, or a Digital-to-Analog converter trim. Connect the test setup as shown below:
 Performing a 4 - 20mA Trim on a Smart Transmitter

Use a hand-held digital communicator to put the smart transmitter into a fixed current output mode. The input value for this test is the mA value that you instruct the transmitter to produce. The output value is obtained using a precision milliammeter to measure the resulting current. Calculate the error between the digital mA value produced by the transmitter and the analog mA value measured by the current meter. The desired accuracy for this test should also reflect the manufacturer’s accuracy specification. If the test does not pass, then follow the manufacturer’s recommended procedure for trimming the output section. The trim procedure should require two trim points close to or just outside of 4mA and 20 mA. Do not confuse this with any form of re-ranging or any procedure that involves using zero and span buttons on the transmitter. Below are the general guidelines for performing a 4 – 20mA trim:
(a) Execute the “low” output trim test function on the transmitter.
(b) Measure the output signal with a precision milliammeter, noting the value after   it stabilizes
(c) Enter this measured current value when prompted by the transmitter
(d) Execute the “high” output trim test function
(e) Measure the output signal with a precision milliammeter, noting the value after it stabilizes
(f) Enter this measured current value when prompted by the transmitter

After both the input and output (ADC and DAC) of a smart transmitter have been trimmed (i.e. calibrated against standard references known to be accurate), the lower- and upper-range values (LRV and URV) may be set. In fact, once the trim procedures are complete, the transmitter may be ranged and ranged again as many times as desired. The only reason for re-trimming a smart transmitter is to ensure accuracy over long periods of time where the sensor and/or the converter circuitry may have drifted out of acceptable limits. The situation is very different in an analog transmitter, where re-ranging necessitates re-calibration.

Transmitter Damping:
Many HART transmitters support a parameter called damping. If this is not set to zero, it can have an adverse effect on tests and adjustments. Damping induces a delay between a change in the transmitter input and the detection of that change in the digital value for the transmitter input reading and the corresponding output value. It is advisable to adjust the transmitter’s damping value to zero prior to performing tests or adjustments. After calibration, be sure to return the damping constant to its required value.