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Fieldbus Transmitters Calibration and Ranging

Calibration and ranging for a FF device is similar in principle to any other “smart” measurement instrument. Unlike analog instruments, where the “zero” and “span” adjustments completely define the instrument’s calibration and range, calibration and ranging are two completely different functions in a digital instrument.

To begin, we will examine a block diagram of an analog pressure transmitter showing the zero and span adjustments, with analog signaling between all functions inside the transmitter:

Pressure Transmitter Calibration

The “zero” and “span” adjustments together define the mathematical relationship between sensed pressure and current output. Calibration of an analog transmitter consists of applying known (reference standard) input stimuli to the instrument, and adjusting the “zero” and “span” settings until the desired current output values are achieved. The goal in doing this is to ensure accuracy of measurement.

The “range” of a transmitter is simply the input values associated with 0% and 100% output signals (e.g. 4 mA and 20 mA). Ranging an analog transmitter consists (also) of adjusting the “zero” and “span” settings until the output signal corresponds to the desired LRV and URV points of the measured variable. For an analog transmitter, the functions of ranging and calibration are always performed by the technician at the same time: to calibrate an analog transmitter is to range it, and vice-versa.

By contrast, a “smart” (digital) transmitter equipped with an analog 4-20 mA current output distinctly separates the calibration and range functions, each function determined by a different set of adjustments:

SMART Pressure Transmitter Configuration

Calibration of a “smart” transmitter consists of applying known (reference standard) input stimuli to the instrument and engaging the “trim” functions until the instrument accurately registers the input stimuli. For a “smart” transmitter equipped with analog electronic (4-20 mA) output, there are two sets of calibration trim adjustments: one for the analog-to-digital converter and another for the digital-to-analog converter.

Ranging, by contrast, establishes the mathematical relationship between the measured input value and the output current value. To illustrate the difference between calibration and ranging, consider a case where a pressure transmitter is used to measure water pressure in a pipe.

Suppose the transmitter’s pressure range of 0 to 100 PSI translates to a 4-20 mA output current. If we desired to re-range an analog transmitter to measure a greater span of pressures (say, 0 to 150 PSI), we would have to re-apply known pressures of 0 PSI and 150 PSI while adjusting the zero and span potentiometers so 0 PSI input gave a 4 mA output value and 150 PSI input gave a 20 mA output value. The only way to re-range an analog transmitter is to completely re-calibrate it.

In a “smart” (digital) measuring instrument, however, calibration against a known (standard) source need only be done at the specified intervals to ensure accuracy over long periods of time given the instrument’s inevitable drift. If our hypothetical transmitter were recently calibrated against a known pressure standard and trusted not to have drifted since the last calibration cycle, we could re-range it by simply changing the URV (upper range value) so that an applied pressure of 150 PSI now commands it to output 20 mA instead of an applied pressure of 100 PSI as was required before. Digital instrumentation allows us to re-range without re-calibrating, representing a tremendous savings in technician time and effort.

The distinction between calibration and ranging tends to confuse people, even some experienced technicians. When working with an analog transmitter, you cannot calibrate without setting the instrument’s range as well: the two functions are merged in the same procedures of adjusting zero and span. When working with a digital transmitter, however, the function of calibration and the function of ranging are entirely separate.

Fieldbus instruments, of course, are “smart” in the same way, and their internal block diagrams look much the same as the “smart” transmitters with analog current output, albeit with a far greater number of parameters within each block. The rectangle labeled “XD” in the following diagram is the Transducer block, while the rectangle labeled “AI” is the Analog Input block:

Fieldbus Transmitter Calibration

Calibration (trim) values are set in the transducer block along with the engineering unit, making the output of the transducer block a digital value scaled in real units of measurement (e.g. PSI, kPa, bar, mm Hg, etc.) rather than an abstract ADC “count” value. The analog input function block’s Channel parameter tells it which transducer output to receive (Note 1) as the pre-scaled “Primary Value”, which it may then translate to another scaled value based on a proportionality between transducer scale values (XD_Scale high and low) and output scale values (OUT_Scale high and low).

Note 1 : Fieldbus transmitters often have multiple channels of measurement data to select from. For example, the multivariable Rosemount 3095MV transmitter assigns channel 1 as differential pressure, channel 2 as static pressure, channel 3 as process temperature, channel 4 as sensor temperature, and channel 5 as calculated mass flow. Setting the Channel parameter properly in the AI block is therefore critical for linking it to the proper measurement variable.

To calibrate such a transmitter, the transducer block should first be placed in Out Of Service (OOS) mode using a handheld FF communicator or the Fieldbus host system. Next, a standard (calibration-grade) fluid pressure is applied to the transmitter’s sensor and the Cal_Point_Lo parameter is set to equal this applied pressure. After that, a greater pressure is applied to the sensor and the Cal_Point_Hi parameter is set to equal this applied pressure. After setting the various calibration record-keeping parameters (e.g. Sensor_Cal_Date, Sensor_Cal_Who), the transducer block’s mode may be returned to Auto and the transmitter used once again.

To range such a transmitter, a correspondence between sensed pressure and the process variable must be determined and entered into the analog input function block’s XD_Scale and OUT_Scale parameters. If the pressure transmitter is being used to indirectly measure something other than pressure, these range parameters will become very useful, not only proportioning the numerical values of the measurement, but also casting the final digital output value into the desired “engineering units” (units of measurement).

Ranging in Fieldbus transmitters is a somewhat confusing topic due to the unfortunate names given to the different L_Type parameter options. Here is a list of the L_Type parameter options along with their meanings:

  • Direct = the AI block will publish the signal output by the XD block, regardless of the specified OUT Scale range
  • Indirect = the AI block will mathematically scale the signal from the XD block into a range specified by OUT_Scale parameters using a linear equation (e.g. y = mx + b)
  • Indirect square root = same as above, except that a square-root function is applied to the percentage of range (useful when characterizing flow transmitters based on differential pressure measurement)

The terms “direct” and “indirect” are unfortunate, because they often cause people to interpret them as “direct” and “reverse” (as though L_Type described the direction of action for the function block). This is not what these terms mean for the AI block! What a “direct” value for L_Type means is that the raw value of the XD block is what will be published onto the Fieldbus network by the AI block. What an “indirect” value for L_Type means is that the XD block’s signal will be scaled to a different range (specified by the OU_ Scale parameter). In summary, the technician must set the XD_Scale range according to the primary signal sensed by the transmitter’s sensing element, and set the OUT_Scale range according to what the rest of the control system needs to see proportional to that primary signal.

Example :

The concept of ranging a FF transmitter makes more sense when viewed in the context of a real application. Consider this example, where a pressure transmitter is being used to measure the level of ethanol (ethyl alcohol) stored in a 40 foot high tank. The transmitter connects to the bottom of the tank by a tube, and is situated 10 feet below the tank bottom:

Fieldbus Transmitter Calculations

Hydrostatic pressure exerted on the transmitter’s sensing element is the product of liquid density (γ) and vertical liquid column height (h). When the tank is empty, there will still be a vertical column of ethanol 10 feet high applying pressure to the transmitter’s “high” pressure port. Therefore, the pressure seen by the transmitter in an “empty” condition is equal to:

Pempty = γ. g. hempty = 49.3 lb/ft3 ) (1) (10 ft)

Pempty = 493 lb/ft2  = 3.424 PSI

When the tank is completely full (40 feet), the transmitter sees a vertical column of ethanol 50 feet high (the tank’s 40 foot height plus the suppression height of 10 feet created by the transmitter’s location below the tank bottom). Therefore, the pressure seen by the transmitter in a “full” condition is equal to:

Pfull = γ. g. hfull = 49.3 lb/ft3 ) (1) (40 ft + 10 ft)

Pfull = 2465 lb/ft2  = 17.12 PSI

Thus, the transducer (XD) block in this Fieldbus transmitter will sense a liquid pressure ranging from 3.424 PSI to 17.12 PSI over the full range of the tank’s storage capacity.

However, we do not want this transmitter to publish a signal to the Fieldbus network in units of PSI, because the operations personnel monitoring this control system want to see a measurement of ethanol level inside the tank, not hydrostatic pressure at the bottom of the tank. We may be exploiting the principle of hydrostatic pressure to sense ethanol level, but we do not wish to report this measurement as a pressure.

The proper solution for this application is to set the L Type parameter to “indirect” which will instruct the AI function block to mathematically scale the XD block’s pressure signal into a different range. Then, we must specify (Note 2)the expected pressure range and its corresponding level range as XD Scale and OUT Scale, respectively (Note 3) :

Fieldbus Transmitter Parameters

Note 2 : It is important to note that you must correctly calculate the corresponding XD Scale and OUT Scale parameter values in order for this to work. The Fieldbus instrument does not calculate the parameters for you, because it does not “know” how many PSI correspond to how many feet of liquid level in the tank. These values must be calculated by some knowledgeable human technician or engineer and then entered into the instrument’s AI block, after which the instrument will execute the specified scaling as a purely mathematical function.

Note 3 : When configuring the XD Scale high and low range values, be sure to maintain consistency with the transducer block’s Primary Value Range parameter unit. Errors may result from mis-matched measurement units between the transducer block’s measurement channel and the analog input block’s XD Scale parameter.

Now, the ethanol tank’s level will be accurately represented by the FF transmitter’s output, both in numeric value and measurement unit. An empty tank generating a pressure of 3.424 PSI causes the transmitter to output a “0 feet” digital signal value, while a full tank generating 17.12 PSI of pressure causes the transmitter to output a “40 feet” digital signal value. Any ethanol levels between 0 and 40 feet will likewise be represented proportionally by the transmitter.

If at some later time the decision is made to re-locate the transmitter so it no longer has a 10 foot “suppression” with regard to the tank bottom, the XD_Scale parameters may be adjusted to reflect the corresponding shift in pressure range, and the transmitter will still accurately represent ethanol level from 0 feet to 40 feet, without re-calibrating or re-configuring anything else in the transmitter.

If we wished, we could even mathematically determine the liquid volume stored inside this ethanol tank at different sensed pressures, and then scale the AI block’s OUT_Scale parameter to report a volume in units of gallons, liters, cubic feet, or any other appropriate volume unit. Using the “indirect” mode with appropriate XD_Scale and OUT_Scale parameter values gives us great flexibility in how the transmitter senses and represents process data.

In summary, we set the XD_Scale parameter to the physical range of measurement directly sensed by the transducer, we set the OUT_Scale parameter to the corresponding range of measurement we wish the transmitter to report to the rest of the control system, and we set L_Type to “indirect” to enable this translation from one range to another. We should only use the “direct” L_Type setting if the raw transducer range is appropriate to output to the rest of the control system (e.g. if the transmitter directly senses fluid pressure and we wish this very same pressure value to be published onto the Fieldbus network by the transmitter, with no scaling).

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