Instrument error can occur due to a variety of factors: drift, environment, electrical supply, addition of components to the output loop, process changes, etc. Since a calibration is performed by comparing or applying a known signal to the instrument under test, errors are detected by performing a calibration.

An error is the algebraic difference between the indicated value and the actual value of the measured variable. Typical errors that occur are as given below.

1. Span error
2. Zero error
3. Combined zero & span error
4. Linearization error

Zero and span errors are corrected by performing a calibration. Most instruments are having facility for adjusting the zero and span of the instrument, along with instructions for performing this adjustment.

The zero adjustment is used to produce a parallel shift of the input-output curve. The span adjustment is used to change the slope of the input-output curve. Linearization error may be corrected if the instrument has a linearization adjustment. If the magnitude of the nonlinear error is unacceptable and it cannot be adjusted, the instrument must be replaced or repaired.

To detect and correct instrument error, periodic calibrations are performed. Even if a periodic calibration reveals the instrument is perfect and no adjustment is required, we would not have known that unless we performed the calibration. And even if adjustments are not required for several consecutive calibrations, we will still perform the calibration check at the next scheduled due date. Periodic calibrations to specified tolerances using approved procedures are an important element of any quality system.

Why we should calibrate?

• Testing a new instrument
• Testing an instrument after it has been repaired or modified
• Periodic testing of instruments
• Testing after the specific usage has elapsed
• Prior to and/or after a critical measurement
• When observations are not accurate or instrument indicators do not match the output of a surrogate instrument
• After events such as:
  • An instrument has had a shock, vibration, or exposure to adverse conditions, which can put it out of calibration or damage it.
  • Sudden weather changes

Risk Involved in Not Calibrating an Instrument

• Safety procedure: In case of instruments involving perishable products such as food or thermometers with area of sensitive nature, uncalibrated instruments may cause potential safety hazards.
• Wastage: If the instrument is not perfectly calibrated, it might lead to potential wastage of resources and time consumed in the operations, resulting in an overall increase in expenses.
• Faulty or Questionable Quality: If the instrument is improperly calibrated, the chances of faulty or questionable quality of finished goods arises. Calibration helps maintain the quality in production at different stages, which gets compromised if any discrepancy arises.
• Fines or litigations: Customers who have incurred damage may return the product against a full refund, which is still alright; but if they go for litigation due to damages, you could be up for serious costs in terms of reputation and restitution payments.
• Increased downtime: Poor quality of finished goods is the first indicator of disrepair in your equipment. Regular calibration programs identify warning signs early, allowing you to take action before any further damage is caused.

Accuracy:

The magnitude of the error of the total output scale or the error rate to the output, expressed in percentage time or percentage reading, respectively.

Tolerance:

Permissible deviation from specified value; can be expressed in units of measurement, space percentage, or reading percentages.

As you can see from the definition, there are subtle differences between the terms. It is recommended that tolerance, specified in the measurement units, be applied to the measurement requirements made in your facility. By specifying the actual value, errors caused by calculating the percentage of space or reading are eliminated. Also, tolerance should be specified in units of measurement.
For example, you’ve been assigned to do
previously mentioned 0-to-300 psig transmitter with calibration tolerance of ± 2 psig. Withdrawal tolerance can be:
2 psig
÷ 300 psig
× 16 mA
—————————-
0.1067 mA
The calculated tolerance reached 0.10 mA, because

close to 0.11 mA will exceed the calculated tolerance. It is recommended that the tolerance of both ± 2 psig and ± 0.10 mA on the measurement data sheet when distances to the milliamp signal are recorded.

Accuracy ratio:

This term has been used in the past to describe the relationship between test level accuracy and test metal accuracy. This term is still used by those who do not understand the uncertainty calculation (uncertainty is described below). A good rule of thumb is to ensure a 4: 1 accuracy ratio when performing measurements. This means that the metal or scale used must be four times more accurate than the tool being tested. Therefore, the test equipment (such as field level) used to measure process process should be four times more accurate than process process, laboratory standard used to measure field quality should be four times more accurate than field field, and so on.
With today’s technology, a 4: 1 accuracy ratio becomes much more difficult to achieve. Why the recommended 4: 1 ratio? A 4: 1 rating confirming will reduce the result of the level of accuracy to the full measurement accuracy. If a high level is found to be intolerable in pairs, for example, the measurements made using that standard are not significantly reduced.

Suppose we use our previous example of the test equipment with a tolerance of ±0.25°C and it is found to be 0.5°C out of tolerance during a scheduled calibration. Since we took into consideration an accuracy ratio of 4:1 and assigned a calibration tolerance of ±1°C to the process instrument, it is less likely that our calibration performed using that standard is compromised.

The out-of-tolerance standard still needs to be investigated by reverse traceability of all calibrations performed using the test standard. However, our assurance is high that the process instrument is within tolerance. If we had arbitrarily assigned a calibration tolerance of ±0.25°C to the process instrument, or used test equipment with a calibration tolerance of ±1°C, we would not have the assurance that our process instrument is within calibration tolerance. This leads us to traceability.

Traceability:

All measurements should be tracked to a level that is known nationally or internationally. For example, in the United States, the National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS), maintains national standards. Tracking is defined by ANSI / NCSL Z540-1-1994 (which replaces MIL-STD-45662A) as a measurement asset that may be compliant with applicable standards, usually national or international standards, for a continuous series of comparisons. ”Note that this does not mean that the stock market needs to be measured by its standard levels. It means that the measurements made are followed by NIST at all levels used to measure standards, regardless of how many levels exist between the store and NIST.

Uncertainty:

Parameter, associated with the result of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measurand. Uncertainty analysis is required for calibration labs conforming to ISO 17025 requirements. Uncertainty analysis is performed to evaluate and identify factors associated with the calibration equipment and process instrument that affect the calibration accuracy.

calibration is a comparison of measuring equipment against a standard instrument of higher accuracy to detect, correlate, adjust, rectify and document the accuracy of the instrument being compared.

Typically, calibration of an instrument is checked at several points throughout the calibration range of the instrument. The calibration range is defined as “the region between the limits within which a quantity is measured, received or transmitted, expressed by stating the lower and upper range values.” The limits are defined by the zero and span values.

The zero value is the lower end of the range. Span is defined as the algebraic difference between the upper and lower range values. The calibration range may differ from the instrument range, which refers to the capability of the instrument. For example, an electronic pressure transmitter may have a nameplate instrument range of 0–750 pounds per square inch, gauge (psig) and output of 4-to-20 milliamps (mA). However, the engineer has determined the instrument will be calibrated for 0-to-300 psig = 4-to-20 mA. Therefore, the calibration range would be specified as

0-to-300 psig = 4-to-20 mA. In this example, the zero input value is 0 psig and zero output value is 4 mA. The input span is 300 psig and the output span is 16 mA. Different terms may be used at your facility. Just be careful not to confuse the range the instrument is capable of with the range for which the instrument has been calibrated.

Transmitter

A transmitter is a device that converts the electrical signal from the transducer into a compatible electrical signal that can be sent over a long distance to a PLC or a DCS. The transmitter output signal is usually a range of voltage (1 to 5 V) or current (4 to 20 mA) which represents the 0 to 100% of the actual sensed physical measured variable.

Pressure Transmitter

A Pressure Transmitter is an instrument connected to a Pressure Transducer or directly to sensor. The output of a Pressure Transmitter is an analog electrical voltage (0-5V) or a current signal (4-20mA) representing 0 to 100% of the pressure range sensed by the sensor or transducer.

The real part of the transducer that makes contact with actual pressure is based on a lot of technology and building materials such as Strain Gauge, Capacitance, and Potentiometric.

The type of sensor selected is determined by the application and the environment in which it is used.

Pressure measurement can be absolute pressure, gauge pressure, or differential pressure.

Absolute pressure

Absolute pressure is referenced to a perfect vacuum which is normally called 0 psi! We express vacuum pressure as 0 psi (a) and Atmospheric pressure is usually about 14.7 psi (a).

Gauge pressure
The most common pressure measurement is gauge pressure which is the total pressure minus the atmospheric pressure. Atmospheric pressure is 0 psi (g).

Summary

• Please note Transducer is a device that converts one form to another.
• A transmitter is a device that converts an electrical signal from a transducer to a larger electrical signal that can be transmitted over long distances to the PLC or DCS.
• Pressure Transmitter is a device connected to the Pressure Transducer.
• Pressure Transmitter output is an analog electric current or current signal representing 0 to 100% of the pressure range generated by the transducer
• Pressure transmitters can measure absolute, gauge, or differential pressures.

In the world of process control, Transmitter is a device that converts sensor-generated signal into a standard instrumentation signal representing a process variable being measured and controlled.

Pneumatic vs electrical signal

In the early days of process control, the standard instrumentation signal was pneumatic signal where today it is likely to be an electric signal.

• The typical pneumatic signal is 3 to 15 psi.
• Typical electrical signals are 1 to 5 volts or 4 to 20 mA.

In process control, it is understood and goes without saying that the transmitter output range represents the 0 to 100% of the sensed physical variable. For example, the transmitter would produce an output current range of 4 to 20 mA for a measured temperature range of 0 to 300 degrees Fahrenheit (0 to 100%).

Let’s check where the transmitter fits into a process control loop. As already stated, the Transmitter converts the signal from the sensor to the Process Variable (PV) signal which represents the physical measured variable. The Controller is the device that looks at the difference between the Process Variable (PV) and the Set-point (SP). The Controller then determines what action to take place and generates an output signal that is a function of the result of this comparison. Controllers are either a DCS or a PLC in process control today. The Final Actuator is the device such as a valve that exerts a direct influence on the process as directed by the controller.

Types of transmitter

The four major process variables measured and represented by a transmitter are Pressure, Level, Temperature, and Flow. Transmitters are also used in industry to measure other variables such as Position and Speed and chemical properties such as pH and Conductivity.

4-wire and 2-wire transmitters

A 4-wire transmitter has 2 wires connected to a power supply and 2 signal wires connected to the PLC. The power supply can be AC or DC depending on the vendor and model. A 2-wire transmitter has only 2 wires. These 2 wires provide power for the transmitter and are also the signal lines!

Smart Transmitters

Smart Transmitters not only produce the 4 to 20 mA process variable signal, but also transmit and receive digital information such as Instrument Tag Names, Calibration Data, and Sensor Diagnostics. Protocols such as HART are commonly used on Smart Transmitters.

Summary

• In the world of Telecommunications, Transmitter is a device that produces radio waves from an antenna.
• Transmitter in process control is a machine that converts a signal generated by a sensor into a standard instrumentation signal representing process flexibility measured and controlled.
• The typical pneumatic transmission signal is 3 to 15 psi.
• Four major variables measured and represented by the sender pressure, level, temperature and flow.
• Instrumentation Transmitters can be connected to 4-wire or 2-wire configuration.

The instrument index will include the tag number of all physical objects (e.g. field instrument, visual alarm and indicator) and counterfeit tools commonly referred to as “soft markers” (e.g. DCS indicator, alarm, controller).

The instrument index will be built at the beginning of the project and considered as a live document that should be kept updated even if the plant was active.

The instrument index will be updated if a plant or system modification contributes to the addition, removal, or reset of the instrument

In the instrument reference text, the following information should be stated but not limited to:

• Tag number
• Loop Number
• Type of Instrument
• Location
• Service description
• P&ID Number
• Line number or equipment number
• I/O Type
• Control System
• Range or set point along with engineering unit used
• Applicable reference Document (Instrument Data Sheet Number, Hook-up Drawing Number, Instrument Layout Number, Loop Drawing Number)
• Package Number
• Manufacturer
• Model Number

The following guidelines are required in preparing the instrument index guide to complete the information: Reference diagram: P&ID, HMB

P & ID (Piercing and Metal Drawing)
From P & ID all tool tag number is collected with its associated details such as service description, instrument type, line / machine number, set point. A good quality P&D can have a distinctive feature that separates the control system to which each device is connected.

HMB (Temperature Balance and Object)
We can see the width of the metal scale based on the process data for each broadcast from HMB (Temperature and Balance Material).

Reference document: Cause and effect

The instrument index should include the number of tags related to fire and gas. Fire and gas devices usually do not appear in P&ID, they are said to have an effect and effect instead.

The purpose of the instrument index

From its descriptive name, the instrument index will be sent as a reference for many purposes as follows:

1. As a basis for preparing an I / O list by simply removing the I / O tag number
2. Searching, Listing, Sorting tag number

The formula is:

SPAN = URV – LRV

Where

PV          =  Process Variable
LRV       =  Low Range Value
URV      = Upper Range Value
mA        = milli ampere

Consider left side of formula for Process variable, LRV of transmitter lower range, Span is the difference between LRV & URV of transmitter ranges.

Consider right side of formula for current (ma), LRV of standard current range .i.e is 4ma,

Span is the difference between LRV (4ma) & URV (20ma) of standard current range

.i.e. 20 – 4 = 16, Using this formula we can calculate mA from pv and as well as pv from mA.

Differences

Performance

Significant improvements in smart transmitter accuracy, static pressure effects, temperature performance, and time stability provide a complete improvement in performance.

Rangeability

The power switch is increased to 400: 1 with a smart transmitter against a standard transmitter from 6: 1

The most noticeable variation helps to

1. Reduce the cost of backup management,
2. Increase the transfer of transmission equipment through crop rotation, too
3. It offers other benefits such as the freedom to define the distances directly when ordering units.
   A clear example of real savings with increasing variability is in applications where two or more standard units are used on a single orifice plate to improve the accuracy of various flows.

The difference in durability of intelligent transmission means that only one unit is required, thus only the other units and the cost of installing and maintaining them can be maintained and also prevent the low reliability that occurs in two or three loops against each other. In the case of a smart transmitter, the range can be changed manually using the speaker for various plant operating conditions.

Remote control adjustment

The range of the smart transmitter, water drain, etc., can now be adjusted from 1500 m away by contact over the 4-20 mA signal line. Adjustments can be made by connecting a smart field communicator (SFC) to all two signal lines at any location in their path.

1. The output can be seen as a straight or square root
2. The reduction time can be changed in several steps
3. LRV / URV can be adjusted to any value in the pressure range.
   Adjustments are made to any engineering units that can be selected in PSI, m bar, bar, k Pa.

Redesigning the smart transmitter without applying pressure is possible. Measurement corrections can be made with minor errors caused by ascending angle effects. Zero testing, and adjustments if necessary, can be done from the control room where the plants are closed or during regular maintenance. Remote diagnoses and remote control can now be performed back in the control room, thus taking a fraction of the previous maintenance period that usually requires two men several hours to do any field testing or switch.

All of the above can be done by one man from easily in the middle.

Reliability

The main purpose of intelligent transfer is built on reliable and stable work as a prerequisite. Few materials are used, and protection is provided against harmful influences, such as reverse polarity, over-pressure, and high voltage.

The SMART transmitter represents a Single Modular Auto-based Remote Transducer.

It is a smart transmitter with analog effect and simultaneously provides digital communication signal in accordance with the HART protocol or FOUNDATION FIELDBUS or PROFIBUS. Typically, a simplified drawing of a smart transfer is shown in the image below. Contains sensor or input circuits, microprocessor, memory, and communication block.

The term multi-variable transmitter is sometimes used especially in a device like a smart flow measurement tool. This measures the total pressure, power difference, and temperature process. It calculates the average flow rate and the volumetric flow rate of a liquid process.

SMART Transmitter

The result from the smart transmitter is no longer the same as the main variable process, but also includes secondary process variables, sensor health, sensory performance features, measurement power details, and real-time diagnosis. (This data can be accessed through the HART protocol or through fieldbus communication)

All of this information is used to improve the process, improve the performance of the metal while extending its life span, and increase productivity of workers. With the advent of the Internet, high data speeds, and with these smart tools and field systems are being transformed into part of the IoT (Internet of Things).

Advantages of SMART Transmitters

Below are a few advantages for Smart Transmitters:

1. Smart transmitters contain microprocessors and have bi-directional connections.
2. Smart transmitters include secondary sensors, which can measure and compensate for environmental disturbances.
3. Signal adjustment will be performed along with analog to digital conversion.
4. Smart Transmitters contain multiple sensors and cover various measurement distances and allow automatic selection of the required range.
5. Smart self-propelled transmitters allow for the removal of zero drift errors and sensitivity drift.
6. Smart transmitters will have the ability to self-test and one can set maintenance requirements.
7. It can adapt to offline lines and provide line output.
8. Smart transmitters provide improved accuracy and reliability.
9. Long-term stability can be improved, the frequency of re-balances can be reduced.
10. Reduced maintenance costs.
11. It allows interaction and is able to select your preferred vendors.
12. Allows remote remodeling and rearrangement.
13. Reduced in handling the number of inactive transmitters, because a single transmitter can be adjusted to cover any range and make it possible to replace transmitters.
14. Some transmitters such as the temperature gauge have a center for connecting unwanted sensors, which does not happen with standard transmitters. In the event of a failure of one sensor, the other sensor helps to detect heat that improves plant integrity.
15. Preparation data stored under the same tag can be transferred from handheld contacts or DCS programs.
16. Most smart senders have great power (two cables).

Disadvantages of SMART Transmitters.

As part of preventative maintenance, measurement or validation is unavoidable, although SMART senders.