Key concepts about process instrumentation

Overview of key concepts about process instrumentation: Measurement accuracy (a static characteristic that determines the degree of agreement between the instrument indicator and measurement properties in measurement or monitoring), measurement uncertainty, measurement verification, and calibration.

Measurement forecast

The purpose of accurate measurement:

• According to ISO-IMV (International Vocabulary of International Assessment): “… Proximity of agreement between the marks or measured values ​​obtained by similar measurements in the same objects or under certain conditions”.
• According to IEC-IEV (International Electrotechnical Glossary): “.. a quality that indicates the ability of a measuring instrument to provide a specified value close to a true measured value”. Accuracy);
• Or we can deduce the following practical definition from the previous ones: “آزمایش By testing a measuring instrument under certain conditions and by specified methods, the maximum positive and negative deviations from a given characteristic curve (usually a straight line)”.

Thus, the concept of linearity is also inherent in the term precision measurement (which is currently very limited in digital precision instruments), while the concept of hysteresis is not included (although this is considered because in the maximum positive and Negative included (deviation found)

 

In addition, the concept of measurement reproducibility is not included (which instead takes into account the accuracy of accuracy in several measurement cycles. Therefore, in the practical verification of the accuracy of measuring instruments with a single up and down measurement cycle) Which is usually done for instruments with hysteresis, such as pressure gauges, pressure transducers, load cells, etc.) is obtained from the calibration curve, the type in Figure 1, where the concept of accuracy can be tested (measured accuracy). Which should be inferred in the term nominal accuracy (called accuracy) or the ranges in which the inaccuracy of a tool with its specifications is guaranteed .. Confirmation Maintains the measurement process over time.This concept of inaccuracy is sometimes referred to as the accuracy class for some common types of instruments (such as gauges, resistance thermometers, thermocouples, etc.), which according to the international ISO-IMV and IEC-IEV reference terms: Measurement or measuring systems in accordance with the stated measurement requirements, which intend to keep the measurement errors or measurement uncertainties of the instrument within a certain range under certain operating conditions ”(ie, the measured accuracy must be less than the accuracy.

Figure 1 – Description of the concepts of measurement accuracy

Unreliability

Uncertainty of measurement The measuring instrument is a new concept that not only finds errors or deviations during calibration, but also shows its sharpness and also considers the uncertainty of the measurement standard used in the calibration itself.

Measurement of uncertainty:

According to ISO-GUM (Measurement Uncertainty Guide): “The result of an estimate that determines the amplitude of the field in which the actual value of a measurement should be located is generally determined by a certain probability, that is, by a level. Self Confidence “

• According to ISO-IMV (Internat. Metrology Vocabulary): “A non-negative parameter that assigns the scatter of quantitative values ​​to a measurement based on the information used.”
• According to ISO-GUM (Measurement Uncertainty Guide): “The result of an estimate that determines the amplitude of the field in which the actual value of a measurement should be located is generally determined by a certain probability, that is, by a level. Self Confidence “

From the above definitions, we can measure two basic concepts of uncertainty:

1- Uncertainty is the result of an estimate that is evaluated according to the following two types:

• Category A: When the assessment is done by statistical methods, ie through a series of repeated observations or measurements.
• Category B: When the evaluation is done using non-statistical methods, ie data that you can find in booklets, catalogs, specifications, etc.

2. The uncertainty of the estimate must be presented with a certain probability, which is usually presented in the following three terms (see also Table 1):

• Standard uncertainty (u): at the level of probability or uncertainty of 68% (exactly 27%).
• Combined Uncertainty (UC): The standard uncertainty of the measurement when the result of the estimate is obtained using values ​​of different values ​​and is consistent with the summation in the quadrangle of standard uncertainties in different values ​​related to the measurement process.
• Wide Uncertainty (U): Uncertainty at the 95% probability or confidence level (exactly 45%), or 2 standard deviations, assuming a normal or bull probability distribution.

Standard uncertainty (x) (a)

Uncertainty of the measurement result expressed as a standard deviation (x) º s (x)

Type A assessment (uncertainty

Method of estimating uncertainty by statistical analysis of a series of observations

Type B assessment (uncertainty

Uncertainty assessment method other than statistical analysis of observational series

Uncertainty of standard uc (x

The standard uncertainty of a measurement result when that result is obtained from the values ​​of a number of other values ​​is equal to the positive square root of a value of the terms, these terms are the variance or variability of these other values ​​depending on how they weigh. Making is different by changing these quantities

Coverage coefficient k

A numerical coefficient used as a multiplication on a standard compound uncertainty to obtain extended uncertainty (typically 2 for probability @ 95% and 3 for probability @ 99)

Uncertainty U (y) = k is extended. (uc (y) (b

The value of the time interval definition for the measurement result that one might expect is to obtain a large part of the distribution of values ​​that can logically be assigned to the receiver size (including definitely multiplied by the uncertainty of the hybrid standard with a coverage factor of k = 2 Brought). Ie with 95% coverage probability)

(A) The standard uncertainty u (y), i.e. the mean square deviation (x), if not experimentally detected by the normal or Gaussian distribution, can be calculated using the following equations: u (x) = a / Ö3 , For rectangular distributions, with amplitude changes ± a, for example sign errors u (x) = a / Ö6, for triangular distributions, with amplitude changes ± a, for example interpolation errors (b) measurement uncertainty The measurement U (y), unless otherwise specified, must be prepared or calculated from the uncertainty resulting from the coverage factor 2, ie with a probability level of 95%.

Metrological verification

Calorimetric verification is a routine operation and control that verifies that the measuring instrument (or equipment) maintains the accuracy and uncertainty necessary for the measurement process over time. Measurement verification is in accordance with ISO 10012 (Mgt Measurement System) standard: “A set of intertwined or interlocking elements required to achieve measurement verification and continuous control of measurement processes”, and in general Includes the following:

Calorimetric verification is a routine operation and control that verifies that the measuring instrument (or equipment) maintains the accuracy and uncertainty necessary for the measurement process over time. Measurement verification is in accordance with ISO 10012 (Mgt Measurement System) standard: “A set of intertwined or interlocking elements required to achieve measurement verification and continuous control of measurement processes”, and in general Includes the following:

• Calibration and verification of tools;
• Any necessary adjustments and new conclusions,
• Comparison with measurement requirements for intended use of equipment;
• Labeling in confirmation of positive positive assays.

 

Measurement verification should be ensured through a measurement management system, which basically includes the steps in Table 1.

NORMAL PHASES	PHASES IN CASE OF ADJUSTMENT	PHASES IN CASE OF IMPOSSIBLE ADJUSTMENT
0. Equipment scheduling
1. Identification need for calibration
2. Equipment calibration
3. Drafting of calibration document
4. Calibration identification
5. There are metrological requir. ???
6. Compliance with metrological req.	6a. Adjustment or repair	6b. Adjustment Impossible
7. Drafting document confirms	7a. Review intervals confirm	7b. Negative verification
8. Confirmation status identification	8a. Recalibration phase (2 to 8)	8b. State of identification
9. Satisfied need	9a. Satisfied need	9b. Need not satisfied

• Table 1 – Main steps of measurement verification (ISO 10012)

  Table 1 highlights three possible ways to validate sentimentality:

  • The left path, which normally achieves the satisfaction of a positive result of the measurement confirmation without any adjustment of the instrument in the confirmation to step 6.
  • The first left and then the middle path from phase 6a to 9a, if positively adjusted or restored by the validator, will satisfy that confirmation: therefore, in this case, only the reduction of the approval distance is necessary;
  • The first path on the left and then on the right from phases 6b to 9b, in the event of a negative adjustment or repair in the confirmation, which does not satisfy the approval result: therefore the instrument must be lowered or alienated.

  Verification of measurements can usually be done in two ways:

  Comparison of maximum relief error (MRE) with maximum tolerance error (MTE), ie: MRE <= MTE

  Comparison Max. Uncertainty (MRU) with tolerance uncertainty (MTU, ie: MRU <= MTU

  Based on the previous content and taking into account the existing cases of calibration, related to the evaluation of calibration results in terms of error and uncertainty of a manometer, are equal, respectively:

  • MRE: ± 05 bar
  • MRU: 066 bar

  If the maximum error and the bearing uncertainty are both 0.05 bar, then the manometer corresponds if it is evaluated in terms of MRE, while if it is evaluated in terms of MRU it does not correspond and therefore it has to follow path 2 of table 1 or path 3. ; If it does not enter, it goes down.

  Tool calibration

  Instrument calibration is the practice of obtaining, under certain conditions, the relationship between the measured values ​​and the corresponding output signals of the device in the calibration.

  Order of calibration:

  • According to ISO-IMV (International Vocabulary of International Assessment): Establishes a related measurement and in a second step, uses this information to establish a relation to obtain the measurement result of the token.
  • Or we can deduce the following practical example from the previous example: “An operation performed to establish a

   

  relationship between a measured value and the corresponding output values ​​of an instrument under certain conditions.”

Calibration should not be confused with setting, meaning that: “A set of operations performed on a measuring system in such a way that the prescribed marks correspond to the given values ​​of a value for measurement (ISO-IMV) Offer. Hence, setting is usually the initial operation before calibration, or the next operation if de_calibration is found from the measurement tool. Calibration should be performed at 3 or 5 suitable measurement points to increase (and decrease) the values ​​for instruments with a hysteresis phenomenon (eg manometer): Figure 1 shows the calibration setting, while Table 1 presents the calibration results. Has been.

Calibration of manometer adjustment

Figure 1 – Calibration adjustment of a manometer