Instrument calibration

Continuing with the theoretical foundations of instrumentation today, I would like to deal with a very important subject on which the entire measurement process is based. I am referring to the calibration of instruments by means of which we tune them to give correct measurements on the basis of which a process is automated.

Measurement Process
DEFINITION

It is the action of measuring a variable quantity or condition and transmitting it to the error detector. The error detector is where the variable (PV) is compared with the set point, resulting in the corrective signal.

Process Variables

DEFINITION

In the processing of a material, it is any measurable characteristic or attribute whose value changes with variations in the prevailing conditions. Common variables are flow, level, pressure and temperature. It can also be defined as the quantity or condition of the process varied by the final control element(s).

CLASSIFICATION
Thermal variables
Temperature
Specific heat
Thermal energy
Entalpy
Entropy
Calorific value
Radiation variables
Nuclear radiation
Electromagnetic radiation
Heat radiation
Visible and ultraviolet light
Gamma and beta radiation
X-rays

Force variables
Total force
Torque or momentum
Force per unit area
Empty

Electrical variables
Electromotive force
Electric current
Resistance
Conductivity, Impedance
Capacitance.

Instrument Calibration
DEFINITION
It is the adjustment of the output of an instrument to desired values within a specified tolerance for particular values of the input signal.

TRAITS
A representative instrument is considered to be well calibrated when, at all points of its measuring range, the difference between the real value of the variable and the value indicated, recorded or transmitted, is within the limits determined by the precision of the instrument.
In an ideal instrument (without error), the relationship between the real values of the variable within the measuring range and the reading values of the device is linear.

BASIC PATTERNS
There are different types of measurement and calibration standards classified according to their function and application:

• Internationals
• Primary
• Secondary Schools
• Working

PROCEDURE

1. Set the variable to the minimum value of the measuring range, and adjust the zero screw of the instrument to this value until the index points to the base point.

2. Set the variable to the maximum value of the measuring range and adjust the multiplication screw until the index points to the base point.

3. Repeat the previous points 1 and 2 successively, until the readings are correct at the minimum and maximum values.

4. Set the variable to 50% of the measuring range and at this point adjust the angularity screw until the index moves five times the value of the error in the direction of the error (the actual curve is flattened).

5. Readjust the zero and multiplication screws successively until the desired or required accuracy is achieved. If necessary, make a new angularity correction.

This procedure is general, with the exception of replacing the word "index" with "pen" and output signal in the case of pneumatic (3 to 15 psi output signal) or electronic (4 to 20 ma d.c.) recording instruments and transmitters respectively. The position of the zero, multiplication and angle adjustment screws varies from instrument to instrument; some types do not have any of them.

Measuring Instrument Errors

THE ANGULARITY ERROR

Definition
Deviation of the points of the curve of the instrument's output values from the line that relates the input variable to the output of an ideal instrument without error and coinciding the two at points 0 and 100% of the measuring range.

Impact on the instrument display
The angularity error causes an uncertainty about the true value of the measurement, being the uncertainty the dispersion of the values that can be reasonably attributed to the true value of the measured magnitude.

Correction
The angularity error is corrected by making the transmission of the movement of the primary element by levers to be exactly square with the variable at 50% of its value by adjusting the angularity screw on analog instruments.

THE ERROR OF ZERO

Definition
Constant displacement of all the instrument output values in relation to the line that relates the input variable to the output of an ideal instrument without error.

Impact on the instrument display
The error of zero causes an uncertainty about the true value of the measurement, being the uncertainty the dispersion of the values that can be reasonably attributed to the true value of the measured magnitude.

Correction
The zero error is corrected with the so-called zero screw, which directly modifies the position of the index or registration pen by changing the variable-reading curve in parallel to itself, or by removing the index and fixing it to the reading axis in another position.

THE AMPLITUDE ERROR
Definition
Progressive increase of all instrument output values relative to the line that relates the input variable to the output of an ideal instrument without error.

Impact on the instrument display
The error of zero causes an uncertainty about the true value of the measurement, the uncertainty being the dispersion of the values that can reasonably be attributed to the true value of the mean magnitude.

Correction
It is corrected by acting on the amplitude or multiplication screw that directly modifies the amplitude of movement ratio of the variable to the index or pen, i.e. it progressively increases or decreases the readings on the scale.

All images used in this article are my own, photographs taken during my professional work. I allow the use of these images as long as they are mentioned to me in the article and the source is cited.

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