# The accuracy of the measuring instruments

**Author:**vacorda **Source:**vacorda.com **Date:** 2018-09-28 15:10:50

It is an important indicator that the accuracy of the measuring instruments when we selecting them. Today we are going to tell you the details about the error how to generate, calculate and calibrate in several different working situations, hope it can help you choose the right instrument.

**First, the definition of measurement error**

The measurement error is the difference
between the measured results minus the measured true value, referred to as the
error. Because the true value (also called the theoretical value) cannot be
accurately obtained, the actual value is actually agreed. The agreed true value
needs to be characterized by the measurement uncertainty, so the measurement
error cannot be accurately obtained.

Measurement Uncertainty: Indicates that the
dispersibility of the measured value is reasonably given. It is related to the
degree of knowledge of the person being measured, and is an interval obtained
by analysis and evaluation.

**Measurement error:** It is the difference
indicating that the measurement result deviates from the true value. It exists
objectively but cannot be determined.

**Second, the generation of errors**

The error is divided into random error and systematic error.

The error can be expressed as: error = measurement result - true value = random error + systematic error

Therefore, any error can be decomposed into algebraic and systematic errors of systematic and random errors:

**Systematic error: **The measurement error
caused by the inherent error of the measuring tool (or measuring instrument),
the measurement principle or the theoretical flaw of the measuring method itself,
the experimental operation and the psychological and physiological conditions
of the experimenter itself.

**Random error:** Random error is also called
accidental error. Even in the ideal case of completely eliminating systematic
error, repeated measurement of the same measurement object repeatedly will
cause measurement error due to various accidental and unpredictable uncertain
factors. It is called random error.

**Third, precision, precision and accuracy**

Use the same measuring tool and method to
measure multiple times under the same conditions. If the random error of the
measured value is small, that is, the fluctuation of each measurement result is
small, indicating that the measurement repeatability is good, that is, the
measurement precision is good, and the stability is good. Therefore, measuring
the magnitude of the accidental error reflects the precision of the
measurement.

Instrument accuracy is referred to as
accuracy, also known as accuracy. Accuracy and error can be said to be twin
brothers, because of the existence of errors, the concept of precision. The
accuracy of the meter is simply the accuracy of the meter's measured value
close to the true value, usually expressed as a relative percentage error (also
known as relative folding error).

Meter accuracy is not only related to
absolute error, but also to the meter's measurement range. The absolute error
is large, the relative percentage error is large, and the accuracy of the meter
is low. If two instruments with the same absolute error have different
measurement ranges, the relative percentage error of the meter with a large
measurement range is small, and the accuracy of the meter is high. Accuracy is
a quality indicator that is important for the instrument. It is commonly used
to standardize and represent accuracy levels. The accuracy level is the maximum
relative percentage error minus the sign and %. According to the national
unified regulations, the grades are 0.05, 0.02, 0.1, 0.2, 1.5, etc. The smaller
the number, the higher the accuracy of the instrumentation.

**Fourth, the choice of application accuracy**

In the actual application process, the
range and accuracy of the instrument should be selected according to the actual
situation of the measurement, and the instrument with a small accuracy level is
not necessarily the best measurement effect. Take the application of the
multimeter as an example, and use a multimeter with different accuracy to
measure the error caused by the same voltage.

**For example: **there is a standard voltage of
10V, measured with two multimeters of 100V, 0.5 and 15V, and 2.5. Which one has
a small measurement error?

**Solution:** Maximum absolute allowable error
of the first block

ΔX1 = ±0.5% × 100V = ±0.50V.

The maximum absolute allowable error of the second table

ΔX2 = ± 2.5% × 15V = ± 0.375V.

Comparing △X1 and △X2, it can
be seen that although the accuracy of the first block is higher than that of
the second block, the error caused by the measurement of the first block is
larger than the error caused by the measurement of the second block. Therefore,
it can be seen that when the instrument is selected, the higher the accuracy,
the better. Also choose the appropriate range. Only when the range is correctly
selected can the potential accuracy be achieved.

**Five, the accuracy of the calibration
method**

In addition to the nationally stipulated
level, with the wide application of electronic technology, depending on the
performance, there are several precision calibration methods as follows.

1. Display value ± X:

Applied in an electronic display
instrument, indicating that there is an error of X words at the lowest bit of
the current display value. If the display value is Y, the error △X=X/Y×100%

2. Display the X% of the value:

Applicable in electronic display
instrumentation, indicating that X% of the current display value is the current
error range. If the display value is Y error ΔX=X%, the error value is ±X%×Y

3. Segmented range calibration:

For wide-range instruments, the instrument
uses different error calibration methods in different measurement intervals.
For example, when measuring 0.01~1 volt, the error is 5%. When measuring 1~10
volts, the error is 1%. For the segmentation calibration method, when applying
the segmentation calibration instrument, it is necessary to select a suitable
range and carefully check the error calculation and calibration method of the
range.

4. Mathematical model error calibration:

The error calculation formula F(X) of the
instrument is given, and the current error ΔX=F(Y) is calculated according to
the current measurement result Y of the instrument and other relevant
conditions. The corresponding relationship between the error result and the
measured value measured by this method is mostly a curve. Since the error of
each point of this method is different, the application should pay special
attention and be carefully calculated.

**To sum up, **it is not difficult to conclude
from the above conditions that the actual measurement accuracy of the results
is different for different measurement values. When selecting, it is necessary
to analyze the measurement conditions and the allowable error of the instrument
at the measurement point. It is not necessarily the lowest level of
instrumentation that will have the best measurement results. It is necessary to
select the appropriate instrumentation and range according to the specific
situation, so as to minimize the measurement error.

**Sichuan Vacorda
Instruments Manufacturing Co., Ltd**

21 Years Focused
on Level Measurement in Extreme Process Conditions

Tel: +0086 28 8701 3699

Email: sales@vacorda.com

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