Stability of a measuring instrument

In metrology, stability represents a static characteristic of a measuring instrument and the ability of an instrument to retain its performance throughout is specified operating life. The stability of a measurement is its ability to maintain a constant value.

The variations of the read value can be discontinuous, with random directions and amplitudes, or even slow, continuous and monodirectional (commonly called drifts). The causes of these variations can be different, for example: poor quality of instrumentation, presence of environmental disturbances, real variations of the measurand, etc..

Normally in the metrological field we refer to three types of stability:

  1. stability of the reading;
  2. measurement stability (over time);
  3. instrumentation stability.

Reading stability of a measurement

Reading stability is the ability of a measurement to maintain (during the survey) the read value within the resolution limits of the instrument. This concept overlaps with that of measurement repeatability.

During the reading of a measurement, especially with high-resolution instruments, it is common to find that the read value varies from instant to instant.

If the variations are discontinuous, with random directions and amplitudes, you can look for possible sources of disturbance to try to eliminate or reduce them; but if this is not possible or not successful, you will be forced to revise the resolution of the measurement or to take into account the presence of a repeatability error.

If the variations indicate a drift, one can look for a possible influence quantity to stabilize, otherwise, even in this case, one is forced to take into account a repeatability error dictated by the drift of the read value.

In some cases, if the drift has sufficiently stable characteristics, it is possible to take repeatable measurements by checking the influence quantity; for example, if a dynamometer drifts constantly following the application of a load, it is possible to obtain repeatable measurements by reading the load always after a precise period of time from its application. It should be noted, however, that most of the instrumentation needs a certain period of set-up, before taking a measurement, following the power on or the application of the measurand, in some cases it is kept permanently on in an environment with controlled temperature and humidity.

Measurement stability over time

It is common to call measurement stability the ability of a measurement to remain of constant value over a long period of time. Such considerations arise from monitoring a measurand observed over time (sometimes for years).

For these types of measurements, the quality of the instrumentation used must be taken care of in advance, in order to ensure that the variations read over time are properly referable to the observed measurand.

Normally two types of phenomena emerge from these measurements:

  1. drifts linked to some effect that varies the characteristics of the measurand over time. For example, in the case of measurements on a tool, the mechanical wear it undergoes during its use may show up as a continuous variation of some dimensional parameter.
  2. sudden changes in the characteristics of the measurand due to accidental causes. For example, in the case of strain gauge measurements on a bridge girder, sudden changes in the measured stress may indicate the failure of a part of the structure.

Stability of measuring instrumentation

The stability of a measuring instrument is its ability to maintain constant metrological characteristics.

An ideal instrument, once made, should maintain its characteristics constant. In reality, due to internal causes (e.g. deterioration and consumption of components, defects in manufacturing) or external (e.g. change of environmental conditions, misuse), the instrument varies its metrological characteristics in a more or less significant way.

In practice, one never speaks of “instrumental stability” in a generic sense, but one always refers to variations dictated by some influence quantity. Therefore, the concept of stability overlaps with the concept of instrument reproducibility.

Commonly, when we talk about the “stability” of an instrument, without specifying anything else, we mean the stability of its characteristics in the long run (also known as time drift). However, when we refer to the stability of an instrument when another quantity changes, it is necessary to specify which quantity we are referring to.

Among the most significant influencing quantities is ambient temperature (hence temperature stability). Among the specifications of the instrumentation it is common to find information related to the thermal drift of the same. This information is important in determining the portion of the measurement uncertainty associated with the change in ambient temperature.

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