Metrological characteristics of resistance strain gauges and evaluation of deformation measurement errors
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METROLOGICAL CHARACTERISTICS OF RESISTANCE STRAIN GAUGES AND EVALUATION OF DEFORMATION MEASUREMENT ERRORS N. P. Klokova
UDC 531.781.087.92.088
According to [i], the set of standardized characteristics of resistance strain gauges, which are transducers of linear deformation into resistance variations, includes the static conversion characteristic at normal temperature, the sensitivity at normal temperature, the effect of temperature on sensitivity, as well as creep, hysteresis, resistance temperature characteristics, output-signal drift, and insulation resistance. Description of metrological properties in terms of a set of standardized metrological characteristics as suggested in [2] is more convenient since each characteristic reflects a definite physical property of the strain gauge, the supervision of metrological characteristics is simplified, the characteristics can be checked against specified norms, and in case of differences the reason for discrepancies can be determined. Metrological characteristics of resistance strain gauges are determined statistically and are essentially expressed as mean values or standard deviations in a sample extended to the whole batch. The set of characteristics makes it possible to represent the nominal conversion characteristic (NCC) for a batch of resistance strain gauges in the form I
= - - ~ (~-- L-- ~c- ~(t) --D~-- hm),
(1)
where r is the measured deformation, ~ is the average effect of temperature on sensitivity, K is the average sensitivity at normal temperature, ~ is the output signal due to deformation c, ~o is the initial output signal, ~C is an output signal due to average strain-gauge creep in the time interval between the application and measurement of the deformation, ~(t) is the average temperature characteristic of resistance, D t is the average drift at constant temperature in the time between the measurement of ~o and ~, and ~ins is the output signal due to variations of insulation resistance. The first two terms in (i) represent an output signal increment measured with the aid of secondary instruments with a conversion factor Kin , and can be written as
~ - ~ o = ( ~ . - ~o) K m ,
(2)
where ~ -- ~0 is the difference in readings of the secondary instruments. The subsequent terms and the function # are determinate values of systematic errors and are thus included in the standardized characteristic as corrections [3]. The number of such corrections depends on the desired accuracy, and on the conditions and method of deformation measurement. For example, if circuit compensation is used, the correction for temperature dependence can be neglected; corrections for creep and drift cannot be applied if the time Between the instant the load or temperture begin to act to the instant the deformation is actually measured is unknown, etc. Thus, in the measarement of deformation by means of a concrete resistance strain gauge under specific conditions, the actual conversion characteristic differs stochastically from the nominal one as a result of the difference between
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