General Specifications of Instrumentation Equipment
May 20, 2018Threaded Connection Safety Valves
June 10, 2018
General Specifications of Instrumentation Equipment
Measurement Zero :
A specific point within the measurement range is usually defined as the zero point. For example, in temperature measurement, the zero point is the temperature at which water freezes, while in pressure measurement, atmospheric pressure is considered as zero. However, at the zero point, the instrument’s output is not necessarily zero and may have a certain value.
Zero Drift – Offset :
Measuring instruments are usually adjusted so that the output at zero input equals zero (or another specified value). However, over time or due to other factors, the output at the zero point may change. This phenomenon is called zero drift and can lead to significant errors in the control loop.
Zero Drift is divided into two categories:
This drift is caused by external and environmental factors such as temperature or power supply variations.
A deviation caused by aging or changes in the properties of the measuring element over time.
Response Time:
Response time is the duration between a change in the measured quantity (input) and the moment this change is reflected by the instrument (output). A good instrument should measure the quantity quickly. In practice, however, instruments have a time constant and sometimes a delay, so that if the input changes suddenly (step input), the output display will respond with a delay.
Hysteresis :
Hysteresis is a type of nonlinear behavior in measurements. It can be defined as the phenomenon where the output differs when the measured quantity approaches the same value from higher versus lower values.
For example, if a process pressure reaches 5 bar once from 10 bar and another time from 1 bar, and the instrument displays 4.8 bar in the first case and 5.2 bar in the second, hysteresis is present.
Hysteresis occurs due to the non-elastic properties of the measuring system or instrument components. For instance, backlash in the gears of instrumentation devices can cause hysteresis.
