01 March 2005

ISA Certified Control Systems Technician (CCST) Program

This question is from the Level I study guide, Domain 1, Calibration.

Level l represents a professional who has a 5-year total of education, training, and/or experience.

CCST question

A 0-30 psig gage is undergoing calibration. Starting at 0 psi, the technician applies a pressure of 15 psi. The gage reads 14.9 psi. Then the technician applies 30 psi of pressure. The technician then reduces the calibration pressure to 15 psi. The gage reads 15.2 psig. What type of error do these readings suggest?

A. hysteresis error

B. span error

C. zero shift error

D. linearity error

CCST answer

The correct answer is A, hysteresis error.

Hysteresis is that phenomenon of an element, or in this case the instrument, whereby the value of the output (14.9 psi) for a given excursion of the input (calibration tool's pressure-15 psi) depends upon the history of the prior excursion (increasing values from 0 psi) and the direction (increasing or decreasing) of the current traverse.

In this example, the first excursion is the increase from 0 to 30 psi stopping at 15 psi along the way. The second excursion is the decrease from 30 psi to 15 psi. The reading on the instrument going up in pressure at 15 psi was 14.9 psi. The reading coming down from 30 psi to 15 psi, at 15 psi, was 15.2 psi.

mar10

A hysteresis curve is here. The x-axis is the calibrator input. The y-axis is the reading of the instrument which we are calibrating.

Span error, when displayed on an input/output calibration graph, translates as a line that is not parallel to the ideal line. That is to say, both lines are straight, but their slopes are different.

Zero error will show up on a calibration graph as two lines that are parallel because the shift is not only present at zero, it is constant throughout the range of the instrument.

Nicholas Sheble (nsheble@isa.org) explains the answers to CCST questions. For information about the CCST program, go to www.isa.org/ccst.