Optimation in Control Design Magazine
Created 2 years 251 days ago
by Jennifer Palumbo
Jennifer Palumbo is Public Relations Manager at Optimation. Her primary responsibilities include corporate communications, media relations, website content development and maintenance, marketing collateral, writing articles and project profiles, and sales support.
Optimation Systems Developer Jim McCarty has written an article, "Process variables and the art of calibrating instruments" for Control Design Magazine.
McCarty lays out the critical points for performing a good calibration, providing details and relating them to real-life examples. His conversational tone makes this this a fantastic read for anyone looking to learn more about calibration.
The article is expected to also run in Control Design's October print issue.
Below is an excerpt:
Let’s start with the basics of calibration: the output of an instrument is measured under one or more known conditions (for example, the current output of a pressure transducer may be measured at 0 and 100 psig), and then a function of the sensor output (typically a linear function, so just a slope and an intercept) is generated to calculate the measured value of the sensor anywhere in that measurement range by knowing the output of the sensor.
Theoretically, in the example of a pressure transducer, this means that you can throw a digital multimeter (DMM) on a transducer and measure it sitting in the open, connect it to a shop air line, measure it with the DMM again, and be done after the calculation of the slope and intercept of the line that you draw through the points on a pressure vs. output current plot. However, there are critical points for not only performing a good calibration, but also making sure that it remains good:
- The stimulus (input—pressure, temperature, flow) is very accurately known.
- The calibration procedure yields repeatable and reproducible results.
- The stimuli/conditions are stable.
- The conditions represent the entire test range (if you expect to measure 0-100 psig on your test system, then you should calibrate at or near 0 and 100 psig) and test the sensor linearity.
- The calibration is performed under the same conditions as the measurement is made using the test system.
- The analysis is performed correctly, with passing and failing criteria.
Let’s step through each of these and discuss how they are relevant to a cleaning skid, so that we can build an effective and efficient calibration procedure. I’ll spend the most time on the first two topics, as they are arguably the most important, as well as a good chunk of time on the last topic, as I feel that it’s often overlooked. I’ll be using flow meters, pressure transducers, and thermocouples as examples.
Before diving into the details, first determine what your measurement accuracy must be, as calibration accuracy has a huge influence on measurement accuracy.
Read the full article.