Poor performing instruments can cause safety incidents, production downtime, or lower product quality. For those reasons and others, accurate instrument measurements are important in any industrial environment.
Industrial facilities typically have a maintenance plan in place to ensure that all equipment is calibrated at the right time. However, these days, plants are focusing more on lowering operational expenditures, and time management has become more important. Understanding the factors that can influence instrument performance is essential in determining an appropriate calibration interval, and making sure your plant can run as efficiently as possible.
Join us for a webinar that will introduce you to the best practices in maintaining pressure transmitters and how it can improve your plant operations.
In this webinar you will learn:
What needs to be considered to determine the real performance of a pressure transmitter
What is “over pressure” and how does it relate to the performance of a pressure transmitter
How to determine a calibration interval
Pressure thermometer
Pressure transmitters in almost any industry
Industrial plants need to ensure that all of their instrumentation is measuring accurately within a certain tolerance. Poor performance from instruments can cause problems concerning safety, production downtime and issues with quality.
P = f/a , psi , lbs typically expressed as a force, mg, square inch
(inH20, rho * g * h , rho = m/V = m/h^3 , -> f/a)
Abs – not affected by atm pressure changed, barometer if left open
Capsule for various pressure ranges
Need to define before we talk about real world performance
The reference accuracy guaranteed on specification sheets is just an accuracy based on laboratory conditions.
Typical/common reference conditions might be:
- Temperature: 25 o C or 77 o F
- Static Pressure: Zero psi
- Relative Humidity: 10 to 55%
Manufacturing – characteristic (calibration) curve, raw sensor has unique output, characterization curve created and tested, optimized, turn characterization curve to ideal is goal, always some error
out of scale
% of span typically
IEC 61298-2 – not defined standard for pressure transmitter accuracy, but defines factors that make up accuracy - must include effects of linearity, repeatabilty and hysteresis
We define reference accuracy as the maximum deviation from an ideal characteristic line including the effects of zero point errors, full scale errors, linearity, repeatability, and hysteresis; expressed in % of span.
Assuming characteristic curve is linear, transition to linearity
How close characteristic curve is to a straight line
IEC does not define how quantified
Terminal based – straight line through actual zero point and full scale values, it is max deviation of characteristic curve
Best fit straight line – straight line to limit max deviation, generally = ½ * TB
Terminal based more conservative,, be aware
Repeatability – Change in output (characteristic lines) when same pressure is applied consecutively, same conditions, same direction
Repeatable not necessarily accurate
But if very accurate, are some what repeatable
Not understandable from name, Disease cured in the 1950s
Hysteresis- difference in output (characteristic lines) increasing pressure vs decreasing pressure from full span pressure cycle.
5/9 point calibration
Again, zero point errors, full scale errors, linearity, repeatability, and hysteresis all are used to compose reference accuracy. Value for each, % of span
Errors cannot be added, some errors will affect others, visa versa, errors are compounded, root sum square common, IEC does not define
So, manufactures who claim 3 sigma specification conformance, means 99.7 % of devices are manufactured to meet specifications.
The reference accuracy guaranteed on specification sheets is just an accuracy based on laboratory conditions.
Temp ambient and process – zero and span
SP- changing process conditions – zero and span
This calculation OMITS Static Pressure Zero Effects and is therefore not correct.
Zero trim at line pressure cannot eliminate SP-Zero effect, since the line pressure is changing constantly in the process
Root sum squared for compounding of errors
Root sum squared for compounding of errors
Overpressure another measurement error, mainly for DP transmitters
Surge pressure can be generated by water hammer action (start up, shut down), caused by wrong sequencing of three-valve manifold, opening the vent/drain plug of D/P Tx, process upsets.
Not always noticeable
Stability – drift, degradation of components and sensor over time,
