The transmitter-sensor matching feature of the Rosemount Model 3144, 3244MV field mount and the 644 headmount temperature transmitters can reduce the error in a temperature point measurement by over 75%.
By accepting Callendar-Van Dusen constants, generated from a specific RTD's calibration data, the transmitter creates a custom curve to match the sensor's unique curve, eliminating sensor interchangeability error and significantly reducing the total measurement error.
Most inaccuracies in a temperature point measurement do not come from the transmitter. Although the transmitter will inevitably introduce some uncertainties into the measurement (reference accuracy, ambient temperature effects, and drift over time) these inaccuracies tend to be minor, compared to sensor errors. For most process conditions, the main source of temperature measurement inaccuracy is sensor interchangeability error. For example,
Sensor interchangeability error is a result of differences in the resistance vs temperature behaviour of a 'real' RTD compared to the resistance versus a standard, theoretical temperature curve programmed into every temperature transmitter. Sensor interchangeability error is introduced into the temperature point measurement because transmitters translate the resistance signal from the RTD into a temperature reading using a theoretical RTD curve.
Fortunately, most smart transmitters can be programmed to approximate any 'real' RTD resistance vs temperature curve, reducing sensor interchangeability error in the temperature point measurement. There are multiple ways to accomplish this, varying in accuracy and ease of implementation. These methods include one-point trim, two-point trim - and multipoint approximation.
Although one and two-point trims are simple to perform, they are limited in their ability to reduce error, compensating for only a constant offset and slope change while ignoring errors introduced from differences in the curvature. Multipoint approximations are more successful at error reduction but are impractical and hard to use. To significantly reduce error in the transmitter, 20 data points (40 numbers entered into the transmitter) are commonly needed. Using the multipoint approximation, it would take many data points to effectively eliminate sensor interchangeability error.
Rosemount's system permits total elimination of sensor interchangeability error - only requiring the entry of four numbers. These are called Callendar-Van Dusen constants. The Rosemount Model 3144 and 3244MV temperature transmitters will calculate and use sensor-specific curves, eliminating sensor interchangeability error in the temperature point measurement. This results in the transmitter translating the resistance signal from the RTD into a temperature reading using the true RTD curve.
To illustrate the significance of this accuracy, in a typical North American fuel oil/petrol cutpoint process, each degree of accuracy gained can save thousands of dollars per year. On an average 100 000 BBL/day unit in summer (when petrol is worth $2,10 per BBL more than fuel oil), each 0,6°C of error eliminated is worth $75 000 per year.
| Tel: | +27 11 249 6700 |
| Email: | rfq@acs-autocon.com |
| www: | www.acs-autocon.com |
| Articles: | More information and articles about Automation Control Solutions |
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version