When selecting an instrument for an HVAC application, whether it be a differential pressure transmitter, flow transmitter, temperature sensor, etc., it is important to choose the appropriate level of accuracy. To do this, one must understand that accuracy statements are nuanced and require careful attention. This article describes the key concepts and applications of accuracy statements so that the reader may confidently understand the specified accuracy of the instruments they choose.
Components of Accuracy Statements
Complete HVAC accuracy statements must include the following three components:
- Measurement uncertainty
- Reference
- Range
Measurement uncertainty is the deviation between the measured value and the actual underlying value being measured (sometimes referred to as error). It is often described in percentage terms with a ± descriptor, indicating that the error may be low or high. The ±2.0% is an example of the measurement uncertainty portion of an accuracy statement.
Reference is the portion of the accuracy statement that gives meaning to the uncertainty. Two references are common – percent of full scale (or span) or percent of reading. This apparently subtle distinction has significant impact on the actual performance of the device. Consider a generic instrument that has a full scale of 100 units and an accuracy ±2% of reading.
As you can see, a simple change in wording leads to significant differences in accuracy, especially at lower values. In a percentage of full-scale statement, the numeric magnitude of the error remains the same throughout the range. The percentage error goes up as the measured value goes down. In the two tables above, a slight change in wording caused the actual error to go from 2% to 20% at 10 units.
This problem is magnified if the selected instrument is oversized. If an instrument with a full scale of 200 units is chosen for an application for a maximum measured variable of 100 units, the errors shown above double.
Lastly, an accuracy statement must include the Range over which the accuracy statement applies. The range can be simple or complex. Consider the following examples:
An accuracy statement can only be relied upon if it contains the three components described in this article. Subtle omissions in the accuracy statement can have significant consequences.
If you’d like to discuss air and liquid flow measurement in more detail, please contact your Accuspec representative at 203-261-8100 or email us with any questions. We look forward to serving you!
