What is the Difference Between Relative and Absolute Units?Primary Software: LabVIEW Development Systems>>LabVIEW Full Development SystemPrimary Software Version: 6.1 Primary Software Fixed Version: N/A Secondary Software: N/A
Problem: In LabVIEW, I have the option to assign units to controls and indicators. Several of these units seem to exist in two forms, such as DegC and Cdeg. What is the difference between these? Solution: Think of a Celsius degree (Cdeg) as the difference between two temperatures. It is always 9/5 of a Fahrenheit degree, regardless of whether you are talking about temperatures near freezing, near boiling or absolute zero. 1 Celsius degree (Cdeg) has no bearing on how hot or cold it is, it is just a relative value. However, Degrees Celsius (DegC) is an absolute quantity. 1 degree Celsius is just above freezing. To convert to degrees Fahrenheit, you not only multiply by 9/5, but you add 32 degrees as an offset. Internally to LabVIEW, degrees Celsius (DegC) is stored as Kelvins. (0 K is absolute zero. 0 DegC is 273.15 K. However, 1 Kdeg is the same as 1 Cdeg.) Therefore, adding 25 DegC to 25 DegC is the same as adding 298.15 K + 298.15 K. This returns 596.3 K (596.3 Kelvin), and converting back to DegC returns 323.15 DegC, and not 50 DegC as you might expect. However, you probably do not want to add two absolute quantities. In most cases, you add a change (relative value) to an existing (absolute) value. For example, if you have a block that is 10 degrees C (DegC) and you heat it up by 5 degrees (Cdeg), the block is now 15 degrees C (DegC). Refer to the LabVIEW Help, available by selecting Help»Search the LabVIEW Help... for more information about using units in LabVIEW. (In the Contents tab, navigate to Building the Block Diagram»Using Units.) Refer to the Polymorphic Units in LabVIEW Application Note (linked below) for more information about how LabVIEW handles unit conversion. Related Links: Attachments:
Report Date: 11/29/2000 Last Updated: 12/20/2011 Document ID: 23SEJ7DM |
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