Understand unit of measure conversion for steam functions

The default units of measure (UOM) for the steam functions are SI units. Any inputs to the steam functions defined in English units are automatically converted to the corresponding SI units. Similarly, if a steam function is configured with an output attribute, its output is automatically converted to the UOM configured for that attribute.

Note: Automatic conversion requires that all UOM have been defined properly in the UOM database. If the definition of conversion from one unit to another of the same type is missing from the UOM database, conversion will fail.

In the expression editor, when you evaluate a steam function expression, results are displayed in the default SI units. To display the results in other UOM, you can use the Convert function.

The following example expressions use numeric values, variables, and attributes to show how steam functions handle UOM for input and output quantities.

Numeric input values

Numeric input values are recognized as quantities in the default UOM. In the following example for Steam_VPT, the numeric inputs are recognized as 300 kPa and 200 °C, and the output value is in cm3/g, the default UOM for specific volume.

Input and output attributes

In the following expression, Steam_VsatT takes as its only input the attribute 'TempSat', whose current value is 300 °F. The temperature is automatically converted to °C to evaluate the expression, and the result is displayed in the default SI UOM for Steam_VsatT.

The result is also mapped to the 'OutputVolume' output attribute. This attribute is configured in English UOM (ft3/lb), so the result is automatically converted to that UOM. (On the Attributes tab in PI System Explorer, 'OutputVolume' would be listed with a value of 6.466627 ft3/lb.)

Variables and the Convert function

This example uses variables and the Convert function to specify quantities in English UOM as steam function inputs.

1. In the P and T rows of the expression below, Convert assigns numeric values to English UOM for pressure and temperature.

2. In the B row, the variables P and T are automatically converted to SI units for the Steam_VPT calculation; the result is displayed in default SI UOM. The result is also mapped to the output attribute 'OutputVolume'. That attribute is configured in English UOM (ft3/lb), so the result is automatically converted to that UOM.

3. In the last row, Convert is used to convert the result (B) to English UOM and display it in the expression.

   Name	Expression	Value	Output Attribute
   P	Convert(2000, "psia")	2000 psia	Click to map
   T	Convert(800, "°F")	800 °F	Click to map
   B	Steam_VPT(P, T)	19.206 cm3/g	OutputVolume
   Volume	Convert(B, "ft3/lb")	0.30762 ft3/lb	DisplayedVolume