The National Institute of Standards and Technology (NIST) is a part of the U.S. Department of Commerce. NIST provides a wealth of scientific information, most of which is beyond the scope of this article. Of particular interest to those who deal with chemical processes is the NIST Chemistry WebBook which contains a great deal of information regarding the properties of a broad range of chemicals. In order to make full use of the WebBook, one must spend a good deal of time exploring its content and capabilities.

It is often of interest to process engineers, and to engineers who need to do control valve sizing calculations, to know the actual density of a gas rather than simply calculating the density based on the Ideal Gas equation using the molecular weight, the pressure, and the temperature of the gas. The most common of the ISA/IEC control valve gas sizing equations requires the pressure, temperature, and molecular weight of the gas. To account for the difference between the Ideal Gas density and its actual density the ISA/IEC equation also requires the Compressibility Factor (symbol Z), which, unfortunately, is usually not easily found.

In this article, Jon Monsen has outlined the procedure for finding the actual density of a gas using the NIST Chemistry WebBook. For this example, he will find the density of propane gas at a temperature of 113F and a pressure of 20 psia.

### Open The Chemistry WebBook

When the WebBook opens, you will see the first screen pictured below.

On this opening page, under "**General Searches"** click on "**Name"** (or if instead of the name of the gas you know its formula, click on "**Formula"**).

**NIST Chemistry WebBook **

### Enter The Name of The Gas

The screen pictured below will appear. (If you selected "**Formula"** in the above step a similar page will open asking for the formula) Enter the name of the gas on line 1. Here, he has typed in the name "propane."

**Search For Species by Chemical Name **

### Select The Appropriate Data

Clicking on the "**Search"** button opens the screen pictured below.

**Permanent Link **

Click on "**Fluid Properties"**

The screen pictured below will appear.

**Thermophysical Properties of Propane **

### Select The Appropriate Units

For this example, he has selected degrees F, psia, and lbm/ft3. He also selected cP for viscosity because it will show an optional calculation for viscosity. If you are only planning on finding the density, you do not need to select viscosity units or any of the other units listed. After selecting the units you wish to use, click on the "**Press to Continue"** button. The screen pictured below will appear.

Here you need to enter the temperature. In the example it is 113F. The NIST program will generate a graph of density vs. a range of pressures that you specify. In the example, we are only interested in one pressure, 20 psia, so he has selected a graph range of 19 to 21 psia, putting the desired 20 psia in the middle of the graph that will be given. He also told the program that he wants the graph to be in increments of 1 psia, therefore, the graph will show densities for 19, 20, and 21 psia. If you need the densities over a wider range of pressures, enter the range between PLow and PHigh along with a PIncrement that seems reasonable for your needs.

**Isothermal Properties **

After entering the desired parameters shown above, click the "**Press for Data"** button.

The graph shown below will appear. It shows the requested densities for the range of pressures we have specified, in this case, 19 to 21 psia. If you point the mouse pointer at the dot that represents your specified pressure and the resulting density, a label will appear stating the coordinates for your calculation. In this case, 20.000 psi and 0.14613 lbm/ft3

**Pressure Density Graph **

This concludes the demonstration of using the NIST Chemical WebBook to determine the density of a gas.

The screen shown below illustrates the fact that other properties are available from this same graph if you need them.

Below the graph are the parameters used for the X-axis and the Y-axis. If you click on the "down arrow" next to the Y-axis definition you will see a number of other parameters that are available for the Y-axis of the graph. Here he has selected "Viscosity (cP)." Placing the mouse pointer on the point on the graph representing 20 psia and the corresponding viscosity, a label will appear stating the coordinates for your viscosity calculation. In this case, 20.000 psia and 0.0086824 cP. The reason he selected cP for viscosity units previously was so he could demonstrate this optional calculation. If his only goal had been to calculate the density at 20 psia, he would not have needed to specify an engineering unit for viscosity.

**Pressure Viscosity Graph **

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