Vol. 5 No. 3 Subject: Flow Measurement News Date: 3-27-03 From: McCrometer, "The Flow Measurement Specialists" ........ http://www.mccrometer.com ........ ******************************************* This newsletter is e-mailed by McCrometer monthly to subscribers worldwide. It consists of flow measurement information of interest for professionals in this field. For More information on our complete line of flowmeters go to: http://www.mccrometer.com ******************************************* McCrometer Newsletter - March 2003 DENSITY UNITS Density is an important fluid property in flow measurement. Density is defined as mass divided by volume. The units for density are usually pounds per cubic foot in the English system or kilograms per cubic meter in the SI system. As a fluid property, the density of a fluid depends on the type of fluid and the affects of environmental conditions on the fluid. For instance, water will have a different density than oil under the same conditions. Cold water will also have a different density than hot water. When density is needed in flow measurement applications, there are three ways to obtain the value. 1. Measure the density directly. Densitometers accomplish this and can directly measure either liquid or gas density. 2. Measure the components of the fluid. A good example of this is measuring the components of natural gas. A gas chromatograph will analyze a sample of natural gas and show the amounts of each major component in the sample. A density for the sample is then calculated using the densities of each component. 3. Infer the density of a fluid from the measurements of pressure and temperature. This is the most common method because of its simplicity. The cost of pressure and temperature measurements is much smaller than a densitometer or chromatograph. Since most liquids are non-compressible, pressure does not affect them significantly and therefore temperature is only required for density compensation in liquid applications. This last method assumes that the composition of the fluid is constant. If the composition changes during measurement, the compensation method will calculate the density incorrectly. The amount of inaccuracy depends on the shift in composition. Like most fluid properties, density will influence different flowmeters in different ways. Some volume flowmeters, such as magnetic and turbine meters, will not be affected by changes in density. The usual signal output from these meters is in volume units such as gallons per minute or liters per minute. When mass flowrate measurement is necessary, density must be multiplied into the flowrate such as: Volume flow * Density = Mass flow (cubic feet/minute) * (pounds/cubic feet) = (pounds/minute) Differential pressure meters, such as orifice plates and V-Cones, are dependent on density because of the Bernoulli theorem on which they are based. This is true whether the meter is measuring volume or mass flowrates. Mass flowmeters, such as Coriolis and thermal mass meters, measure mass flowrate directly. These meters have signal outputs in units such as pounds per minute. If volume flowrate units are necessary, the mass flowrate must be divided by the density, such as: Mass flow * Density = Volume flow (pounds/minute) / (pounds/cubic feet) = (cubic feet/minute) In most applications, density is frequently changing. Generally, this is due to changes in pressure and/or temperature in the line. The fluid composition changes less often. Because of the constant changes in density, people have developed different ways to express density. "Specific gravity" is often used in place of density. For liquids, specific gravity refers to the density of a liquid at a certain temperature as compared to the density of water at a certain temperature. Since both temperatures are necessary for the proper calculation, three values are necessary: specific gravity and both temperatures. However, temperature values are often forgotten or assumed. This makes the density value uncertain and a poor substitute for actual density. For compressible gasses, specific gravity refers to the density of a gas as compared to the density of air, both at "standard" temperature and pressure conditions. Standard conditions can vary but are usually near ambient conditions. Gas specific gravity is usually sufficient for estimating fluid density. Users that require the highest accuracy should use densitometers or chromatographs. Put your trust in McCrometer. To find out more about our range of flow measurement products, contact us at 951-652-6811 or visit our web-site at http://www.mccrometer.com. ********************************************** Copyright 2003 McCrometer If you have comments or suggestions for newsletter topics you would like to see, please address them to: Bob Peters or Richard Steven mailto:newscomment@mccrometer.com ********************************************************************************* Know anyone looking for a steady dose of flow measurement information? ********************************************************************************* To subscribe send e-mail to mailto:news-request@mccrometer.com with a keyword SUBSCRIBE in the subject field. 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