Lab 3 Introduction to Quantitative Analysis: Pumps and Measurements of Flow

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1 Georgia Institute of Technology School of Earth and Atmospheric Sciences EAS 4641, Spring 2008 Lab 3 Introduction to Quantitative Analysis: Pumps and Measurements of Flow Purpose of Lab 3: 1) To gain a basic understanding of how to measure (control) a sample air flow. In this lab you will calibrate a critical orifice and will use it in the next lab to control the air-flow through a filter. The particles collected on this filter will be analyzed with the IC calibrated from the previous lab. In practically all situations in atmospheric air quality monitoring, a known quantity of air is extracted from the atmosphere through an inlet, conducted to the detector through a sampling line, and then analyzed for constituents of interest. Among other things, this requires 1) a pump to move the air and 2) a way to monitor or determine the amount of air sampled (e.g., a flow meter). The schematic below shows a typical arrangement of components. In this laboratory we will focus mainly on the measurement of flow. Inlet Sample Transmission line Detector Flow Meter or Flow Control Vacuum Pump Flow Meters To determine concentrations of atmospheric constituents, a known quantity of air must be analyzed. Typically, this requires a measurement of a flow rate (i.e., how much air was sampled over a period of time). Flow meters can be divided into two types, mass and volumetric, and the corresponding flow rates given on a mass or volumetric basis. Measurements of trace gases typically involve mass flow rates, whereas aerosol measurements are most often done on a volumetric basis. Volumetric flow rate (e.g. unit, cm 3 /s) depends on the gas T and P. However, concentrations are often reported at some reference condition, i.e., standard T and P (20 C, 1 atm). One can convert between different states by the ideal gas law. That is Q s = Q a P a /P s T s /T a where s = standard conditions; a = ambient conditions The concentration will then be: C s = C a (Q a /Q s ) 1

2 Mass flow meters (e.g. unit, g/cm 3 ) and mass flow controllers are ubiquitous and more readily available then volumetric meters and flow controllers. Using mass flow rates trace gas concentrations are reported as mixing ratios (e.g., ppbv, pptv, etc see any chemistry or atm. chemistry text for more info on mixing ratios). Flow rate Measurement Methods. (Reference: Aerosol Measurement; Principles Techniques and Applications, Editor Willeke and Baron, Chapter 22) Also see Appendix Methods to measure flow rates include: Pitot tube (measures velocity that can be converted to a flow rate) Hot wire or film anemometer (velocity measurement) Obstruction Meters o Venturi or orifice meter (measure ΔP across calibrated resistance) o Critical orifice (used to maintain constant volumetric flow) o Rotameter (variable area) Laminar flow meter (useful since can transmit particles/gases efficiently) Positive displacement meters o soap bubble o piston, which includes gas meters Mass flow meter A note of caution when measuring volumetric flow rates; care must be taken as to where the flow meter is placed since volumetric flow depends on P. Typically a volumetric flow meter is situated so that one side of the flow meter is at ambient P. In this way the measurement is of the flow rate at ambient conditions. Details on Some Specific Flow Meters (See web page file: Appendix 1 Meas of Flows.pdf for more details) Positive displacement meters are primary standards because their calibrations can be determined by direct physical measurement. The simplest and most accurate of these meters use a water surface (spirometer), or a soap bubble film (bubble flow meter) to produce a sealed chamber with variable volume. A more common positive displacement meter is the gas meter, which involves a piston moving in a cylinder. The movement of the piston, and hence the volume swept out by the piston, is recorded on a dial. A flow rate is determined from the volume of air the passes through the gas meter over a certain period of time. Laminar Flow meters are commonly employed in aerosol science since the flow meter is simply a straight narrow-bore tube that will efficiently transports particles (minimal wall losses). Thus a laminar flow meter can be used to monitor a sample flow upstream of the detector and the air to be sampled can be passed through the meter. The device is based on measuring the pressure drop through a known length of tube under fully developed laminar flow conditions. Under so-called Hagen Poiseuille flow pressure drop is directly proportional to volumetric flow rate. For circular tubes, laminar flow requires a Reynolds Number less than ~ Laminar flow meters can typically only measure flow rates up to approximately 2 L/min 2

3 A critical orifice is a small circular restriction placed in a tube to maintain a constant flow rate. If the absolute pressure downstream of an orifice is less than 0.53 times the upstream pressure the flow in the orifice throat will be sonic and further reduction in pressure does not change the flow rate. These devices are useful for taking constant flow rate samples with a vacuum pump. The are often used in integrated filter measurements of aerosol chemical composition. Flow Controllers: A flow controller combines a flow measurement with a metering valve. A feed back loop is used to maintain a constant user preset flow rate by automatic adjustment of the valve. Mass flow controllers are common and used extensively in trace gas measurement systems. 3

4 Experiment No 3: Measurement/Control of Volumetric Flow In the following experiment you will calibrate a critical orifice. An uncertainty analysis should be applied to all calibrations. Calibration of a Critical Orifice with a gas meter Set up the experiment shown in Lab Figure 1. Start with the valve totally closed and make a series (say 5 or so) repeated measurements with the gas meter and stop-watch at each valve setting. Record all pertinent data for each valve setting (e.g, volume, elapsed time, ambient pressure P 1 (use local met data) and pressure P 2. Make measurements for a total of 5 or so different valve settings. As you perform this experiment construct a graph of the absolute pressure ratio P 2 /P 1 vs Q. Note, P 1 is the ambient pressure, use 1 atm. The P 2 meter may measure gauge pressure, make sure you convert this to absolute pressure. Q is the volumetric flow rate, use units of L/min. For the last data point, remove the valve and determine the flow rate Pressure Gauge Flow Meter P 2 P 1 Valve Lab Figure L/m Critical Orifice Vacuum Pump (vane pump) 1. Make a graph of Q versus the pressure ratio (P 2 /P 1 ), include error bars and state how they were determined. 2. Is the orifice s behavior as expected, explain? (see Appendix 1 Meas of Flows.pdf). If the orifice is used in an experiment to maintain a constant flow, how would you know if it is working properly (what would you monitor). Can you think of problems when using a critical orifice when measuring aerosols. 3. Math Problem: Assume a critical orifice with 0.4 mm diameter is fabricated for air sampling purposes and used downstream of a filter. The flow rate is measured to be 1L/min when the upstream pressure is close to the ambient pressure (760 mm Hg, 20 C). 4

5 (See Appendix 1: Measurement of Flows for equation). What size of orifice (diameter) must be fabricated if the sampling flow rate is now 2 L/min, assuming the downstream/upstream pressure ratio is still less than 0.53? 5