Laboratory #1Calibration of Air Flows

Background:

Proper calibration of air flow is extremely important for eliminating bias in the collection and measurement of air pollutants. Air flow calibration devices can be broken down into primary and secondary standards. Primary devices provide a direct measurement of air flow. Secondary devices provide an indirect measure and must be periodically calibrated with a primary calibration device. Refer to FIH 6e, Chapter 16, pages 539-541, for additional details on air flow standards.

A linear regression produces the slope of a line that best fits a single set of data. Regression lines can be used as a way of visually depicting the relationship between the independent (x) and dependent (y) variables in the graph. In addition to visually depicting the trend in the data with a regression line, you can also calculate the equation of the regression line. The standard format of a linear regression equation is: y = mx + b, where m is the slope of the line and b is the zero intercept. The slope indicates the change in y for a change in x. The intercept indicates the point where the line crossed the vertical (y) axis. In this type of calibration the intercept often indicate a systematic error in the form of an offset from zero in the instrument being calibrated. This equation can be used to predict the value of a variable based on the value of the other variable.

How well this equation describes the data (the "goodness of fit"), is expressed as a correlation coefficient, R² (R-squared). The closer R² is to 1.00, the better the fit. Linear regressions are commonly used to provide a correction to the value of secondary standards as compared to a primary standard. For use as a calibration equation, the R² value is expected to be 0.95 or greater.

Materials:

37 mm cassette Backup pad

Membrane filter Bubble meter

Rotameter Tubing

Quick connects Buck calibrator

Accuflow calibrator Soup solution

Air sampling pump

Procedure:

Practical Exercise #1: Calibration of a rotameter with a bubble meter

Arrange apparatus as shown below.

Table l Rotameter Volume Time Volume Time Liters! minute Reading (milliliters) (seconds) (liters) (minutes) (1pm) \fTable 2 Rotameter Volume Time Liters/minute Volume Time (milliliters) (seconds) Reading (liters) (minutes) (Ipm) 2.3 1000 23.45 2.3 1000 22.78 2.3 1000 22.31