Conventional lab hoods (also known as traditional, chemical, simple, bypass, and lab fume hoods) typically have a moveable sash or door in the hood face, an exhaust plenum with slots or other openings at the rear and near the top of the hood, and an air duct takeoff mounted on the top of the hood. The following are hood parameters of importance to OEHS professionals (note that this article uses IP or U.S. units of measurement):
Hood Face Area (Af):
The area of the open hood face with the sash placed at an acceptable position as defined by the user, usually wide open. Af is measured in units of square feet (sq. ft.).
Hood Face Velocity (Vf):
The average velocity of air into and through the hood face, measured in feet per minute (fpm). Lab ventilation standards such as ANSI Z9.5 usually call for specific face velocities to ensure optimum emissions control and containment (for example, Vf = 80–120 fpm). The owner of the hood chooses a face velocity that will meet standards; achieve a cost-effective balance among energy consumption, operating costs, and compliance with standards; and provide adequate emission and exposure control for the process being performed in the hood. 
Volume Flow Rate (Q):
The air volume flow rate through the hood face and into the exhaust duct, measured in cubic feet per minute (cfm).
Duct Diameter (D):
The diameter of the exhaust duct.
Hood Static Pressure (SPh):
The static pressure in the duct serving the hood. The appropriate SPh can create airflow rates sufficient to provide needed hood face velocities. SPh is measured in “inch wg”; the term “wg” stands for “water gauge.” “Inch wg” assumes the pressure would create a water column of that height in a vertical manometer. (Similarly, the atmospheric pressure at standard conditions is often given as “29.92 inches of mercury,” which is equal to 407 inches of water, or 407 inch wg.) Note that SPh is a “suction” or “negative” pressure. SPh is less than atmospheric pressure, but it is often referred to as a positive number: “The hood static pressure is about 1.5 inches wg.” These important parameters are linked by complex equations, which many OEHS professionals have likely forgotten. This article provides a simple nomogram (see Figure 1) that can be used to quickly and easily determine approximate values when two or more parameters are known. Computer programs can be purchased for this purpose, but they’re costly and time consuming, and require user training. And users must know the equations to use the programs. For those who aren’t familiar with the equations or don’t have access to the appropriate digital tools, the nomogram is a good choice. And even if you don’t need the nomogram immediately, chances are the issues covered in this article will surface one day.
Industrial Ventilation: A Manual of Recommended Practice
, 30th ed. (2019). AIHA:
Lab Ventilation Guidebook
, 2nd ed. (2017). American National Standards Institute/American Society of Safety Professionals:
Laboratory Ventilation,
ANSI Z9.5 –2018
draft standard, 2018). DiBerardinis, Louis, et al.:
Guidelines for Laboratory Design: Health and Safety Considerations
, Wiley (1987). Molnar, John:
Nomographs: What They Are and How to Use Them
, Ann Arbor Science, (February 1982). Scientific Equipment and Furniture Association:
Recommended Practices for Laboratory Fume Hoods
, 2010).

On the nomogram, note that columns 1, 3, and 5 are linked, and columns 2, 3, and 4 are linked. The common parameter is Q, in column 3, which allows us to associate terms in columns 1 and 5 with terms in columns 2 and 4.

The nomogram assumes hood air at “standard conditions” (STP, or Standard Temperature and Pressure), which is air near room temperature (70 F) and at an elevation near sea level. (For the more experienced: At STP, the air density correction factor, df, is near 1.0.) For more information, see my
Lab Ventilation Guidebook
, published by AIHA. The nomogram also assumes a hood Coefficient of Entry (Ce, also known as Coefficient of Flow) of 0.58, which is a measure of the hood’s efficiency converting pressure (SPh) into airflow (Q). A Ce of 0.58 is close to that of many traditional lab fume hoods. You should check with the hood supplier or look in the hood specifications to determine the actual estimated Ce for your lab hood. (The Ce for most hoods can also be determined by system testing. For more details, see chapter 11 in the
Lab Ventilation Guidebook
.) Because most ventilation estimates and measurements are not perfect, these assumptions can vary slightly without requiring correction of the nomogram answers. For example, for temperatures between 60 F and 80 F, elevations between sea level and 1,000 feet, and Ce values between 0.57 and 0.59, corrections aren’t necessary. Corrections for more significant variations are found in Table 1.
Because ANSI Z9 and other standards call for “real-time airflow monitors” on lab hoods, many hoods are equipped with an SPh monitor in the duct close to the hood. If not, one can usually be installed quite easily, or a measurement can be taken with a simple water manometer. (Chapter 11 in the
Lab Ventilation Guidebook
also covers the usefulness and measurement of SPh.) Duct diameter, D, can easily be measured, or found on the hood specifications. The following examples illustrate how the nomogram can be used to identify potential problems with lab hood performance and assist with design and operation of lab hoods.
Example 1A: Determining Operating Conditions at an Existing Hood
The measured parameters of an existing conventional lab fume hood located in a San Diego-based lab are D = 10” and SPh = 0.30 inch wg. The dimensions of the open hood face are 48” by 27”, and the face area, Af, is 9.0 sq. ft. The desired face velocity, Vf, is 100 fpm, which would meet the requirements of the ANSI Z9.5 standard. What is the approximate actual flow rate, Q, through the hood? What is the actual average air velocity, Vf, at the face of the hood?  The hood specifications indicate Ce = 0.58 and we are near STP air conditions—that is, conditions suitable for the nomogram. First, mark and draw a straight line on the nomogram between D = 10 (column 1) and SPh = 0.30 (column 5). The line passes through column 3, Q, at 700 cfm. Now draw a straight line between Q = 700 and Af = 9 (column 2) and extend the line to Vf (column 4). The face velocity, Vf, is 75 fpm.
Example 1B: Fixing the Problem
The hood face velocity of Example 1A does not meet the desired face velocity of 100 fpm. What Q and SPh would provide a Vf of 100 fpm?  Again, use the nomogram. Mark and draw a straight line connecting Af = 9.0 (column 2) and Vf = 100 (column 4). The line shows that Q (column 3) is approximately 900 cfm. To determine which SPh in the ductwork would provide 900 cfm, mark and draw a straight line between D = 10 and Q = 900, and extend the line to SPh. The SPh needed to provide a face velocity of 100 fpm is about 0.50 inch wg.  The nomogram has provided quick approximations of both the existing flow parameters at the hood and the new parameters that would bring the hood closer to compliance with ANSI Z9.5. Your next call might be to the lab’s maintenance department to rebalance the ductwork or adjust the fan speed to achieve a suction pressure of SPh = 0.50 inch wg in the ductwork near the hood. 
Example 2: Monitoring Hood Operation
John, an OEHS professional working for a fruit canning facility in Florida, wants to track the behavior of a lab fume hood in the cannery’s lab. He has an inexpensive static pressure tap installed in the hood’s exhaust duct and a simple water manometer installed on the side of the hood so the lab technicians can check the hood’s SPh daily. The proper operating specifications for the hood include: Ce = 0.58, Af = 12 sq. ft., D = 10”, Vf = 100 fpm, Q = 1,200 cfm, and SPh = 0.90 inch wg. John instructs the lab technicians to check the manometer daily and, if the SPh varies by plus or minus 0.10 inch wg or more, to call his office.  About one month later, a lab technician calls and says, “Everything seems normal, but the manometer is reading low at about 0.60 inch wg.” John, using the nomogram, quickly estimates the new airflow rate. He marks D = 10 on column 1 and SPh = 0.60 on column 5, then draws a line between these two points and determines that Q in column 3 is approximately 950 cfm, or 250 cfm less than the required flow rate. Now connecting column 2 (12 sq. ft.), column 3 (950 cfm), and column 4, he sees that the face velocity, Vf, looks to be about 80 fpm. John tells the lab technician to use another hood for now, and that he will send someone to check the equipment.
Example 3: A New Hood
The U.S. Navy in Norfolk, Va., intends to move a conventional lab hood to a new maintenance shop. The ductwork will connect to an existing exhaust plenum that has a suction static pressure of about SP = negative 4.0 inch wg. (The plenum serves other hoods in the facility and should be able to accommodate the new hood.) The hood face dimensions are 42” by 31” with the sash fully open. To meet its internal standards, the naval station desires an average face velocity of 120 fpm for this hood. The hood is sized for an exhaust duct diameter of 8”. A damper downstream in the duct can be adjusted to provide the necessary SPh at the hood in the new 8” duct. What flow rate, Q, and hood static pressure, SPh, are required to meet the desired specifications?  First, using the nomogram and assuming air at or near STP and Ce = 0.58, estimate the hood face area: Af = 42” x 31” = 1,302 sq. in., or approximately 9.0 sq. ft.  Mark and draw a straight line between Vf = 120 (column 4) and Af = 9.0 (column 2). Read Q = 1,100 cfm on column 3. Then mark and draw a line between D = 8 and Q = 1,100 and extend the line to SPh. Read SPh = about 2.0 inch wg. This suggests that the operating parameters should be Q = 1,100 cfm and SPh = 2.0 inch wg (which is well within the available 4” suction static pressure in the exhaust plenum). The damper in the duct should be adjusted to provide a pressure at the hood of SPh = 2.0 inch wg.
Example 4: A Hood and a Location Not at Nomogram Conditions
A traditional lab hood that houses a small chemical operation is to be installed near Salt Lake City, Utah, at an elevation of about 4,000 ft., a hood air temperature of about 75 F, and a hood Coefficient of Entry of 0.56. The hood face area is 12 sq. ft. when the sash is fully open; the desired face velocity is 120 fpm; and the exhaust duct diameter is 10”. What is the required airflow, Q, and hood static pressure, SPh, to achieve a face velocity of 120 fpm? On the nomogram, find and mark Vf and Af on columns 2 and 4. Draw a line connecting the two points and read from column 3, Q = 1,450 cfm. Now mark D = 10 inches on column 1, draw a line between D and Q on column 3, and extend it to SPh on column 5. Note that the suggested SPh = 1.3 inch wg.  Corrections for determining SPh are found in Table 1 under the heading “When Estimating SPh from Q.” The correction for Ce is 1.07, and the correction for elevation = 0.87. We won’t worry about correcting the slightly higher temperature of 75 F.  Now multiply the nomogram-estimated SPh by the correction factors, 1.07 and 0.87: SPh = 1.3 x 1.07 x 0.87 = 1.2 inch wg (rounded for significant figures), a total correction of about 8 percent less than the value provided on the nomogram.

In closing, can the nomogram be used for hoods other than lab hoods? Yes, if the hood’s Ce is near 0.58. If not, provide a correction. Also, keep in mind that the nomogram provides approximate answers, not unlike most ventilation measurements and estimates. For example, given today’s velometers, ventilation measurement results are plus or minus 5 percent at best.  
(former CIH, CSP), is an industrial hygiene engineer with broad experience in ventilation used for emission and exposure control. He is the author of many books and training courses, and is current chair of the ANSI Z9.2 and Z9.10 subcommittees. His full biography can be found one his
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Table 1. Approximate Correction Factors for Different Hood Ce Values and Air Conditions
Tap the table to open a larger version in your browser.
Figure 1. Conventional lab fume hood nomogram, developed by D. Jeff Burton and originally published in
Occupational Health & Safety Magazine
. Tap or click the figure below for a larger version (

Editor's note: This figure, and its corresponding
version, were updated March 23, 2019, to correct a problem with the proportions of the original image.
Simple Visual Tool Provides Quick Approximations of Operating Conditions

A NOMOGRAM  for Lab Hoods