Ventilation Testing – Determining Air Changes Per Hour (ACH) Using Tracer Gas Technology With Sulfur Hexafluoride

If you need ventilation testing discussed in this article, call us at 1-800-344-4414 or e-mail us at info@atlenv.com for details and a free estimate.

 

Written by Robert E. Sheriff, MS, CIH, CSP, President

November 9, 2018

 

It is important for most commercial operations whether office, commercial, hospital, health care facility, public buildings, warehouse or industrial locations to determine if the ventilation system is operating as it should and within the Air Exchange Rate Guidelines.

  1. There are two principal approaches to ventilation testing:For human occupancy, ASHRAE has set recommended air volumes based upon the number of occupants in the building. It is titled ASHRAE Standard 6.2.1, Ventilation for Acceptable Indoor Air Quality.
  2. The other method is to determine the air changes per hour (ACH) for different types of activity. For example: The Engineering Tool Box (engineeringtoolbox.com) has identified recommended ACH’s for various types of buildings/rooms.

 

Banks 4 – 10 ACH Air Changes per hour (ACH)
Boiler Rooms 15 – 20
Classrooms 6-20
Computer Rooms 15-20
Engine Rooms 4-6
Factory Buildings 10 – 15
Foundries 15-20
Hospital Rooms 4 – 6
Libraries 4
Lunch Rooms 12-15
Machine Shops 6 – 12
Medical Clinics 8 – 12
Paper Mills 15-20
Municipal Buildings 4 – 10
Museums 12-15
Private Offices 4
Police Stations 4-10
Post Offices 4 – 10
Retail Stores 4 – 12
Paint Shops 15 – 20
Theaters 8-15
Warehouse 2

 

There are several ways to measure ACH.

  1. Measure actual supplied air and/or exhaust air using an airflow measuring device (the most common are a velometer or thermoanemometer).
  2. Another method is by using a tracer gas injected into the supply system—or the space itself and measure the reduction in tracer gas over a fixed time. The most common gas to use is sulfur hexafluoride (SF6) since it is relatively non-toxic (OSHA PEL is 1,000 ppm) and is not naturally present in the air so it is easy to detect. Also, most instruments for detection of FS6 can give readings in the parts per billion (ppb) range and some instruments are capable of measuring in the parts per trillion range. ASTM (American Society of Testing and Materials) has established a method for determining airflow using tracer gas – ASTM E741-11.

 

There are simple formulas for calculating the ACH for either the direct airflow method or the tracer gas method (to be presented and discussed in later articles).

In commercial industrial buildings, generally the most practical method is to use tracer gas. A tracer gas can be injected into the air supply system or just released into the air. Upon reaching a stable level, the measurements and times can be easily recorded.

Measurement of actual air flows in buildings is more difficult because of difficulty in measuring all the air intakes and exhaust points which are often spread out, in high ceilings, or a large number of test points.  Rise of a tracer gas only involves release of the tracer gas at an air intake or even within the building itself.  The only real issue is to allow the tracer gas time to be evenly distributed though out the building before taking the “start” test.

There are other uses for tracer gas measurements as well.

  1. They can be used to test the effectiveness of individual supply or exhaust systems.
  2. They can measure if exhaust air is being re-entrained back into the building.
  3. As a leak detector in both open and closed systems.

In a later presentation, the exact methods of testing and the formulas for calculating ACH will be discussed.

 

Our primary service areas for Ventilation Testing are:  NJ, NY, NYC, PA, CT, DE, (Boston) MA, RI, Wash DC, WI, MD, MI, (Chicago) IL, VA, IN, (Atlanta) GA, AL, NC, SC, TN, (Dallas, Ft Worth) TX, OK, DC, AR, we can service most other areas of the U.S. but with some added travel charges.