Ask the Expert Question-and-Answer Archive

by Ron Joseph

April, 2004

Determining VOC/HAP Flow Rate by Mass or Volume

Q. I am calculating emissions from a compressed air/siphon spray gun. I first determined a mass flow rate through bench testing (lb/hr) and then I used this to determine the volume flow rate of the various paints that are used (lb/hr/lb/gal=gal/hr). I then multiply the gal/hr rate for the various paints times the VOC or Hazardous Air Pollutant contents in each paint in lb/gal to get lb/hr.

Question: What is more accurate with such a paint gun, the volume flow rate or mass flow rate? Thank you.

A. EPA and state regulations want to know how much VOC/HAP go into the air on an hourly or daily basis. Since you obviously have an accurate scale to measure the flow rate by mass there is no reason for not using it. To determine VOC emissions you presumably used the VOC content that was provided on the MSDS and this gave it to you in lbs/gal . If you had been able to accurately measure the volumetric flow rate in gal/hr without having to do any conversions then this would also have been acceptable. I'm assuming that it was easier for you to measure flow rate by mass than by volume since you may not have had an accurately graduated container from which to work. On the other hand if you were working from a measuring cylinder graduated in small increments it would make no difference if you used mass or volume.

The VOC content in lbs/gal is not 100% accurate nor is it 100% reproducible since there are experimental errors that you encounter when you perform EPA Method 24 . Therefore, you should not be concerned with minor differences that you might get by measuring the flow rate either on a mass or volume basis.

Best wishes,

Ron Joseph