What Ambient Factors Contribute to Changes in Differential Pressure? Part 1


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What Ambient Factors Contribute to Changes in Differential Pressure? Part 1

By: Craig A. Cox, CPG

USEPA and others in the vapor intrusion (VI) field have been evaluating a variety of Indicators, Tracers, and Surrogates (ITS) to assess their use in predicting the best time to collect representative indoor air samples for vapor intrusion studies (Schuver, et. al., 2018).  The idea is that if a predictive combination of easily obtainable, low cost ITS can be identified, they could be used to improve the collection of actionable analytical data at a lower cost. 

For VI assessments, typical indicators include seasons of the year, wind speed, the difference between the indoor and outdoor temperatures (differential temperature), barometric trends, and the difference between sub-slab and indoor air pressure (differential pressure).  Last month, I reviewed the usefulness of one such indicator – sub-slab to indoor air differential pressure. This month I will look at two ambient factors that are potential contributors to changes in differential pressure – barometric pressure and temperature.

Differential pressure is generally measured with a hand-held manometer.  Recently, however, sensitive differential pressure sensors have become available as part of the “Internet of Things” (IOT) revolution.  These sensors can be connected to permanent sub-slab monitoring points, such as the Vapor Pin®, to collect and transmit differential pressure, temperature, and barometric pressure readings to the web at preset intervals.  They can also be used to set alarm point that will notify users of system faults or other unacceptable conditions.

This month (June 2020), I installed a differential pressure sensor at our warehouse.  The sensor was connected to a Vapor Pin® and allowed to run continuously. The warehouse consists of 4,500 square feet, slab-on-grade with a common moisture barrier.  The ceiling height is approximately 22 feet, the walls and roof are constructed of insulated steel, and the building interior is heated by two gas fired furnaces hanging from the ceiling.  For this experiment, the differential pressure sensor measured and logged differential pressure and internal temperature.  External temperature and barometric pressure readings were obtained from monitoring data published through a weather site on the internet, a common source of data for our industry.

The two graphs below plot a segment of the collected data as differential pressure, exterior temperature, and interior temperature on one graph; and differential pressure and barometric pressure on the other graph.

Upon inspection of the graphs, two conclusions can be made with respect to differential pressure: 1) during the week of June 7 through 13, the differential between the sub-slab environment and the indoor air space was slightly greater than 0, indicating that the flow of soil gas was upward into the warehouse most of the time; and 2) there is a slight diurnal fluctuation in differential pressure during the week.  

The graph of temperatures is quite revealing.  Based on this week of data (which was consistent with readings collected throughout the month), the differential pressure tends to mirror the changes in internal temperature, but is less affected by the changes in external temperature. The diurnal internal temperature and maximum differential pressure occurs around noon of each day.  I suspect that this pattern is due to solar heating of the building, and would indicate, in our situation, that external temperature is not a dominant factor in effecting the differential pressure.

The graph of barometric pressure is also revealing.  I had suspected that differential pressure would be most sensitive to changes in barometric temperature.  However, to my surprise, there is no immediately discernable patterns in the differential pressure and barometric pressure readings. Could this really be the case?  I have, for instance, measured large fluctuations in water levels in confined aquifers due only to changes in barometric pressures.  Why not here?  

It may be that we relied on weather data available through the internet from a remote weather station for barometric pressure (and temperature) data rather than site data. We may need to collect our own data locally to see if patterns emerge.

Next month, we will revisit the external temperature and barometric pressure data with locally placed weather station data.  In addition, we will examine the potential effects of wind speed on differential pressure.

We must remember, however, that the conditions measured in the spring and early summer months of Ohio, may not be representative of those in the fall and winter months.  Stay tuned.

Schuver, H, Lutes, C, Kurtz, J, Holton, C, Truesdale, RS. Chlorinated vapor intrusion indicators, tracers, and surrogates (ITS): Supplemental measurements for minimizing the number of chemical indoor air samples—Part 1: Vapor intrusion driving forces and related environmental factors. Remediation. 2018; 28: 7– 31. https://doi.org/10.1002/rem.21557  


Craig Cox is a principal and co-founder of Cox-Colvin & Associates, Inc., and holds degrees in geology and mineralogy from the Ohio State University and hydrogeology from the Colorado School of Mines. Mr. Cox has over 30 years of experience managing large environmental project implemented under CERCLA and state voluntary action programs. Mr. Cox is the inventor of the Vapor Pin® and has developed a variety of software products including Data Inspector, an internet-enabled environmental database application. Mr. Cox is a Certified Professional Geologist (CPG) with AIPG and is a Certified Professional (CP) under Ohio EPA's Voluntary Action Program.