Tracer Gas Testing

Introduction

You can perform Sulphurhexaflouride Tracer Gas Testing to quantify the operating ventilation efficiency of the building's HVAC Systems, and correlate this to potential contamination problems. Suggested mitigation procedures can also be developed by utilizing this procedure:

To perform the tests, the following items are normally utilized:

1)            Computerized Gas Monitor Detector

2)            Computerized Gas Doser for the Tracer Gas

3)            Laptop computer

4)            Application Software

5)            Data base software

Computer real time graphic presentations are developed utilizing database software and the application software. The application program processes the real-time data at the site for on site utilization and curve plotting through the database program.

Procedure

The Tracer gas technique provides the most accurate and, in some case, the only means of measuring several ventilation parameters which are critical to airborne contamination control. The versatility of tracer gas methodology permits accurate airflow measurements to be performed in situations in which Pitot tubes, air velocity meters or temperature sensors cannot be applied.   

These traditional measurement techniques are completely incapable of determining the extent of any external or internal re-entrainment of building exhaust into building supply, or the dilution of outdoor air at the workers breathing level. These airflow parameters are important components of a building's fresh air supply characteristic and must be considered. The following test procedure explains the tracer gas techniques utilized in order to accurately quantify building ventilation characteristics.

The overall test procedure required for this application involves dosing tracer gas at a constant rate at one point in the system and then monitoring the tracer concentration at other positions of interest.

The dosing point is located in the main return coming from the zone. Dosing must be done at a point where good mixing of the tracer gas with the return air will occur before the duct branches between building spill air and the mixing box. The best dosing points are located upstream of the return fan or before any bends in the duct work and as far upstream from the branch as possible. When adequate mixing has been achieved, the tracer gas concentration will be the same in the recirculated and exhaust air streams.

Tracer gas concentrations are monitored in the return air duct both before and after the dosing point. Additionally, tracer measurements are made in the outdoor supply, in the supply to the zone, at intakes to induction or fan powered boxes, one foot below diffusers and at occupants' breathing level.

Dosing Procedure

When we measure airflow, or the concentration at the breathing level, the tracer gas procedure utilized is the constant emission method. Before dosing begins air samples are taken for 15 to 30 minutes. This sampling is done to establish a baseline for each sample point. Dosing is then started at a constant rate until approximately steady-state concentrations are obtained at each sample point. The amount of tracer gas dosed and the time required to reach steady-state concentrations can be estimated using an approximation of the air flow rate expected and the volume of the zone.

The actual amount that is dosed depends on the outside air flow and the concentration of tracer gas which we aim to maintain.  Normally we try to get up to a concentration in the range of 0.1 to 2 ppm. To calculate the amount to dose from the size dosing nozzle we use, we work from an estimate of the amount of outside air and zone volume.

Measuring Procedure

As the dosing reaches steady state, the computerized sensor starts to measure the concentration of Tracer Gas at each of the predetermined sensor points. Up to six sensing points can be measured at the same cycle each about 15 seconds apart. The  concentrations  are  measured  and  plotted  versus  time  for  each  point,  in accordance with instructions and calculations inserted into the software at the time the dosing is begun.

After spot checking an adequate sampling of terminal boxes, diffusers, fans, etc., the data and curve plots are utilized to calculate and determine essential information such as:

1)            Cross circuiting of exhaust air to fresh air intakes.

2)            Quantity of outside air reaching occupants' breathing level

3)            Ventilation effectiveness in a sampling of typical offices and at typical work stations.

4)            Air movement at worker's breathing level.

5)            The rate at which normal contaminants are flushed from a worker's breathing level.

Principle of Operation

The tracer gas survey is performed with a computerized microprocessor controlled quantitative gas analyzer. The measurement principle is based on the photoacoustic infra-red detection method. In effect, this means that the instrument can be used to measure almost any gas which absorbs infra-red light. Appropriate optical filters are installed in the filter carousel so that the instrument can selectively measure the concentration of up to 5 different tracer gases in any air sample pass. The detection threshold is gas-dependent but typically in the 10-3 ppm region.

Reliability of measurement results is ensured by the regular self tests which the instrument performs to check that all is functioning correctly. Accuracy is ensured by the ability to compensate any measurement for temperature fluctuations, water vapor interference and interference  from  other gases  which are  known  to be present.

The selectivity of the Multi-gas Monitor is determined by the optical filters we utilize and place in the filter carousel. A wide range of narrow-band optical filters is available. By studying the absorption spectra of the gases to be monitored, as well as the absorption spectra of any other gases which are likely to be found in the ambient air in the same area, the most appropriate optical filters are selected.

Water vapor, which is nearly always present in ambient air, absorbs infra-red light at nearly all wave lengths so that, irrespective of which optical filter is being used during the measurement sequence of the monitor, water vapor will contribute to the total acoustic signal in the analysis cell. The higher the concentration of water vapor in the cell, the more it contributes to the measured signal. However, a special optical filter is permanently installed in the filter carousel of the monitor which allows water-vapor's contribution to be measured separately during each measurement cycle. The monitor is thus able to compensate for water vapor's interference.

Any other interferant gas which is known to be present in the ambient air can be compensated for in a similar fashion. By installing an optical filter to selectively measure the concentration of the interferant gas, we can "set up" the monitor to compensate for the interferant gases contribution, thereby assuring accurate and reliable results.

After installation of the relevant optical filters, the Multi-gas Monitor is zeropoint calibrated (using clean, dry air), humidity-interference calibrated (using clean, wet air) and then span-calibrated (using a known concentration of each of the gases it is to monitor).

Additionally, Sulphurhexafloride is an EPA approved tracer gas for use in occupied buildings. For detailed tracer gas procedures see my PUB #29.

A very popular method for performing tracer gas studies is by the use of CO2 seeding. This method is relatively quick and inexpensive, details can be found in my PUB #29. Following are the highlights of the method. It is important to always keep in mind when you want to know the ventilation effectiveness of an air conditioning system for the purpose of controlling the spread of infectious contaminants you must design for the effective air changes at the occupants breathing level. To do this you must know the rooms mixing factor and use this in the calculation of the rooms effective ventilation rate. The formula for this is as follows:

Actual Required ACH = Recommended ACH x Mixing Factor

Mixing factors normally range from 1 to 10

The CO2 seeding method basically uses a dry CO2 gas with the room being tested seeded with the CO2 until the concentration reaches 5000 ppm. At this point the CO2 flow is shut off and the concentration is allowed to decay(decay method), as it decays concentrations are measured about every 2 minutes and the decay is plotted. From this plot you can determine the rooms mixing factor and use this to determine the actual air flow needed to control the spread of the infectious material at the occupants breathing level.

CO2 Tracer Gas Testing Check List

1.  Use the concentration decay method for determining the mixing factor.

2.  Test the ambient gas level for at least 15 to 20 minutes.

3.  Inject the tracer gas.

4.  Stop injection when you reach the maximum level you desire.

5.  Test and plot at least five or more test points during the decay.

6.  The semi-log plot of the net data collected during the decay should be a straight line.

7.  The negative slope of the plot of the natural log of the net tracer gas concentration vs time equals the local air change rate.

8.  Calculate approximate mixing factor as follows:

a.  Figure the actual delivered air changes to the room:

(CFM of air supplied into the room) X 60 minutes = cu. Ft. per hour divide cu. Ft. per hour by room volume = ACH delivered.

b.  Take the ACH delivered and divide this by the local air change rate. This equals the approximate mixing factor.

9.  To calculate the amount of CO2 you need for a test take the net PPM you will seed up to, times the CFM of provided to the room divided by the tracer gas specific vol.(cu. ft./lb)

10.   That take the result you get from #9 above and multiply it time to reach the test ppm(about 15 minutes is a good starting point). This will equal pounds of tracer gas required. See the example below:

Example: (Using CO2 in a small protective room) (5000 -500)ppm X 200cfm/8.76 = .103 lb/min

.103 lb/min X 15 min = 1.543 lbs of CO2 required.

5000 ppm = The initial seeding concentration to start the test. 500 ppm = ambient CO2 level in room at start of test.

200 cfm = air supplied to the protective room.

When a protective room has its own dedicated HVAC system a tracer gas survey can be made to see if you are providing the proper amount of HEPA filtered and adsorption treated air to the breathing level of the occupants.  This is something that must be done for CP systems that are to be used on a continuous basses as may be done for schools, government buildings and some corporate headquarter buildings.