The Ins and Outs of Air Flow Controllers

Flow controllers (FC), also called regulators, are critical to the collection of grab air samples in canisters for analysis by EPA Method TO-15. This post will help you navigate through the Vapor Intrusion (VI) process so that you achieve accurate, usable indoor air samples. Accurately collected indoor air sample results are critical as they may confirm a complete exposure pathway or an exceeded inhalation exposure risk, resulting in mitigation with potentially significant financial implications. Flow controller failures during a time averaged sampling event can cause a serious data quality problem which requires resampling.

There is much published information on sampling protocols. However, this post focuses on the role of the FC in collecting data relevant to site conditions and ongoing exposure risk.

How does the FC work?

Time weighted average FCs have two components that work together to restrict the sample flow rate to a flow that meets the sample collection timeframe. First a critical orifice, which gives a physical resistance to flow, and second a stainless steel diaphragm that regulates the vacuum pressure at the orifice to ensure a near linear flow for the active sampling period (i.e. 8 hours or 24 hours).  The flow rate is set to leave approximately negative 5 inches of mercury at the end of sample collection period.

What is the FC set-up process?

In the laboratory, the FCs go through the following quality control check:

1. Back flushing with purified nitrogen

2. Flow is then measured to confirm there are no blockages in the unit.

3. The unit is then set for a specific flow rate using a mass flow meter. The flow rate is set to meet the sample collection timeframe for the sample canister size that is being used.

4. If there is visual damage or the FC cannot be calibrated, it is pulled from service for additional servicing.  This process is conducted at room temperature (approx. 70 degrees F) as most indoor air sampling events will be performed close to this temperature.

When should you collect samples?

The typical practice is to collect samples in the breathing zone over the time period that represents the most conservative site conditions. For residential property, sample time is typically a 24-hour period reflecting the diurnal cycle for the location or an 8-hour period for a commercial location when employees are present. The typical practice is for collection of an indoor air sample is in an evacuated pre-cleaned 6-liter passivated canister for a 24-hour sampling event, or a 2.7-Liter or 6-Liter passivated canister for an 8-hour sampling event.

The flow controller is pre-set at the laboratory to collect a sample for the selected time period while leaving vacuum (negative 4 – 6 inches Hg) at the end of the sampling event.  A residual vacuum is the confirmation of sample collection in the planned timeframe.

What field sampling issues are typically reported?

The most common reports are of a flow rate that are too fast or too slow.  If the flow rate is too fast, this can happen if the FC has not be attached to the canister with a gas tight seal, or the flow rate is too fast and depletes vacuum before scheduled sampling duration. If the flow rate is too slow, this often indicated that there is too much vacuum at the end of the sampling period. A dilution is needed which may report data above the risk screening levels (RSLs).

What should be done to reduce the possibility of a FC malfunction?

Pre-set, pre-cleaned FCs are shipped to the field in bubble wrap with end caps on the unit to prevent issues that may impact performance issues. However, certain conditions in the field can cause malfunction. For successful sampling, avoid the following:

• Dropping, knocking or rough treatment during handling in the field;

• Exposure to extreme temperature changes at the sampling site.  

• Removing the filter on the inlet

• Allowing water to enter the unit

• Using the FC in a dusty environment

• Using a wrench to tighten fitting.

How can I confirm that the FC is working properly?

• Review the instructions on how to setup FC and be familiar with the procedure in advance of the field sampling event.

• Plan sample collection to start a few hours before leaving the site.

• Carefully remove the FC from the shipping crate and inspect the unit for any signs of damage during shipping.

• Attach FC and canister per the instructions provided.

• Record the time that sampling is started.

• Check and record vacuum pressure while sample collection is in progress.

The pressure should be dropping at around 1 in of mercury per hour (for a 24 hour sample).  This will indicate the equipment is working properly.

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