Passive Canister Sampler – Part I: How a flow controller works and how to select the proper critical orifice (featuring the all new Critical Orifice Calculator V1.03)18 May 2015
If you have been following my George Lucas-inspired, passive canister sampler trilogy, then you know how to calibrate your passive sampler and collect a sample; and assemble your passive sampler and leak check it. Today we will cover how the passive canister sampler (i.e., flow controller) works and how to select the proper critical orifice for your sampling scenario.
A passive canister sampler (see figure below) maintains a constant sample flow over an integrated time period, despite changes in canister vacuum. The critical orifice acts as a flow restrictor, upstream of a constant back pressure (i.e., vacuum in the canister). This constant back pressure is established by the balance between the mechanical spring rate of the diaphragm and the pressure differential across the diaphragm. The latter is established by the pressure difference between the atmospheric pressure (hence the atmospheric reference), the vacuum in the canister, and the flow through the critical orifice. The net result is a constant flow (up to ~-5” Hg and then the sampling rate drops off). The critical orifice determines the flow range. The adjustable piston is used to set a specific, fixed flow rate within the flow range. An adjustment to the position of the piston changes the back pressure, which changes the pressure differential across the critical orifice. If the piston is lowered away from the diaphragm, the flow rate will increase. If the piston is raised toward the diaphragm, the flow rate will decrease. And there you have your crash course in passive sampler theory of operation.
So the take away message for today is that the critical orifice is responsible for the flow range. Now how do we select the proper orifice? Let us start off with the scenario discussed in part III, in which we have a 6 L canister and we would like to sample for an 8 hour period.
We have the following:
- An evacuated canister starting at 29” Hg
- We are shooing for 29” Hg sampled down to 5” Hg
- This means 24” Hg (~83%) consumed
- 83% of 6 L = 5 L
- 5000 mL / (8 hr * 60 min/hr) = 10.3 mL/min
Now that we know we want a flow ~10.3 mL/min, we go to the chart below and see that we need a critical orifice of 0.0020”.
*Kit part #s are for the entire passive air sampling kit.
*Orifice part #s are for the orifice only.