Gaseous Pollutants mini-courseTAMS Center February 2009

Creating a Test Atmosphere

Creating a test atmosphere using cal gas, zero air generator and a calibrator

What total flow do I need?What concentration do I want?What concentration is the cal gas?What cal gas flow do I need?What zero air flow do I need?

Cal Gas

Zero AirTest Atmosphere

Basically, it’s mostly about mixing ratios.

Gas/Titration/ Ozone

Concentration & Flow

2 parts this

15 parts this

FlowConcentration & Flow

From API Teledyne manual (blue oblongs added)

Zero air10 slpm10,000sccm

Cal Gas100 sccm

Cal Gas50 sccm

Ozone100 sccm

Mass flow controllers that may be inside gas calibrators (gas calibrators are often called diluters).

Flows aren’t the same. Volumetric liters per minute isn’t the same as standard liters per minute and standard cubic centimeters per minute (sccm) isn’t the same as volumetric cubic centimeters per minute. Most calibrators and analyzers used in ambient air monitoring are set to EPA standard flow rates. Some calibrators are initially set to 0 degrees as the temperature standard instead of 25 degrees. This doesn’t impact mixing ratios as long as all mass flow controllers are set to the same flow type. But, this can affect calibrator and analyzer flow checks and calibrations.

Volumetric Flow – EPA STP Flow: Conceptualize Balloon Impact

Temperature(increasing)

PressureVolume

0 º Kelvin

298 º K

325º K 0 mmHg

760 mmHg

790 mmHg

Note: pressure decreases with elevation; to illustrate that, low

pressure is on top in this graphic

Converting System Flow TO EPA Standard

Volumetric Flow X Temperature Correction X Pressure Correction

Volumetric Flow Meter

Temperature Pressure

Is the balloon getting bigger as it moves from system to EPA standard? The correction fraction should be greater than 1

Smaller? The correction fraction should be less than 1

EPA Standard for ambient air monitoring is 25º C (298º K) &1 atmosphere (760 mmHg)

X298 760

Xsystem system

10.0 mL

Temperature Pressure

X293

730

Example: The pressure is 730 mmHg and the room temperature is 20º C where you are checking flow.

If the volumetric flow meter reads 10 mL/Min what is the flow rate at STP?

298

(greater than 1)

760

(less than 1)

730 mmHg (system) is increasing pressure to 760 mmHg (EPA STP) so the balloon

gets smaller

= 9.77 mL/min X

20º C = 273 + 20 = 293 º K (293 is colder than 298, so from system to EPA STP, the temp is increasing and the balloon gets bigger)

Min(2.3% different)

Step 1. How much total flow do I want?

I need enough flow for each analyzer and some excess to ensure outside air won’t mix with my test atmosphere

Analyzer 1 draws 0.5 slpm – Analyzer 2 draws 0.6 slpm - analyzer 3 draws 1.0 slpm

0.5 + 0.6 + 1.0 = a total analyzer draw of 2.1 slpm - add excess

Let’s make total flow 3.1 slpm. 3.1 standard liters per minute is the same as 3100 standard cubic centimeters per minute

(Flow of cal gas) X [cal gas bottle concentration] [concentration of gas out ] = Flow of zero air + Flow of cal gas + other flows (note: the denominator adds up to total flow)

What do we know? Say we want a cal gas out of .090 ppm; we can pick that and “concentration of gas out” is then known. We decided we wanted a total flow of 3.1 slpm (3100 sccm) so we know the total of the denominator, total flow. We also know the calibration gas concentration (you can read it from the cal bottle – let’s say it reads 50.1 ppm). What don’t we know? We need to know the flow of the cal gas. Think algebra and solve for flow of cal gas in the numerator in sccm units.

Step 2 – Calculate from this formula

(Flow of cal gas) X (50.1 ppm) 0.090 ppm = 3100 sccm

(0.090 ppm) X (3100 sccm) = (Flow of cal gas) = 5.569 sccm PROBLEM!!!!!!!! 50.1 ppm

We did the calculation right so what’s the problem?

Mass flow controllers aren’t always accurate below 10% of their flow maximum.The calibrator likely has a 100 sccm mass flow controller. 10% of 100 is 10 and we ended up with 5.569. Some calibrators will flag this flow as not trusted. We need to have a higher total flow, a smaller mass flow controller for the cal gas (a 50 sccm flow controller would work in this instance) or a lower concentration cal gas. The easiest might be to just increase total flow, but there are limits. It’s a balancing act between total flows, cal gas concentrations and calibrator mass flow controllers. Let’s explore this further.

Let’s find out what the minimum total flow would need to be in order to keep the cal gas flow at 10% of the 100 sccm mass flow controller. This time we’ll solve for total flow

(Flow of cal gas is 10 sccm) X (50.1 ppm)

0.090 ppm = Total Flow

(10 sccm) X (50.1 ppm)

Total Flow = = 5567 sccm 0.090 ppm

What is the lowest concentration we can get from a 50.1 ppm cal gas bottle concentration with 10 liter zero air mass flow controller and a 100 sccm cal gas mass flow controllers in a calibrator?

Lets only use 90% at the top end of the zero air mass flow controller, or 9000 sccm (9 slpm) and the minimum flow of 10 sccm for the cal gas mass flow controller (10% of 100 sccm).

(Flow of cal gas is 10 sccm) X (50.1 ppm)

Output ppm = = .056 ppm 9010 for total flow

NO2 actual = % NO2 ConvertedNO2 expected

Gas Phase Titration (GPT)

Ozone calibrator: Scrubbed air (zero air) mixes with ozone generated from a UV lamp. The output gas is fed through a photometer and ozone is measured. A feedback loop adjusts the voltage/current through the ozone generator lamp and adjusts ozone output until it matches the measured ozone in the photometer.

Graphic is from Ecotech 9811 Service Manual

Graphic is from Ecotech 9811 Service Manual

Ozone

Note: most of the gas in a calibration bottle is actually N2 Gasses flow differently and calibrators sometimes ask what the source gas is in order to make small adjustments to flow. If the cal gas is 50 ppm NO and 50 ppm SO2, almost all the gas is actually the mostly inert carrier gas N2. In this case flow would be set for N2 and not SO2 or NO. The concentrations entered for concentration calculations would be NO and SO2.

NO2 is generated by reacting ozone with the nitrogen oxide gas (NO) from a cal bottle in a diluter/calibrator. To determine your expected NO2 you stabilize then record an NO concentration (NO Initial), then you add ozone until your NO concentration has dropped to your precision concentration (NO initial – Precision Concentration).