# Presentation: 128334

## Gas Calibration SystemsOverview

- Dennis Mikel, EPA Office of Air Quality, Planning and Standards

## Outline 

- Overview
- Mass Flow Controllers (MFC) Calibrators
- Compressed Gas Cylinders
- Zero Air Generators
- Calculating the Concentrations
- Cautions
- Summary

## Overview 

- The combination of the MFC Calibration instruments with NIST traceable compressed gas cylinders and a “clean” zero air supply _will enable the monitoring organization to create a dynamic calibration system_ that can support an NCore Level II monitoring station.
- How do you get such a system?
-  I am glad you asked!!

## MFC Theory

- MFC technology works on a very simple principle!
- Each MFC has a _Thermister (a Thermal Resistor)_
- Thermisters are sensitive to heat
- As air passes over it, it loses heat, the more air flow, the more heat that is lost
- This action changes the resistance of the thermister
- The change in resistance is monitored by a sensitive electronic feedback loop.  The resistance is converted to voltage and controlled by computer.

## MFC Theory-Calibrator Diagram

- Zero Air MFC

## MFC Calibrator Features

- Thermo (TEI) Model 146			Environics Model 9100

## MFC Calibrator Features

- Programmable Scheduled tasks
- Integrated Switch Closure System
- _NIST Traceability +/1 % accuracy!!_
- Built in ozone generator – Ozone SRP traceable
- Some MFC Calibrators are compatible with Data Acquisition Systems

## MFC Calibrator Issues

- Calibration should be provided (new)
- Certify your MFC against a  NIST traceable flow device
- NIST Traceable flow device should be certified annually (or if you suspect a problem)
- Perform quarterly calibrations of your MFCs (or until establish trend)
- Calculate True flow vs Flow Set Points
- Air Flow: 0 – 20 lpm (recommended)
- Gas Flow 0 – 100 cc/min (recommended)
- Corrections should be made to STP (25o C and 760 Torr)

## MFC Calibrations

| Setting | Display | BIOS |
| --- | --- | --- |
| 5.000 | 4.986 | 5.187 |
| 6.000 | 5.995 | 6.194 |
| 8.000 | 7.995 | 8.106 |
| 10.000 | 10.000 | 10.160 |
| 12.000 | 12.000 | 12.104 |
| 14.000 | 14.000 | 14.080 |
| 16.000 | 15.990 | 16.170 |
| 18.000 | 18.000 | 18.154 |
| 20.000 | 19.990 | 19.328 |

| Setting | Display  | BIOS |
| --- | --- | --- |
| 10.000 | 9.933 | 7.616 |
| 13.000 | 12.913 | 10.546 |
| 20.000 | 19.957 | 17.073 |
| 30.000 | 29.975 | 26.616 |
| 40.000 | 39.941 | 36.540 |
| 50.000 | 49.958 | 46.424 |
| 60.000 | 59.976 | 56.122 |
| 70.000 | 69.994 | 67.697 |
| 80.000 | 80.090 | 78.670 |
| 90.000 | 89.978 | 89.505 |
| 100.000 | 99.995 | 97.403 |

- Air Flow Results (lpm)		         Gas Flow Results (cc/min)

## MFC Calibrations

- Air Flow Calibration
- Y = 0.9694x +0.424
- R2 = 0.9981
- Gas Flow Calibration
- Y= 1.0138x -3.323
- R2 = 0.9992

## Compressed Gas Cylinders - Features

- Come in variety of sizes (size 50 or 150)
- Recommend using Aluminum cylinders
- Get EPA Protocol certification
- Use Stainless Steel regulators and cylinder valves
- Use Stainless steel or Teflon lines from Regulator to MFC (Teflon recommended)

## Compressed Gas Cylinders - Features

- Recommended Cylinder Concentrations for the NCore Level II sites:
- SO2:  	10 – 15 ppm
- NO: 	20 – 25 ppm
- CO:	200 – 300 ppm
- Oxygen free - Balance Nitrogen.

## Compressed Gas Cylinders - Issues

- Recommend reputable vendors
- Purge your lines well!! (Demo later)
- Handle with Care
- Make sure the cylinders are secure!!
- Read your MSDS Sheets
- Leaks!!

## Zero Air Source - Features

-             TEI Model 111			           T_API Model 701

## Zero Air Source - Features

- Materials utilized for pollutant removal
- UV light source (converts NO to NO2)
- Activated Charcoal (scrubs SO2, O3 and NO2)
- Purafil (scrubs NO)
- Hopcalite, Palladium or Carolite (scrubs CO)

## Zero Air Source - Issues

- Ultra Pure Cylinder Air vs. Zero Air Generators
- Trade off
- Quantity Used vs. Costs for cylinders
- High flow rates
- Humidity issues
- Purity of air generated
- Use Ultra Pure Cylinder air to verify Generator purity (recommended)

## Zero Air Source - Issues

**The Zero Generator or Cylinders should be able to provide air below the stated Lower Detection Limits (LDL) of the instruments you are testing.  **_**How clean is clean??**_
**NO ****	****50 ppt**
**SO2 ****	****100 ppt **
**CO ****	****20 ppb **
**Check the specifications before you purchase.**

## Let’s Tie it all together!!!

- The MFC Calibration instrument is certified to a NIST Traceable flow standard
- We have our NIST Traceable Cylinder hooked up
- Our Zero Generator is hooked up and warm
- How do we use this equipment to calibrate our PG instruments and transfer our Traceability?
- I am glad you asked!!

## Calculating the Concentrations 

- _Calibration Output Gas Law_
**C****1**** x V****1**** = C****2**** x V****2**
- Where:
**C****1 ****= Concentration in the Cylinder**
**V****1 ****= Flow of Gas from Cylinder to MFC **
**C****2 ****= Concentration from output of MFC**
**V****2 ****= Final Volume of mixture from output of MFC (zero air flow + cylinder gas flow)**

## Calculating the Concentrations and Flows

- _Start with:_     **C****1**** x V****1**** = C****2**** x V****2**
** **
- To solve for  V1:  ** V****1**** =(C****2**** x V****2****)/ C****1**
- To solve for  V2:  ** V****2**** =(C****1**** x V****1****)/ C****2**
- To solve for  C2:  ** C****2**** =(C****1**** x V****1****)/ V****2**
-  To solve for  C1:  ** C****1**** =(C****2**** x V****2****)/ V****1**

## Calculating the Concentrations and Flows

- _Start with:_     **C****1**** x V****1**** = C****2**** x V****2**
**C1 = 200 ppm CO**
**V1 = 20 cc/min**
**C2 = 250 ppb (precision point)**
**V2 = ?? lpm**
**V****2**** =(C****1**** x V****1****)/ C****2 **** = (200 ppm x 20 cc/min)/0.250 ppm **
**V****2**** = 16,000 cc/min = 16 lpm Total flow**
_**Remember, V**__**2**__** = gas flow and air flow so:**_** **
**Air flow = 16,000 cc/min – 20 cc/min = 15,980 cc/min**

## Calculating the Concentrations and Flows

- _Start with:_     **C****1**** x V****1**** = C****2**** x V****2**
**C1 = ?? ppm CO**
**V1 = 75 cc/min**
**C2 = 4000 ppb = 4.00 ppm (Span Point)**
**V2 = 5.00 lpm (5,000 cc/min)**
**C****1**** =(C****2**** x V****2****)/ V****1 **** = (4.000 ppm x 5,000 cc/min)/ 75 cc/min**
**C****1**** = 267 ppm **
_**Remember, V**__**2**__** = gas flow and air flow so:**_** **
**Air flow = (5,000 cc/min – 75 cc/min) = 4,925 cc/min**

## MFC Features –Environics 9100

**Concentration Mode:** User enters Target Output Gas Concentration (ppm or %) for the span gas. The actual concentration is displayed during mixing.
**Flow Mode:** User enters Target Flow Rate (cc's or liters per minute) for the span gas and dilution gas. Actual flow rates are displayed after mixing is initiated.
**Volts Mode:** User enters a command voltage to the mass flow controller and the ozone generator. The actual voltage is displayed during operation.
**Program Mode:** Permits multi-event programming and unattended automatic operation of the instrument over a seven-day schedule.
**Calibrate Mode:** User enters new calibration data for each mass flow controller and ozone generator. (Can be password protected)
**Maintain Ports:** User enters the name of the span gas in the source cylinder, its concentration (ppm) and the port to which the cylinder is connected.

## Cautions

- _ _

**Cylinders are under pressure -  Handle with Care**
**Do not allow ambient air to back flush into regulator or cylinder**
**Check your lines from cylinders to MFC Calibrator often.  Leaks of gases can accumulate in the shelter**
**Carbon Monoxide is poisonous!!**
**Zero Air Systems using Carolite or Palladium scrubbers are operated at high temperatures.  Hot to the touch!**

## Summary

- _ _

**State of the Art MFC systems offer many features and can be integrated with DAS**
**The MFC systems available today are compatible with the PG instruments **
**Lower concentrations cylinders are required since PG instruments have lower ranges and levels of detection**
**Gas cylinders should be certified**
**Zero air generators should be able to “scrub” below the LDLs of the PG instruments**
**Calculating the flows and concentrations are easy if you use the Calibration Gas Law**