Why the Difference Method (

25A) Should Not be Used f

Analysis on CH

4 Rich 

Em

Why the Difference Method (

25A) Should Not be Used f

Analysis on CH

4 Rich 

Em

Presented by 

Barbara_M

40th Stationary Source Sampling and A

JW Marriott Tucson Star
US Mobile Sources Eng

Categories

 Light Duty: Model YR 2010

– Passenger cars and trucks– Regulated by §86– Tested using §1065 and §1066

 Heavy Duty On-Road

– Regulated by §86, subpart A and parts of sub– Tested using §1065 
EPA Non-Road Engine 

Certification

General 

Compliance 

Procedures 

§1068

Test Procedure

§1065

N

Large nonroad Sparkignition

§1048

Locomotive

§1033

Small sparkignition

§1054

Recreational 

Vehicles

§1051

Non-road 

Diesel

§89

Marine 

Diesel

§94

Marine 

Sparkignition

§91

Large nonroad 

Sparkignition

§60 JJJJ

Locomotive

§92

Small 

sparkignition

§60 JJJ

Recreation

al Vehicles

§1051

Older Organization Newer Organiz
Difference Method Versi

Mobile Source Stationary S
Difference Method

Method 25A, Dual FID with N

EPA Recommends Part 1065.365(

RFCH4[THC-FID]~1.05 to 1.10

Span FID to CH4 thru NMCRF*PFCH4[NMC-FID] = 1

RF*PFC2H6[NMC-FID
NMHC Calculation accord

§1065.365(d) 

Equation 1065.660-2

ܺேெு஼

ൌ

்ܺு஼ ்ு஼ିிூ஽ ௖௢௥ି

்ܺு஼ ேெ஼ିிூ஽ ௖௢௥ ∗ ܴܨ஼ுସ ்ு஼ି

1 െ ܴܨܲܨ஼ଶு଺ ேெ஼ିிூ஽ ∗ ܴܨ஼ுସ ்ு஼ିிூ஽



χNMHC NMHC concentration



χTHC[THC-FID]cor Sample THC corrected to dry on THC-F

 RFPFCH4[NMC-FID] NMC C2H6 penetration fraction

χTHC[NMC-FID]cor Sample THC corrected to dry on THC-F

through NMC

 RFCH4[NMC-FID] NMC Methane response factor
Difference Method Versi

Stationary So
Method 25A

THC Analysis

 Extracted Gas sample 

– Source through heated sample line, glass fib

then sent to a flame ionization analyzer (FIA

– Sampling components shall be heated ≥ 110

– FIA shall be heated ≥ 120°C

 Caution

– Heavy HCs must be heated to 190oC to no

condense

– If you are not running at 191oC 5oC you a

compliant if you are using NMC
NMHC Difference Meth

Version 1 (ppmC1)

Engine 2 CH4/THC THC-FID CH4 NMC-FID NMHC 

Fuel 1 0.93 9517 8865 -58

Fuel 2 0.94 4421 4158 -71

Fuel 3 0.94 3297 3103 -54

Fuel 4 0.93 6909 6440 -47

Fuel 5 0.56 956 539 378

Fuel 6 0.62 649 400 219
NMHC Difference Meth

Version 2 (ppmC1)

Engine 2 CH4/THC THC-FID CH4 FTIR NMHC 

Fuel 1 0.96 9517 9152 365Fuel 2 0.98 4421 4338 83Fuel 3 0.96 3297 3180 117

Fuel 4 0.97 6909 6725 184Fuel 5 0.58 956 552 404

Fuel 6 0.63 649 410 239
NMHC %Relative Differen

between Version 1 and Ver

Engine 2

Version 1

FID (THC), 

NMC (CH4)

Version 2

FID (THC), 

FTIR (CH4)

% Relative 

Difference

|E1-E2|/

(|E1+E2|*0.5)

*100

Fuel 1 -58 365 68.9 

Fuel 2 -71 83 607.6 Fuel 3 -55 117 137.7 Fuel 4 -47 184 84.4 Fuel 5 378 404 1.6 Fuel 6 219 239 2.1 
CNG

Diesel

Analyzer 

Target 

Error 5%

Random Error Growth 

using the Difference Method

Critical 

Point
Speciated Method 

NMHC or VOC

Others

?
Proposed Speciated Gas 

for CNG-based Emissio

Speciated FTIR

Component Comment

RF

EPA Part

1065.8452

RF

SAE3

Methane Needed as interferent for analysis 

Ethane Needed as interferent for analysis for

VOC

Standard NMHC Component

1 

Ethylene Standard VOC Component 0.95 

Acetylene Standard VOC Component 1.2 

Propane Standard VOC Component 1 

Propylene Standard VOC Component 0.95 

Butane Standard VOC Component 

Oxygenated HC

Formaldehyde Standard VOC Component 0 

Acetaldehyde Standard VOC Component 0.5 

Formic Acid Standard VOC Component NA 

Methanol Standard VOC Component 0.63 

Added if Needed During QA/QC

1,3‐Butadiene Not Standard VOC 

Benzene Not Standard VOC 0.9 

Octane Surrogate for heavy HCs if present 

Toluene Surrogate for Aromatic HCs if present 0.9 
Difference FID Method for N

Error Estimate 

Typical 

Conc

5% Overall Erro

(C1)

THC 1575 ppm C1 78.9 ppm C1

CH4 1500 ppm C1 75.0 ppm C1

NMHC 75 ppm C1

(78.9

2+752)

± 108.9 ppm(C1)

Rel Error = 145%
FTIR Speciated Method for N

Error Estimate 

Typical 

Conc 5% Error

Ins

Ethane 30.0 ppm C1 1.5 ppm C1 1 

Ethylene 10.0 ppm C1 0.5 ppm C1 1 

Acetylene 1.0 ppm C1 0.05 ppm C1 1 

Propylene 1.5 ppm C1 0.08 ppm C1 3 

Propane 3.0 ppm C1 0.15 ppm C1 3 

Butane 

(C4+ Surrogate) 2.5 ppm C1 0.13 ppm C1 4 

NMHC 48 ppm C1

 (1.52 +0.5

2 +0.052 

+0.08

2 +0.152 +0.13

2)

± 1.59 ppm (C1)

 (12 +1

2 

± 5.9 

Rel E
‐300

‐200‐100

0

100200300400500

NMHC (ppmv C1)

Engine Test Number

THC ‐ CH4 (FID/NMC) Speciated (FTIR)

Difference Method 1 (NM

Speciated FTIR Meth

CH4/THC >85% CH4/THC ~
Difference Method Versi
EPA Alternate GC-Backflu

Speciated Method 320 F

Large 

Stationary

Engine

NMHC (C3)

NMHC - FID

Method 25A with 

GC back flush

NMHC(C3)

FTIR Speciated

FID-corrected

Method 320

VOC(C3)

NMHC - FID

FTIR Ethane 

FID-corrected

FT

#1 36.3 38.9 2.5 #2 43.2 39.2 9.1 #3 36.1 38.0 3.4 #4 34.2 38.7 0.9 #5 35.1 38.5 3.5 

Average 37.0 38.7 3.9 

StDev 3.2 0.4 2.7 
LATE BREAKING NEW

 EPA Mobile Source Group 

§1065

– Found this argument to be 

persuasive

 New section added 

§1065.266

– FTIR Speciation for CNG and 

LPG emissions

– Dual Fuel blends using CNG 

or LPG can use FTIR in Difference Method

– Added NMNEHC for 

compliance

Speciated FTIR

Component Comm

Ethane Standard NMHC 

Ethylene Standard VOC 

Acetylene Standard VOC 

Propane Standard VOC 

Propylene Standard VOC 

Butane Standard VOC 

Oxygena

Formaldehyde Standard VOC 

Acetaldehyde Standard VOC 

Formic Acid Standard VOC 

Methanol Standard VOC 
FTIR SPECIATION 

COMPARISON TO STANDARD EMISSIONS BENCH ANALYZERS
Speciation of Hydrocarb

 Generally need Surrogate for higher HC s

– Speciate up to C3 plus Butane, Octane for C

 Comparing FTIR to FID Analyzer

– Need to convert number using FID correction– Excellent match with Natural Gas 

≤ 5%

– Very good match with Diesel (5-10%)– Gasoline and Ethanol Blends – issues with 

unconverted oxygenated THC in FID

 NMOG (non-methane organic gases)

– FTIR provides accurate values for non-metha

organic gases which FID can not analyze.
Diesel Emissions Compa

CH4-FID to CH4-FTIR

0

1020304050

160 180 200 220 240 

Time (s)

CH4 (ppm)

MKS MGMKS MGHoriba 70
Comparison of Time-Res

CH4 Data

FTIR = 1.083*RM + 0.0022

R

2 = 0.9964

0.00.20.40.60.8

0.0 0.2 0.4 0.6 

Regulation Method (g/mi)

FTIR Method (g/mi)
Comparison of Time-Res

CH4 Data – Fuel Type

0.000.010.020.030.040.050.06

T1 - E10

T2 - E10

T4 - E10

T5 - E10

T10 - E10

T14 - E15

T15 - E15

T17 - E15

T18 - E15

T19 - E15

T20 - E15

T6 - E50

T7 - E50

T11- E50

T12 - E50

T13 - E50

T21 - E85

T

Test No. / Fuel Type

CH4 (g/mi)BAG FID FTIR
Diesel Emissions Compa

THC-FID to THC-FTIR

0

500

1000150020002500300035004000

0 100 200 300 400 

Time (s)

THC (ppmC)

FTIR THCFTIR FID EquFID THC
FTIR Corrected versus F

Diesel

Comparison FTIR / FID

0

250500750

10001250150017502000

0 250 500 750 1000 1250 1500 1750 2000

THC FID (ppm)

THC FTIR (ppm)
Summary

 What is the Problem?

– It is not with FID Analyzers– It is not with the FTIR – values are 10% or better compared to F

– The problem is the Difference Method

 Difference Method

– Method error increases as the CH4 and C2H6 content rises

– Can get negative concentrations!– Using different versions of the Method give very different answe

picking results

– For Quad J and Quad Z emissions CH4/THC is >50% - method 

 Speciated Methods

– Provides lowest uncertainty when CH4/THC is >50% in emissio– Accepted method for Engine and Vehicle certification

 MKS Offers Free Training on FTIR to Regulators

– April 2016 ASTM D6348 Hands On Method Training at MKS O

TX 

 Webinar broadcast at the same time

– 2015 presented two webinars on how to validate the MKS FTIR 

field and a review of FTIR in general. Presentations available u

– Willing to host more hands on training and webinars – just say t