1

Date: July 28, 2014

Subject: Determination of Representative Detection Level (RDL) and “3 x RDL” Values for 

Carbonyl Sulfide

From: Cindy Hancy

Dave Reeves

To: Susan Fairchild, EPA/OAQPS/SPPD/MMG

Introduction

This memorandum describes the process that was used to develop a representative detection level (RDL) 

as well as “3 x RDL” values for carbonyl sulfide (COS) emission levels measured using FTIR/GC

technology. We based these values on data from emissions testing performed on mineral wool cupolas. 

General Method for Determining RDLs

The RDL is based on the average of the reported pollutant specific method detection levels (MDLs) for 

the best performing units (those in the MACT floor); we consider the resulting average MDL 

representative or characteristic of acceptable performance of source emissions testing and thus an RDL.1

The RDL is multiplied by a factor of three to determine the pollutant concentration at a level where, 

when measured by the compliance test method, the precision of the test method approximates that of 

other EPA methods as estimated by the ASME study; nominally 10 to 20 percent relative standard 

deviation.2,3 This three times the RDL (3 x RDL) value, expressed in units of the emission standard, is 

then compared to the MACT floor value to ensure that the resulting emission limit is in a range that 

would produce measurement results consistent with the test method design. In other words, if the 3 x 

RDL value were less than the calculated floor or emissions limit (e.g., calculated from the UPL), we 

would conclude that measurement variability has been adequately addressed; if it were greater than the 

calculated floor or emissions limit, we would adjust the emissions limit to comport with the 3 x RDL 

value to address measurement variability. 

 

1Note from Peter Westlin and Raymond Merrill to SPPD Management and MACT Rule Writers “Data and procedure for 

handling below detection level data in analyzing various pollutant emissions databases for MACT and RTR emissions 

limits,” revised April 5, 2012.

2

(ReMAP): PHASE 1, Precision of Manual Stack Emission Measurements; American Society of Mechanical Engineers, 

Research Committee on Industrial and Municipal Waste, February 2001.

3The factor of three used in the 3XRDL calculation is based on a scientifically accepted definition of level of quantitation –

simply stated, the level where a test method performs with acceptable precision. The level of quantitation has been defined as 

ten times the standard deviation of seven replicate analyses of a sample at a concentration level close to the MDL (which 

translates to approximately three times the MDL which is defined as three times the standard deviation of seven replicate 

analyses of a sample at a concentration level close to the MDL (40 CFR Part 136, Appendix B)).
2

Determining RDLs for COS

We reviewed all available COS testing provided by mineral wool facilities. Facilities tested COS using 

one of three methods; Method 15, Method 320, and Method 318. The reported method detection level 

for each test was collected and the results are shown in Table 1. Out of 8 COS test reports submitted for 

mineral wool facilities, 3 reported MDL values. 

We recognize that Method 15 is a different analytical approach than Methods 318 and 320 (gas 

chromatograph vs. FTIR) but as it is a valid method for COS measurement and EPA is not intending this 

RDL to be specific to a rule requiring FTIR measurement, we feel it is appropriate at this time to include 

this data in the RDL value. Should future data collection efforts demonstrate that Method 15 detection 

is statistically different than FTIR EPA would consider a separate RDL for Method 15 at that time.

Table 1: COS method detection levels for Mineral Wool facilities

Facility Test Method MDL (ppmv) Units

IIG Phenix City M318 0.105 ppmv

Isolatek M318 1.02 ppmv

Amerrock M15 0.50 ppmv

Average 0.542 ppmv

We calculated the average of these available MDL values to yield the representative detection level 

(RDL) which was 0.542 ppmv. The RDL was then multiplied by the factor of three which resulted in a 3 

x RDL of 1.625 ppmv (see Step 1 below). For the purpose of setting COS emission limits for the 

mineral wool NESHAP, the 3 x RDL had to be converted into units of pound (lb) of pollutant (COS) per 

ton of glass produced (lb/ton of glass). In order to do this, we first converted the 3 x RDL to mg/dscm as 

shown in Step 2 below. We then gathered the run-by-run concentration (mg/dscm) and lb/ton data of all 

the COS testing submitted for both open and closed mineral wool cupolas. For each run we calculated 

the ratio of the results of mg/dscm to lb/ton and we then took the average of all the ratios to get a final 

conversion factor. As shown in Step 3, the average conversion factor (i.e., the average of all ppm to 

lb/ton ratios) was multiplied the 3 x RDL (mg/dscm) which resulted in a 3 x RDL = 0.062 lb/ton of 

glass. 

Step 1 : Calculate the 3 x RDL

3 x RDL (ppmv) = 3 x [average MDL from best performers (ppmv)]

3 x RDL (ppmv) = 3 x 0.542 ppmv = 1.625 ppmv

Step 2: Convert the 3 x RDL from ppmv to mg/dscm

3 x RDL (

mg

dscm) = 3 x RDL (ppmv) (12.187

mole K

atm L

) (

mw COS

T (K)

)

3 x RDL (

mg

dscm) = 1.625 ppmv (12.187) (

60

298.15) = 4.0 (

mg

dscm)
3

Step 3: Convert the 3 x RDL from mg/dscm to lb/ton: 

3 x RDL (

lb

ton) = 3 x RDL (

mg

dscm) x average conversion factor ratio

(lb/ton)

(mg/dscm)

3 x RDL (

lb

ton) = 4.0 (

mg

dscm

) x 0.0155

(lb/ton)

(mg/dscm)

= 0.062 (

lb

ton)