Calculations to Derive Drager Tube Factors for Garrett Gas TrainSulfide (GGT/S=) & Carbonate (GGT/CO3=) Analyses

(1970s-era Tubes and 2006-era Tubes)

Robert L. Garrett

November, 2006

Purpose of this Report:

I was asked by a member of API Committee 13 (Drilling Fluids Standardization) to revisit the Garrett Gas Train, GGT, (after about 30 years time elapsed) and to show how the Tube Factors for the two GGT tests had been calculated originally. The member pointed out to me, as I had heard, that Dragerwerk has changed the scale markings on some of their tubes but had not changed the tube's model number. This model number is used by vendors to make purchases. Since Drager tubes are bought by vendors as part of the GGT test kits, these un-announced, unknown, changes in tube scale rangeshave come as a surprise (often later rather than sooner) to someusers of the GGT kits. Thus, this report has the purpose of doing a complete job of describing the logic behind the calculation of a Drager Tube Factorfor GGT use and doing calculations for the tubes of the "1970s era" and for the current "2006 era".

This report reconstructs the Tube Factor calculations I made originally (1970s-era) - but more importantly, shows what the Tube Factors should be for the current (2006-era) Drager tubes - with their current scale markings.

This report contains four Sections covering the four tubes of present-day concern to API/ISO standardization work. Each Section is intended to stand-alone with information needed to calculate a Tube Factor for that specific tube. Notes also give some "history" regarding work done in Ref. 1 and Ref. 2 on the matter of Tube Factors.

Section A. "Low-Range H2S tube CH 29101"

Section B. "High-Range H2S tube CH 28101"

Section C. "CO2 tube CH 30801"

Section D. "Newer CO2 tube 810181"

Note: The tube CO2 8101811, of Section D, is not one that I had ever used in the GGT testing work, but apparently is being used currently in place of the CO2 tube CH 30801, discussed in Section C. So, it is important to include this newer tube.

Summary of Findings:

In the column on the far right (printed in red) of the Table below, are the "2006-era" Tube Factors that I believe are correct for the four tubes (two H2S and two CO2) that are used - or can be used - in the two GGT test procedures. (These numbers have been rounded to make them more user-friendly in the GGT calculations.) In fact, if the "1450" were rounded to "1500" as is now published, that would likely be within expected accuracy of the test. If other tubes are used, their Tube Factors can be calculated following the logic process I have outline in this report. Also shown are the ScaleRanges and Units of measurements for the four tubes.

Drager Tube Factors, ScaleRanges and Units of Measurement

Drager Tube Model Nos. / 1970-era Tubes / 2006-era Tubes
ScaleRange, Units / Tube Factor / ScaleRange, Units / Tube Factor
H2S CH 29101
(Section A) / "1 to 20" ; 100 to 2000 ppm / 12* / "100 to 2000", ppm / 0.12*
H2S CH 28101
(Section B) / "1 to 17" ; 0.01 to 7.0 vol. % / 600 / "0.01 to 7.0", vol. % / 1450**
CO2 CH 30801
(Section C) / "0.02 to 0.30", vol. % / 25,000 / "0.01 to 0.30", vol. % / 25,000
CO2 8101811***
(Section D) / Not available then / Not available then / "100 to 3000", ppm / 2.50

* Tube Factor of "12, rather than 14 or 15, was determined empirically to be more accurate. This conclusion comes from Ref. 1.basedon datataken during the 1970s in a study of samples which compared the GGT method with other methods of sulfide analysis. Probably that slightly smaller numberof "0.12", rather than 0.14", is best choice for current tubes of the same chemistry. See Section A.

** Since the current API publications use the number "1500", it would likely be within the overall accuracy of the GGT/S= test measurements to use that number rather than "1450" calculated in Section B, here in.

*** CO2 tube Model number 8101811 was not studied in Ref. 2, and was not available, in the 1970 era. It appears to be the same chemistry as Model number CH 30801 but with a ppm rather than vol. % scale. Thus, the difference in Tube Factors between the two similartubes is thefactor 10,000. See Section D.

Background on GGT/S= and GGT/CO3= Methods:

The two SPE/JPT paperswhich I published in 1975 and 1978gave experimental details of the GGT/S=and GGT/CO3=analytical methodsfor sulfide analysis and for carbonate analysis of drilling fluids. See Ref. 1 and Ref. 2 below. I did not showin those parallel papershow I had calculated the Drager tube "GGT Tube Factors" used to convert stain-length tube readingsfrom"ppm" or "vol. %" gas concentration into a liquid "mg/L"concentration. This "Tube Factor" for each tube type used, is amultiplier number to get the sulfide (S=) and carbonate(CO3=) anions reported as mg/L units.

The GGT/S= method -in order to cover a wide concentration range for sulfide ion in drilling fluids (water-base and oil-base) -uses two ranges of H2S gas detector tubes: (1)Low-range H2S Drager Tube (CH 2910)and (2) High-range Drager Tube (CH 28101). The Low-range tube and High-range tube both operate on the same general basis of using 100 mL of a gas sample.The Low-range tube uses lead (Pb) chemistry and the High-range tube uses copper (Cu) chemistry. Both have very stable and predictable stain behavior when contacted by H2Sand excess flow volume (above 100 mL) through them during a GGT testdoes not cause stain elongation or fading.

The GGT/CO3= method, in the 1970s era, usedCH30801 Drager CO2detector tube.It required exactlya 1000 mL (n = 10 pump strokes)gas sample with the CO2 being uniformly distributed in the gas flowing through the tube.. The GGT test protocol for carbonate ion requires an extra step - not neededin the sulfide test - of collecting the GGT effluent in a 1000 mL gas bag and drawing the gas sample through the CH 30801 tube using10 strokes on a Drager suction pump. This exact gas-volume requirement was due to its "redox" type chemistry,making the stain length subject to elongation and fading when over 1000 mL gas flows through.This tube's Tube Factorwas 25,000in the 1970s-era based on CH 30801 tube markings being in vol. %. Today, the tube markings areslightly different but the tubestill covers the same 0.01 to 0.30vol. % range as did older tubes. Therefore, the Tube Factor remains 25,000 for current CH 30801 tube - as before.

A newer CO2 detector tube, 8101811, has become available and seemingly uses the same reagent inside as did the CH 30801 - but with a scale that reads ppm - not vol. %. The tube factor differs from CH 30801 by a 10,000 factor - discussed later.

Stepwise Process for Calculating GGT Tube Factors:

Thethree (or four) step processprovides a Tube Factorappropriate for a specific type Drager tubeand uses Drager's basic technical information. (See Ref. 4.) A Tube Factoris themultiplierin theequation which relates theGGT test parameters to the answer, the result. The input parameters are Sample Volume and Stain Length (the data, read from the tube body). The result is mg/L of the component of interest, which was in a liquid sample being tested, but was converted to the gas. The equation example below is forcomponent "X". The units for each part of the equation must lead to the answer being given in mg/L of "X" in the sample volume that was tested.

"X", mg/l = (Stain Length) x (Tube Factor) / (Sample Volume).

Eachmodel numberDrager tube will have its own Tube Factor. In this report, there are four model number tubes discussed. A Tube Factor will be related to(a) the concentration range of a gas it will measure, (b) therequired volume of gas throughput and (c)numbers on scale, whether it reads in "ppm" or "vol. %". This information can be found in Drager literature for the tube being used.

Also the Tube Factor must be adjusted for the ionic/molecular weight relationship(e.g. H2S-to-S=). In some cases, an adjustment to the Tube Factoris needed to account for the gasvolume change with temperature. There are three and sometimesfour steps in this process to calculate a GGT Drager tube's Tube Factor. Once found, the Tube Factor should not need to be recalculated until Drager's basic tube design or its parameters are changed.

Step 1: Gas Concentration: Calculate from Drager's information, the milligrams (mg) ofa specified gas (e.g. H2S or CO2) that will producea prescribed Stain Length (as markings on the tube's scale)when thespecified volume(either 100 or 1000 mL)is passed into the tube at the gas concentration for the tube reading. Keep in mind that one gram-mole of an "ideal" gas occupies 22,400 mL at standard conditions.It works best to use the maximum Stain Length and thus assumingthe maximum gasconcentration number for the tube in this calculation.Use that calculated"mg" as thenumber in Step 2.

Step 2:Liquid Concentration: From Step 1, use the "mg" number for thegas (e.g. H2S or CO2)to represent"mg/L" (mg/1000 mL) in"hypothetical liquid sample"and apply that mg/L to the equation below. This makes Sample Volumebecome 1000 mL for this hypothetical liquid in the equation below, for species "X".

"X", mg/L = (Stain Length, tube markings)x (Tube Factor) / (Sample Volume, mL).

Up to this pointonly the gaseous component that the Drager tube measures has been considered - not the ionic counterpart that it corresponds to. Go to Step 3 to modify the Tube Factor so the equation applies to the ionic component - which was the precursor that became the gas.

Step 3:Ionic/Molecular-weightadjusted: Step 3adjusts theTube Factor for the differences in the mass of the measured gas andthe mass of the ion of interest - such as H2S gas being generated from S= ions. This is doneby finding theratio ofthe ionic weight of the ion divided by themolecular weight of the gas. For example, going fromH2S = 34 to S== 32, this ionic/molecular weight ratio is (32/ 34) = 0.94. For CO2 = 44 to CO3= = 60, this ratio is (60/44) = 1.36. Theappropriate ratio number is then multiplied withTube Factor found in Step 2 for the final - or almost final Tube Factor. Go to Step 4, if needed, to obtain the final Tube Factor.

Step 4: Gas-volumetemperature adjusted - If needed: An exact volume of gas must be put through theCO2 Drager tubes for accuracy. To accomplish this in the GGT/CO3= procedure, a 1000 mL gas bag is used to collect the effluent gas from the GGT apparatus before it is passed through the CO2 Drager tube. The1000-mL bag holds fewer moles of gas (and fewer mg) atroom temperature than it would at zero degrees C. (Where, in Step 1 above this assumption was tacitly made when 22,400 mL/mole was applied.) To compensate for the temperature effect, aratio of the two absolute temperatures is calculated andmultiplied with the Tube Factor obtained in Step 3. The best ratio for tests at room temperatureis (273/293) = 0.93.

For the GGT/S=test, the Tube Factor from Step 3 need no temperature correction because neither a gas pump nor a gas bag is used to measure gas volume in this test.

Note: Pressure correction for tests different from one-atmosphere is not likely to be analytically significant - unless GGT/CO3= tests are being done at especially high altitude.

Section A: Low-Range H2S Tube Factor (CH 29101)

Given Information for Low-Range H2STube:

The H2S tube, Model number CH 29101 (Ref. 4.) sold by the Dragerwerk in Germany has been around a long time. It measures H2S in air over the 100 to 2000 ppm range. It is safe to assume thatthis tubeisuniform in manufacture and quality control - as tohow the chemical inside the tuberecords the H2S gas.Although the chemistry has probably not changed, the tube markings have been changed - to make it a direct reading tube for gas testing. In the 1970s era the tube scale was marked "1, 2, 3, …… 20" and theTube Factor that I calculated then was "15". Currently,the same type tube is marked "100, 200, 300, ….2000", so one can assume thatthecurrent Tube Factorshould be 0.15 - less by a factor of 100.

Note: The SPE/JPT paper of Ref. 1, showed that experimentally a better Tube Factor was12 - instead of 15 that was calculated. API RP 13B committee chose to adopt the 12 value over the 15 value when GGT/S= was first published. We might now assume the better value today for current tubes would be 0.12 over 0.15.

It also seems that in my original work, I must have not done the Step 3 adjustment for ionic/molecular weight ratio, which would have made my calculated value of "15" come out closer to "14" if I had done it. (Sorry!) Anyway, the lower number seems right.

Calculation of Low-Range H2S Tube Factor (CH 29101):

Step 1: Calculate themgH2Sgasin a100-mL volumeof air that contains2000 ppm H2S.

1. One mole of H2S gas (100% pure) contains 34,080 mg of H2S and occupies 22,400 ml at standard conditions. A 100 mL volume thus contains 100/22,400= 0.00446 moles. This is also 0.0046 x 34,080 = 152 mg of pure H2S.

2. A Drager-tube test sample of 100 mL which contains2000 ppm H2S will contain 0.304 mg of H2S. Math: (2000/1000,000) x 152 = 0.304 mg of H2S.

This 0.304 mg of H2S will give a Stain Length of 20 on the older tubes and 2000 on the newer CH 29101 tubes - which is the maximum reading.

Step 2: Calculate aTube Factor which is the multiplier needed to produce aStain Length of "20" on the old CH 29101 tube or "2000" on the current tube (of the same model number) with 0.304 mg H2S.

1. To have 0.304 mg of H2S in a "hypotheticalliquid sample" - as in a GGT test - the Sample Volumeapplied in the equation belowis1000 mL - to give 0.304 mg/L.

2. The equation for theGGT calculation(for H2S gas at this step) is:

H2S, mg/L = (Stain length) x (Tube Factor) /(Sample Volume, mL).

Plugging in the numbers from this hypothetical liquid sample:

0.304, mg/L = (20) x (Old Tube Factor) / (1000,mL)

0.304, mg/L = (2000) x (Current Tube Factor) / (1000, mL

Solving then for the value of Tube Factor for H2S:

Tube Factor = (Sample Volume, mL) x(H2S, mg/L)/(Stain Length)

Old Tube Factor= (1000) x (0.304)/ (20) = 15.2,

Current Tube Factor = (1000) x (0.304) / (2000) = 0.152

Step 3:Ionic/Molecular weight adjusted: To obtain mg/L results in terms of S= ion rather than H2Sgas molecule, we apply the ratio of ionic/molecular weights: 32/34 = 0.94. Thus the old 1970s-era Tube Factor =15.2as calculated for H2S becomes 14.3when applied to the liquid (e.g. mud filtrate)soluble S= values- as reported in GGT/S=procedures. The factor also adjusts to0.143 for the current CH 29101 tubes. (See the Note below which addresses the adoption of "12" as the accepted Tube Factor.)

Step 4: Gas volume adjustedfor temperature: No adjustment is needed, as discussed above under "Stepwise Process --".

Note: The API RP 13Bcommittee in the 1970s era, adopted theLow-range GGT/S=Tube Factor of "12" over the calculated value of 15 (or 14) due to better statistical fit with laboratory data as published in 1975 in my SPE/JPT paper of Ref. 1. Along those lines, the "current" Tube Factor probably should be adopted to be "0.12"rather than "0.14" until shown otherwise by testing in a quantitative study.

Section B: High-Range H2S TubeFactor (CH 28101)

GivenInformationfor High-Range H2S Tube:

The H2S tube, Model number CH 28101 (Ref. 4.) sold by the Dragerwerk in Germany has been around a long time. It measures H2S in air over the 0.02 to 7.0 vol. % range. It is safe to assume that this tube is uniform in manufacture and quality control - as to how the chemical inside quantitatively records H2S gas. Although the chemistry has probably not changed, the tube markings have been changed - but not the model numberIn the 1970s era this type tube was marked "1, 2, 3, …17" but currently it is marked "0.2, 0.5, 1.0, 1.5.… 7.0", making it direct reading in vol. % scale for gas testing.

The apparent explanation for the change in scale is this. Boxes of old tubes of the 1970 era, with the 0 to 17 scales, had a Batch Factor number stenciled on each box of 10 tubes. From box-to-box this number was always very close to 0.40. (Sometimes it was 0.41 or 0.42.) That Batch Factormultiplied by theStain Length reading- by the user - gave the vol. % H2S that was in the gas sample. For example, if the Stain Length reading was "17" - the tube's maximum - the vol. % number would be (17 x 0.40) = 6.8 vol. % H2S.

My 1970s lab and field work was done with CH 28101 tubes marked "0.40" as theBatch Factor. Thus the 1970s-eraTube Factor in Ref. 1 is "600", which had that "0.40"Batch Factor incorporated into it. The API procedureusing the tubes included a "Batch Factor Ratio" correction factor, so that future tubes could be corrected as needed for variation in manufacture.

This difference in scale units on tubes of the same model number meansthat the GGTTube Factor must be changed too. Thus, newer CH 28101 tubes will differ from old tubes by the ratio of (17/7) = 2.43. The current Tube Factor thus should be 2.43 x 600 = 1457 - approximately. Below are calculations of both 1970s-era and the current Tube Factors.

Calculation of High-Range H2S Tube Factor:

Step 1: Calculate the mg H2S gas present in a 100-mL volume of air that contains 7.0 vol. % H2S- which is also 70,000 ppm H2S- and gives a stain Length reading of either "17" for older tubes or "7" for newer tubes.

1. One mole of H2S gas (100 % pure) contains 34,080 mg of H2S and occupies 22,400 ml at standard conditions. Thus, 100 mL of pure H2S is 0.00446 moles andis also 152 mg H2S. (Math: 100/22,400 = 0.00446 and 0.00446 x 34,080 = 152.)

2. A Dragertube gas sample volume,100 mL, which contains 70,000 ppm H2S contains (70,000/1000,000) x 152 mg = 10.64 mg H2S. This 10.64 mg willgenerate a Stain Length in the 1970s-era tubeat the"17" mark - or at the"7.0"mark on the current tubes. This is the maximum stain length for this model number tube.

Step 2: Calculate aTube Factorfor a Stain Length of "17" on the old CH 280101 tube scale or "7.0" on the current tube that is generated by 10.64mg of H2S in a "hypothetical liquid" sample that contains 10.64 mg/L of H2S.

1. To have 10.64 mg/L of H2S in the liquid sample, the Sample Volume for the equation below would be 1000 mL.

2. The equationfor the GGT calculation and as H2S gas (not yet S= ion) is:

H2S, mg/L = (Stain length) x (Tube Factor) /(Sample Volume, mL).

Plugging in the numbers from this hypothetical liquid sample gives:

10.64, mg/L = (17) x (Old Tube Factor) / (1000, mL)

or

10.64, mg/L = (7.0) x (Current Tube Factor) / (1000, mL)

Solve for Tube Factor - For both the "old" 1970s-era" and the "current" Drager tubes:

Old Tube Factor = (1000) x (10.64) / (17.0) = 625

or

Current Tube Factor = (1000) x (10.64) /(7.0) = 1520

Step 3:Ionic/Molecular weight adjusted: To obtain mg/L results in terms of S= ionrather than H2S molecule, we apply the ratio of ionic/molecular weights: 32/34 = 0.94. Thus the 1970s-era Tube Factor of625 for H2S becomes 588 when used for S= values and the Tube Factor of 1520 becomes 1430 for S= analysis.(See Note below.)

Step 4: Gas volume adjustedfor temperature: No adjustment is needed, as discussed above under "Stepwise Process --".