Troubleshooting: How to correct flow data after choosing the wrong gas

Correcting your flow data after choosing the wrong gas in Gas Select

Like most flow meter technologies, Alicat mass flow meters need to know which gases are flowing through them for their flow readings to be accurate. Our differential pressure-based technology uses the known absolute viscosity of the gas or gas mixture (stored in Gas Select), combined with the measured absolute pressure and temperature to produce flow readings.

If you accidentally selected a preloaded gas calibration that was different from the gas you actually flowed through the instrument, your flow data will be corrupted by approximately the ratio of the viscosities of the two gases. This article shows you how to correct your data by multiplying each reading by the ratio of the selected gas’s viscosity to the desired gas’s viscosity.

How to correct your flow data if you accidentally selected a different gas in Gas Select

Let’s start by defining a few variables:

Qindicated = Flow rate recorded by your device
ηselected = Absolute viscosity of the gas you incorrectly selected
ηactual = Absolute viscosity of the gas that actually passed through your device
Qactual = Actual (corrected) flow rate of the gas that passed through your device

To get to the corrected flow rate of the gas flowing through the unit (Qactual), multiply the indicated flow rate by the ratio of the gas viscosities:

Qactual = Qindicated x (ηselectedactual)

Your Alicat product manual lists the absolute viscosity (η) of each gas that is part of Gas Select, at reference temperatures of both 0 °C and 25 °C. If your process operates near room temperature, the viscosity at 25 °C is a good choice, but note that viscosity does change with temperature.

Example

You flowed a natural gas mixture of 95% CH4/3% C2H6/1% N2/1% CO2 (Gas Select #190, Nat Gas-2), but you accidentally had Gas Select set to air. Your gas was close to room temperature, and you recorded a flow rate of 1.00 slpm.

Qindicated = 1.00 slpm
ηselected = 184.8989 (absolute viscosity of air)
ηactual = 111.5557 (absolute viscosity of Nat Gas-2)

Using the equation above, we come up with the following corrected flow value:

Qactual = 1.00 x (184.8989/111.5557) = 1.6575 slpm

Learn more

Pure Non-Corrosive Gases Gas data (at 25°C)
Gas number (if available) Short Name Long Name RP10 Mol. Wt. RP10 AbsoluteViscosity (uP) at 25°C RP10 Density at 25°C Compressibility at 25°C Adiabatic index (Cp/Cv)
0 Air Air 28.958601 184.899244 1.184011 0.999698 1.401783
1 Ar Argon 39.948 226.239949 1.633864 0.999366 1.669574
2 CH4 Methane 16.0428 110.759546 0.656882 0.998247 1.306166
3 CO Carbon Monoxide 28.0101 177.679915 1.145298 0.999641 1.401279
4 CO2 Carbon Dioxide 44.0098 149.146211 1.80797 0.994955 1.294092
5 C2H6 Ethane 30.06904 92.339546 1.238456 0.992399 1.1939
6 H2 Hydrogen 2.01588 89.002983 0.082348 1.000594 1.405358
7 He Helium 4.002602 198.456139 0.163523 1.000484 1.666546
8 N2 Nitrogen 28.01348 178.047406 1.145245 0.999802 1.401283
9 N2O Nitrous Oxide 44.0128 148.444824 1.808871 0.994533 1.280409
10 Ne Neon 20.179 317.130171 0.824409 1.000471 1.666923
11 O2 Oxygen 31.9988 205.502073 1.308787 0.999353 1.396654
12 C3H8 Propane 44.09562 81.226187 1.832036 0.983805 1.136398
13 nC4H10 n-Butane 58.1222 73.761401 2.449293 0.969949 1.105253
14 C2H2 Acetylene 26.03728 103.202418 1.070692 0.993982 1.228674
15 C2H4 Ethylene (Ethene) 28.05376 103.183898 1.15329 0.994255 1.246119
16 iC4H10 i-Butane 58.1222 74.978459 2.440277 0.973533 1.105199
17 Kr Krypton 83.798 253.405732 3.432281 0.997927 1.67221
18 Xe Xenon 131.293 230.536443 5.395008 0.994712 1.677336
19 SF6 Sulfur Hexafluoride 146.055419 152.202016 6.038304 0.988668 1.098367
210 D2 Deuterium 4.0282 125.869207 0.164553 1.00058 1.398484
Pure Corrosive Gases Gas data (at 25°C)
Gas number (if available) Short Name Long Name RP10 Mol. Wt. RP10 AbsoluteViscosity (uP) at 25°C RP10 Density at 25°C Compressibility at 25°C Adiabatic index (Cp/Cv)
30 NO NO (Nitric Oxide) 30.006 190.0595 1.226716 0.999797 1.386
31 NF3 NF3 (Nitrogen Trifluoride) 71.019 175.425 2.913364 0.996386 1.1876
32 NH3 Ammonia 17.03052 100.8828 0.703516 0.989468 1.316454
33 Cl2 Chlorine 70.906 135.2963 2.935399 0.987333 1.339572
34 H2S Hydrogen Sulfide (H2S) 34.08088 123.8689 1.403755 0.992356 1.330967
35 SO2 Sulfur Dioxide 64.0638 128.5827 2.663574 0.983096 1.281358
36 C3H6 Propylene (Propylene) 42.07974 85.6071 1.745088 0.985606 1.156696
80 1Buten Butylene (1-Butene) 56.10632 77.134 2.359051 0.972125 1.122513
81 cButen Cis-Butene (cis-2-butene) 56.10632 76.28867 2.366072 0.969241 1.13195
82 iButen Iso-Butene 56.10632 78.48763 2.35896 0.972163 1.117015
83 tButen Trans-Butene 56.10632 76.32579 2.365951 0.96929 1.120934
84 COS Carbonyl Sulfide 60.0751 123.6276 2.483215 0.988844 1.260677
85 DME Dimethylether 46.06844 91.96514 1.918212 0.981645 1.156324
86 SiH4 Silane (SiH4) 32.11726 115.944 1.320023 0.9945 1.4
Welding Gases Gas data (at 25°C)
Gas number (if available) Short Name Long Name RP10 Mol. Wt. RP10 AbsoluteViscosity (uP) at 25°C RP10 Density at 25°C Compressibility at 25°C Adiabatic index (Cp/Cv)
20 C-25 25% CO2 / 75% Ar 40.96345 205.1552 1.676578 0.99866 1.505324
21 C-10 10% CO2 / 90% Ar 40.35418 217.6431 1.650904 0.999107 1.592406
22 C-8 8% CO2 / 92% Ar 40.27294 219.3447 1.647492 0.999161 1.60637
23 C-2 2% CO2 / 98% Ar 40.02924 224.5026 1.637267 0.999316 1.652559
24 C-75 75% CO2 / 25% Ar 42.99435 166.7178 1.763527 0.996494 1.340763
25 He-25 25% He / 75% Ar 30.96165 231.5629 1.265458 1.000051 1.66819
26 He-75 75% He / 25% Ar 12.98895 234.606 0.530619 1.000549 1.666839
27 A1025 90% He / 7.5% Ar / 2.5% CO2 7.698687 214.9065 0.31451 1.000531 1.645543
28 Star29 Stargon CS  90% Ar / 8% CO2 / 2% O2 40.11396 219.0373 1.640987 0.999161 1.598911
140 C-15 15% CO2 / 85% Ar 40.55727 213.4274 1.659446 0.998966 1.560182
141 C-20 20% CO2 / 80% Ar 40.76036 209.2651 1.668004 0.998818 1.53132
142 C-50 50% CO2 / 50% Ar 41.9789 185.3551 1.71975 0.997725 1.40648
143 He-50 50% He / 50% Ar 21.9753 236.0761 0.897852 1.000407 1.667353
144 He-90 90% He / 10% Ar 7.597142 222.0966 0.310358 1.000541 1.666637
Refrigerant Gases Gas data (at 25°C)
Gas number (if available) Short Name Long Name RP10 Mol. Wt. RP10 AbsoluteViscosity (uP) at 25°C RP10 Density at 25°C Compressibility at 25°C Adiabatic index (Cp/Cv)
100 R-11 Trichlorofluoromethane 137.368 101.6048 5.823595 0.964145 1.13912
101 R-115 Chloropentafluoroethane 154.4664 125.1478 6.432918 0.981463 1.088332
102 R-116 Hexafluoroethane 138.0118 137.8173 5.700958 0.989501 1.088952
103 R-124 Chlorotetrafluoroethane 136.475 115.9311 5.728202 0.973829 1.103392
104 R-125 Pentafluoroethane 120.0214 129.6174 4.981683 0.98476 1.103591
105 R-134A Tetrafluoroethane 102.032 118.1882 4.257828 0.979481 1.119533
106 R-14 Tetrafluoromethane 88.01 172.4468 3.610848 0.996255 1.160565
107 R-142b Chlorodifluoroethane 100.495 104.2019 4.216309 0.974226 1.124503
108 R-143a Trifluoroethane 84.041 112.1337 3.4945 0.983001 1.12861
109 R-152a Difluoroethane 66.051 100.8132 2.759024 0.978524 1.154922
110 R-22 Difluoromonochloromethane 86.468 126.3039 3.586794 0.985364 1.184702
111 R-23 Trifluoromethane 70.01385 149.1316 2.884034 0.992273 1.201092
112 R-32 Difluoromethane 52.024 126.1314 2.15314 0.987596 1.252257
113 RC-318 Octafluorocyclobutane 200.04 117.123 8.429192 0.970016 1.064227
114 R-404A 44% R-125 / 4% R-134A / 52% R-143A 97.6038 120.8313 4.057387 0.983259 1.118293
115 R-407C 23% R-32 / 25% R-125 / 52% R-134A 95.02751 122.8043 3.9513 0.983008 1.129835
116 R-410A 50% R-32 / 50% R-125 86.0227 133.5442 3.564428 0.986441 1.146601
117 R-507A 50% R-125 / 50% R-143A 98.85916 121.7023 4.108721 0.983463 1.117223