An Analytical Method for Gas Flow Measurement Using Conservative Chemical Elements
Round 1
Reviewer 1 Report
Overall remarks
The work deals with the problem of measuring the volume flow of an unknown gas or gas mixture. This is hardly possible without complex measurements (flow, gas analysis, temperature, pressure), especially when determining exhaust gas flows in chemical reactions, whose composition can change with the reaction conditions.
The paper describes an original approach to take into account the influence of changes in the composition of a gas flow (e.g. due to chemical reactions) on the volume flow to be determined. For this purpose, a constant inert gas flow is fed into the reactor. This is diluted by the resulting process gases. The volume flow of the gas is determined via the detected gas composition. It should be noted that the inert gas used can influence the reaction and thus this method has corresponding limitations.
The manuscript is fairly well written except for section 3.2, which should be revised. There are some questions or inconsistencies (see below) that should be elaborated or corrected in the text.
The manuscript should be considered for publication after major revision.
In the following, some detailed remarks and hints for the authors are given.
Section 3.1
Figure 2 shows the influence of the gas mixture on the measured value of the MFM and the necessity of knowing the gas composition for the measurement. From lines 115 -118 it can be concluded that the gas correction factors GCFs were taken into account both at the inlet (MFC) and at the outlet (MFM). In this respect, it can be expected that the volume flows at the MFC and at the MFM should be almost the same despite the change in the mixing ratio. The volume flows recorded with the MFM (black curve) increase from 100% N2 to 100% CH4. For the two limit values (100% CH4 and 100% N2), the ratio of the volume flows just corresponds to the ratio of the GCFs of both gases. Please check whether the GCF was taken into account at the MFM or only N2 settings were used and note this in the text.
Section 3.2
Here the two measurement methods were investigated for a single gas and a gas mixture at a constant flow rate of 300mL/min. This is also shown in Figures 3a and 3b. In lines 135 - 137, however, a gas flow of 1l/min is referred to (is this an error or what does this information refer to?).
The measurements with the MFM (Figure 3 - brown curves) show fluctuations and jumps during the temperature ramp. There are no explanations and comments on this - please add.
Figure 3a: The reactor temperature (lines 137 -140) influences the MFM measurement although a chiller is used in front of the MFM? Why does the temperature influence decrease at constant temperature (850°C)? Method 2 uses a constant inlet current as a reference for the measurement. In this respect, the output current should also be constant – according to equations (1) and (2)? Why does the temperature have an influence on the flow measurement with method 2? I cannot follow your argumentation. Revise this section!
Section 3.3
In lines 200 -205 it should be explained what the size of the circles in Figure 6 means, in this respect also choose the legend heading appropriately. The unit of measurement in the legend with ml/min does not seem to agree, it is below the MFM measurement error of +/- 3 % (see line 28) for total flow rates of 1 - 4 l/min.
For the captions or designations in the figures (Figure 3, 5 and 6) it is recommended to use uniform designations for the two methods (e.g. method 1 - MFM monitoring, method 2 - conservative element method).
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Some suggestions to the authors:
In section 2.2, authors write "All the elements of Family 18, such as He, N2, and Ar". However, nitrogen does not belong to the noble gases family although it is poorly reactive. Either N2 or the reference to family 18 has to be removed.
Similarly, in line 165 in page 5 they write "using alkali metals (KOH)". It is true that K is an alkali metal, but KOH is the hydroxyde of an alkaline metal and not an alkali metal itself. This has to be changed too. In this same section 3.3 the arrows used for the equations are not the best option. Use a single arrow or the equilibrium double arrow instead.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Figure 6: According to your corrections (unit of measurement of the gas flow; mL/min ==> L/min), it can be seen from the figure that the size of the circles corresponds to the total flow according to method II and thus do not contain any additional information. In this respect, the circles can be replaced by dots and the legend can be omitted.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
