What Does a 4-Gas Monitor Detect? Gases, Limits & Selection

The four readings use different units. Oxygen is reported as percent by volume, while CO and H₂S are reported in parts per million (ppm). The combustible-gas scale is a percentage of the lower explosive limit (LEL) of the calibration gas. ^[S1] These readings answer different questions, so one cannot stand in for another.

For CO and H₂S, a current reading and an exposure average describe different things. A peak shows a concentration reached during monitoring; a time-weighted average includes how long the concentration persisted. ^[S2] That distinction matters when an instrument records a brief high reading but a much lower average. The H₂S peak and TWA record guide works through that comparison.

Before comparing an exposure limit with a reading, identify the time period each represents. The US references below distinguish a time-weighted average (TWA), a short-term exposure limit (STEL) and a ceiling. ^[S2] Those references are not proposed factory alarm settings.

• TWA: a time-weighted average over a stated period. ^[S2]
• STEL: a short-term average, usually over 15 minutes. ^[S2]
• Ceiling: a value that should not be exceeded at any time. ^[S2]

For BTYQ-MS104K-M and BTYQ-MS400-S, our specifications list low, high, range and TWA/STEL alarm modes. ^[S3] The supplied modes and set points belong in the quotation, with the gas combination and measuring ranges. Alarm settings must follow the applicable jurisdiction and site program; the mode list alone does not establish those settings.

Choose a CO range that covers the concentrations the job requires, then assess accuracy near the readings used for alarms or exposure assessment. A wider range can cover higher concentrations, but it does not necessarily give the same absolute error at low concentrations. Our model documents list the following gas and range options for BTYQ-MS104K-M and BTYQ-MS400-S. ^[S4]

The difference is easy to see when accuracy is expressed as a percentage of full scale (FS), the upper end of the measuring range. Our specification table gives a typical accuracy of ±3%FS, with higher-accuracy sensors available as an option. ^[S3] At that rating, 0–100 ppm gives an absolute tolerance of ±3 ppm, while 0–1000 ppm gives ±30 ppm. ^{[S3; calculation]} Display resolution is the smallest displayed step; extra decimal places do not reduce that tolerance.

NIOSH recommends 35 ppm as a CO TWA and 200 ppm as a ceiling. The OSHA permissible exposure limit is 50 ppm as a TWA. ^[S5] The narrower range reduces the calculated error at the same %FS rating, but cannot cover a 200 ppm event. The choice therefore depends on both required coverage and the applicable error specification.

Our CO type-evaluation reports give a different error statement: a maximum permissible error (MPE) of ±5 μmol/mol or ±10% for the 0–1000 μmol/mol configuration. ^{[S6] [S7]} Those reports record tests at 150, 300 and 700 μmol/mol, rather than near 35 ppm. They provide evidence for the wider range under the reported laboratory conditions, but do not establish its error at every lower concentration.

Across the reported CO test points, the results ranged from −1.7% to +5.3% of the reference gas. ^{[S6] [S7]} The tests covered 20–24 °C, whereas our typical operating range is −20 to +55 °C and 10–95 %RH. ^{[S3] [S6] [S7]} A purchase specification that depends on a low CO reading at another temperature needs performance evidence for that condition.

The calculator below answers a narrower question: which listed ranges satisfy a chosen concentration and error requirement if the stated %FS figure applies? With a rating of ±q%FS, the range must satisfy Cmax ≤ FS ≤ 100 × E ÷ q. Here Cmax is the highest required concentration and E is the permitted absolute error in ppm. ^{[S3; calculation]}

For example, covering 200 ppm while limiting absolute error to ±5 ppm gives no solution at ±3%FS. ^{[S3; calculation]} That result rules out the stated %FS specification for the task; it does not rule out a different documented accuracy specification. Any alternative still needs evidence for the concentration and conditions required by the job.

A low %LEL reading cannot establish that solvent exposure is acceptable, because flammability and toxic exposure occur at different concentrations. For n-hexane, NIOSH lists an LEL of 1.1% by volume and a recommended exposure limit (REL) of 50 ppm as a TWA. ^[S8] Using that LEL, even 1%LEL corresponds to 110 ppm. ^{[S8; calculation]} A small reading on the flammability scale can therefore exceed the exposure reference.

At 10%LEL, the calculated n-hexane concentration reaches 1,100 ppm, the NIOSH immediately dangerous to life or health (IDLH) value. ^{[S8; calculation]} OSHA 29 CFR 1910.146 also defines a flammable atmosphere above 10% of the lower flammable limit as hazardous in covered permit spaces. ^[S9] Staying below that flammability boundary does not demonstrate control of toxic exposure.

There is a second distinction: converting a gas property is not the same as predicting an instrument reading. Our %LEL type evaluations used methane; the BTYQ-MS400-S test configuration used a catalytic sensor with an MPE of ±5%FS. ^{[S10] [S11]} A sensor calibrated with methane needs its own response data for other combustible gases. ^[S1] The selector below changes the LEL reference in the calculation, without modelling that sensor response.

The conversion uses the published LEL references for methane, propane and n-hexane. ^{[S12] [S13] [S8]} For an n-hexane application, the flammability requirement and the exposure requirement should therefore remain separate in the enquiry.

Our documents list VOC ranges from 0–20 ppm to 0–10,000 ppm, and both models have a built-in gas library. ^{[S3] [S4]} Those features alone do not establish n-hexane performance: selection still needs the offered sensor’s response data and the required ppm range. On BTYQ-MS104K-M, a PID measurement and %LEL also compete for the same documented sensor position. ^[S3] That combination limit affects the model choice when both measurements are needed.

An O₂ sensor measures oxygen, so its reading cannot tell you how much CO₂ is present. If oxygen-free CO₂ mixes uniformly with air initially containing 20.9% vol O₂, the oxygen fraction falls as the air is displaced. The calculation is O₂ = 20.9 × (1 − CO₂ fraction). ^{[Calculation]} At 3% vol CO₂, the resulting O₂ reading is still 20.27% vol.

At the NIOSH CO₂ IDLH of 40,000 ppm, the same calculation gives 20.06% vol O₂. ^{[S14; calculation]} That is above the 19.5% vol oxygen-deficiency boundary in OSHA 29 CFR 1910.146 for covered permit spaces. ^[S9] This example explains the blind spot; uniform mixing and oxygen-free CO₂ are calculation assumptions, not a prediction of a particular release.

Where CO₂ is a hazard, a separate CO₂ measurement is needed. Range selection then follows the same coverage-and-error question as CO: which concentrations must the instrument read, and how accurate must those readings be? Our documents list the following CO₂ options. ^{[S3] [S4]}

The 0–5,000 ppm option reaches the NIOSH TWA reference only at full scale, while the short-term and IDLH references lie above it. ^{[S4] [S14]} A range that reaches 40,000 ppm covers those higher reference concentrations, but range coverage alone does not establish accuracy. The next section explains why adding CO₂ to O₂/CO/H₂S/%LEL also depends on the model’s sensor arrangement.

For monitoring where gas can reach the instrument, both models support diffusion sampling. For a remote sample, BTYQ-MS104K-M uses an attached external pump, while BTYQ-MS400-S has a built-in pump that can be switched off for diffusion. ^[S3] Choose between those arrangements after deciding where the sample must come from and which gases must be measured together.

BTYQ-MS104K-M: diffusion with an external pump

The standard accessories include an external pump and a filter. ^[S3] The MB20A pump identified in the type-evaluation report is rated at 500 mL/min. ^[S11] The photographs show the instrument by itself and with the pump attached.

BTYQ-MS400-S: built-in pump with diffusion mode

The built-in pump is rated at 0.5–1 L/min with at least 10 flow levels. Standard accessories include a 0.2 m gooseneck sampling probe and a filter. ^[S3] Both models also offer a 0.6 m stainless-steel handle or a 0.9 m telescopic handle, each with 1 m of hose. ^[S3]

Gas combinations can decide the model before sampling does

Our specification table describes both models as configurable for one to four gases, with expansion to five or six. ^[S3] It also gives different sensor-combination rules. A supported gas range is therefore not enough to establish that all required gases can be measured together.

For O₂/CO/H₂S/%LEL plus CO₂, BTYQ-MS104K-M is outside the documented combinations because CO₂ and %LEL use the same sensor position. ^[S3] BTYQ-MS400-S is the model to evaluate for that five-gas requirement; the exact combination and CO₂ range still need confirmation in the quotation. The same reasoning applies when solvent exposure requires a PID measurement alongside %LEL.

Compare runtime for the configuration you will use

The more-than-100-hour figure for BTYQ-MS104K-M applies to diffusion operation without catalytic, infrared or PID sensors. ^[S3] It therefore cannot establish runtime for an O₂/CO/H₂S/%LEL configuration. BTYQ-MS400-S is documented at more than 20 hours in pumped operation. ^[S3] Because the two figures describe different loads, the quotation needs runtime for the proposed gas configuration and sampling mode.

Allow for the complete sampling path

A pump’s flow rating does not establish how quickly the complete kit will detect a target gas. The type-evaluation reports contain measured response times for the tested configurations and modes, as shown below. ^{[S6] [S7] [S10]} T90 means the time to reach 90% of the final reading.

These results were measured at 20–24 °C and do not include an added remote-sampling hose. ^{[S6] [S7] [S10]} For remote sampling, the waiting time must account for the target gas, handle, hose and filters together. A complete-path response result addresses that question; the laboratory response figure alone does not.

The personal gas monitoring guide compares worn and remote monitoring, kit weight and runtime, and pre-use checks. For tubing, filters and sample conditioning, use the wastewater sample-handling guide. Those checks follow from the sampling path selected here.

The approval question concerns the model and version being supplied, not the brand as a whole. The Chinese GB/T records below identify BTYQ-MS104K-M and BTYQ-MS400-S separately. Both carry Ex ia IIC T4 Ga, with an X suffix indicating special conditions. ^{[S15] [S16]} Those conditions need to be included in the project’s certificate review.

Neither of these records is an ATEX, IECEx or US approval. ^{[S15] [S16]} We hold other explosion-protection records with different markings, each limited to its own scope. The RFQ should name the required approval scheme so the offered model and configuration can be matched to the relevant record. A company management-system certificate cannot replace that product-level evidence.

A useful enquiry identifies which gases must be measured together, each range and the permitted error. It also describes the sampling path. Add the required operating period and approval scheme so we can propose a configuration and identify the supporting records. Unknown requirements can remain open.

Copy the worksheet into the configuration RFQ, or use the gas detector quotation checklist for the remaining order details. The BTYQ-MS104K-M product page and BTYQ-MS400-S product page provide the model specifications to review alongside the proposed combination.

Bracketed numbers in the text link to these sources. The four type-evaluation reports are available below as Chinese-language PDFs. Other Eranntex records are available on request; public reference sources link to the issuing body.