Ambient Condition Monitoring

Why We Monitor Ambient Conditions

The uncertainty budgets for all calibration activities bound the ambient conditions to specific ranges. The monitoring system exists to verify that those bounds are met during every calibration — it validates assumptions, it does not contribute to the uncertainty itself. If the ambient conditions drift outside the assumed ranges, the uncertainty statements on calibration certificates are no longer valid.

Three environmental quantities matter for mass flow calibration:

Ambient temperature

Temperature fluctuations affect flow stability through the interconnecting tubing upstream and downstream of the molbloc element. The molbloc design conditions the gas temperature internally, but ambient instability still causes flow instability — particularly at lower flow rates where the tubing volume is large relative to the flow. The environmental limits bound the temperature to 15–25 °C with a rate of change below 0.2 °C per minute.

Barometric pressure

Standard volumetric flow is reported at reference conditions (0 °C, 1013.25 hPa). The conversion from mass flow to volumetric flow uses the ambient barometric pressure. The monitoring system tracks barometric pressure continuously so that any anomalous excursions are visible in the historical record.

Relative humidity

For molbloc-S sonic nozzle calibrations using air, humidity affects the gas density calculation. The monitoring system records humidity alongside temperature and pressure to provide a complete picture of the laboratory environment at the time of each calibration.

This monitoring system implements the requirements of clause 6.3, which requires the laboratory to monitor, control, and record environmental conditions as required by the relevant specifications, methods, or procedures.

The Monitoring Instrument

The laboratory uses an Alicat Scientific AN-M-100SCCM-O-DB9M-IPJ-IB-RH (SN 513646) as the ambient condition monitor. This is a laminar mass flow meter that includes an on-board temperature sensor, barometric pressure sensor, and optional relative humidity sensor — the laboratory uses only these three environmental channels, not the flow measurement function.

The device is wall-mounted at the main calibration station. The main cover has been intentionally removed to expose the circuit board directly to ambient air. Without this modification, the internal electronics generate enough heat to cause the temperature reading to creep upward over time — a systematic bias that would undermine the monitoring function. The DB9 connector and locking power jack prevent accidental disconnection.

Sensor specifications:

Temperature

±0.75 °C accuracy, operating range −10 to 60 °C

Barometric pressure

±0.00725 PSI accuracy, range approximately 900–1100 hPa (pending manufacturer confirmation)

Relative humidity

±1.8% RH accuracy (0–50 °C, below 90% RH); degrades to approximately ±3% RH at humidity levels above 90% RH

Data Pipeline

The monitoring instrument communicates via RS-232 serial to a Moxa RS-232-to-Ethernet converter, which bridges the serial data to the laboratory network. A custom Linux service running on the Grafana virtual machine polls the device every five minutes and writes the readings to an InfluxDB time-series database. Grafana queries InfluxDB and renders the data on a public dashboard.

The dashboard is displayed in the laboratory on a large wall-mounted screen driven by a Raspberry Pi 5 running FullPageOS. The technician sees the current state of the laboratory environment at all times without needing to check a computer.

The dashboard displays:

• Time-series plots of barometric pressure, temperature, and relative humidity
• Current barometric pressure
• Current temperature in °C and °F (the Fahrenheit reading is useful for air sampling pump calibrations where customers specify conditions in imperial units)
• Current relative humidity
• Calibration Go/No-Go traffic light

InfluxDB retains environmental monitoring records indefinitely. The data is accessible through the Grafana dashboard for retrospective review — for example, to verify what the ambient conditions were during a specific calibration if a question arises after the fact.

Calibration Go/No-Go Criteria

The dashboard displays a traffic light that summarizes whether the ambient conditions are suitable for calibration. The traffic light evaluates the ambient temperature against the bounds established by the uncertainty analysis:

Green

Temperature is within 19–23 °C, the rate of change is below 0.2 °C per minute, and these conditions have been stable for at least two hours. Calibration may proceed.

Orange

Temperature and rate-of-change conditions are met, but have been stable for less than two hours. The laboratory is approaching suitable conditions but has not yet demonstrated sustained stability. Wait for the light to turn green.

Red

Temperature is outside 19–23 °C or the rate of change exceeds 0.2 °C per minute. Conditions are not suitable for calibration.

The rule is straightforward: calibration may only begin when the traffic light is green. This applies to all calibration activities — not just molbloc-L, but any measurement where the uncertainty budget assumes controlled ambient conditions. The molbloc-L calibration procedure references this traffic light as a prerequisite in its pre-measurement checklist.

Response to Out-of-Range Conditions

If the traffic light is red, do not begin calibration. Wait for the laboratory conditions to return to the acceptable range and stabilize — the traffic light will transition from red to orange to green as conditions improve and the two-hour stability window accumulates.

If the conditions drift out of range during a calibration that is already in progress, the technician assesses whether to pause and wait or to complete the current measurement point and then pause. In either case, document the condition excursion in the calibration record. No calibration certificates shall be issued for work performed under out-of-range ambient conditions unless the impact on measurement uncertainty has been explicitly evaluated and documented.

Persistent out-of-range conditions warrant investigation. Common causes include HVAC malfunction, the laboratory door being left open, seasonal temperature extremes overwhelming the climate control, or heat-generating equipment operating near the calibration station. Resolve the root cause before resuming calibration work.