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Core Concepts Every User Must Know

These ideas apply to almost every instrument you'll touch. Get them straight now and every technology page afterward will make sense.

Bump test vs. full calibration

These two words get mixed up constantly, and the difference matters.

Bump test (a.k.a. functional test)

A bump test is a quick "are you awake?" check. You briefly expose the sensor to a known target gas — above its alarm setpoint — and confirm two things: the reading moves toward the expected value, and the alarm actually goes off (lights, buzzer, vibration). You are not adjusting anything; you're just verifying the instrument responds. It takes under a minute per gas.

Full calibration

A calibration is a two-point adjustment. You zero the instrument in clean air, then expose it to a certified span gas of known concentration and let the instrument reset its internal scale so the reading matches the certified value. Calibration corrects for the slow drift that every sensor experiences as it ages. It takes longer, uses more gas, and is done on a schedule.

Typical calibration schedule

• Bump test: before each day's use.
• Full calibration: per the manufacturer's manual — commonly monthly, though many programs extend the interval (out to a few months, sometimes six) if a documented daily-bump program shows the instrument stays in tolerance.
• Calibrate immediately (off-schedule) after: a failed bump test, a drop or physical shock, exposure to poisons or very high gas concentrations, sensor replacement, unexplained/erratic readings, or a big temperature swing.

Every technology page lists a calibration & bump schedule specific to that sensor. The numbers above are typical; your manual and SOPs govern.

Zeroing in fresh air

To zero an instrument is to tell it "this is clean air — call this baseline zero." Most multi-gas monitors auto-zero at power-up, which is exactly why you must turn the instrument on in genuinely clean air — not in the rig with the diesel running, not next to the patient, not downwind of the spill.

Note the oxygen channel isn't zeroed the same way; it's calibrated to the 20.9% of fresh air. Some instruments call the fresh-air startup a "fresh air setup," which sets both the toxic/LEL zeros and the O₂ span at once.

Span gas

Span gas is a cylinder of certified gas at a known concentration used as the "high point" during calibration and bump testing. The concentration is printed on the certificate and the cylinder. A few practical points:

• Span gas has an expiration date — reactive gases (Cl₂, NH₃, HCN, H₂S at low ppm) degrade in the cylinder over time. Expired span gas gives you a bad calibration. Check the date.
• Use the correct gas for the sensor and the correct concentration. A common multi-gas mix contains CH₄, O₂, CO, and H₂S; exotic toxics often need their own dedicated cylinder.
• Use the correct regulator and flow rate (a fixed-flow regulator for diffusion instruments, or a demand-flow regulator for pumped ones). Too much or too little flow skews the result.
• Reactive gases like chlorine and HCl "stick" to regulators and tubing, so they need special (often coated) regulators and short tubing runs, or the sensor never sees the true concentration.

Correction factors & response factors

Some sensors — notably PIDs and catalytic LEL beads — respond to many different chemicals, but they can only be calibrated to one. That calibration gas is the reference. A correction factor (CF), also called a response factor, is the multiplier that converts the reading (in "calibration-gas equivalents") into the true concentration of the chemical you're actually facing.

Analogy: imagine a scale calibrated in apples. If you weigh oranges on it, the number isn't wrong — it's just "in apples." The correction factor is the conversion from apples to oranges.

Relative response is the flip side: for a given real concentration, how big a reading does this sensor produce compared to its cal gas? A PID with a low relative response to a chemical (high CF) will under-report it dramatically — some chemicals read only a fraction of their true level, so a small PID number can hide a large hazard.

Sensor lag & response time (T90)

No sensor responds instantly. T90 is the time it takes a sensor to reach 90% of its final reading after being exposed to a step change in gas. Typical T90 values run from a handful of seconds (O₂, CO) to 30+ seconds or more (some toxic and IR sensors). There's also a clear-down (recovery) time — how long to return to zero after the gas is removed, which can be much longer than the rise time for "sticky" gases.

Sampling: pumps vs. diffusion

How does the gas get to the sensor? Two ways:

• Diffusion instruments let ambient air reach the sensor through vents on its own. Simple, reliable, nothing to clog — but they only sample the air immediately around the instrument. Good for personal monitoring clipped in your breathing zone.
• Pumped instruments use an internal (or attached) pump to draw a sample through tubing from somewhere else — down a manhole, into a drum bung, around a corner — before you commit your body. Essential for remote / pre-entry sampling of confined spaces.

Sample tubing & "sticky" chemicals

The tubing isn't neutral. Some chemicals adsorb onto (stick to) the walls of the sample line, so the sensor reads low or delayed until the tubing "saturates," and then reads high on clear-down. Water-soluble and polar gases — hydrogen fluoride (HF), ammonia, chlorine, HCl — are notorious for this.

• Use the tubing material the manufacturer specifies. PTFE/Teflon is far less adsorptive than PVC/Tygon for sticky corrosives.
• Keep sample lines as short as practical.
• For known sticky targets, expect delay and under-reading, and don't trust a fast "clean" result.

Datalogging

Most modern monitors continuously record readings to internal memory (a datalog) with timestamps. This matters after the incident: it documents responder exposure, reconstructs what the atmosphere did over time, supports rehab/medical decisions, and provides a record for the AHJ. Know how your instrument's datalog is downloaded and make sure clocks are set correctly — a datalog with the wrong time is nearly useless for exposure reconstruction.

Alarm setpoints

Setpoints are the concentrations at which the instrument alarms. Most gases have two (a low and a high), and toxics often add exposure-based alarms:

• Low / High alarms: instantaneous thresholds (e.g., LEL low at 10%, high at 20%).
• TWA alarm: trips when the time-weighted average exposure over the shift reaches a limit.
• STEL alarm: trips on a short-term (typically 15-minute) average limit.

Setpoints are usually shipped at sensible defaults, but they can be changed — which is a double-edged sword. Confirm your fleet's setpoints match your SOPs, and never quietly lower an alarm just because it's "annoying."

Exposure limit definitions

These terms turn a raw ppm number into a decision. They come from different organizations and mean different things — mixing them up leads to bad calls.

Full one-line definitions of every acronym live in the Glossary. Next, start on the sensors themselves: Electrochemical Sensors →