Using an infrared thermometer: a field guide with the Fluke 62 MAX
First installment of a series on measurement instruments for compressed air system diagnostics. The infrared thermometer — here the Fluke 62 MAX, a rugged, affordable gun used as the working example — gives a surface temperature instantly, at a distance, without contact. Used well, it is the fastest survey tool there is; used poorly (emissivity ignored, target too small, glass in the way), it reports wrong values with the same confidence. The principles below apply to any infrared thermometer.
What is it good for in a compressed air system?
- Aftercooler: compare inlet and outlet temperatures to judge the heat exchange.
- Dryer: check the air inlet temperature against the nameplate value — overly hot air is the first cause of a degraded dew point (see dew point).
- Compressor: track the heating of a motor, a bearing, a fouled cooler; compare from one visit to the next.
- Hot-spot scanning: with the trigger held (SCAN mode), sweep the surface and the MAX function keeps the highest reading — handy for locating a hot spot on a panel or a line.
- Condensate and drains: spot abnormal temperature differences along a line (see water in the system — diagnostics).
What an infrared thermometer (really) measures
The instrument captures the infrared radiation emitted by the targeted surface (8–14 micron spectral band on the 62 MAX) and converts it to a temperature. Three practical consequences:
- Surface only. Air, steam and gases are practically transparent to infrared: you never measure “the air” or the inside of a pipe — you measure its wall.
- Emissivity governs accuracy. A matte surface emits well; polished metal emits poorly and reflects its surroundings (see below).
- The reading is an average over the whole measurement spot: if the target is smaller than the spot, the background contaminates the reading.
Key specifications (Fluke 62 MAX / 62 MAX+)
| Characteristic | 62 MAX | 62 MAX+ |
|---|---|---|
| Measurement range | −30 to 500 °C (−22 to 932 °F) | −30 to 650 °C (−22 to 1202 °F) |
| Accuracy (≥ 0 °C) | ±1.5 °C or ±1.5% of reading | ±1.0 °C or ±1% of reading |
| Distance-to-spot ratio (D:S) | 10:1 | 12:1 |
| Response time | < 500 ms | < 300 ms |
| Adjustable emissivity (EMS) | 0.10 to 1.00 | 0.10 to 1.00 |
| Spectral band | 8–14 microns | 8–14 microns |
| Ingress / drop rating | IP54 · 3 m drop | IP54 · 3 m drop |
| Power | 1 AA battery (LR06) | 1 AA battery (LR06) |
Below 0 °C the tolerance widens (±2 °C between −10 and 0 °C, ±3 °C below −10 °C). Class 2 sighting laser: never point it at eyes, people or reflective surfaces.
The 62 MAX is used as the example because it is widespread and representative; equivalent guns exist from other manufacturers — Testo, FLIR (Teledyne), Extech, Milwaukee, Klein Tools, among others. The principles (emissivity, distance-to-spot ratio) are identical across brands; only the datasheet numbers change.
The distance-to-spot ratio (D:S): closer is more accurate
The D:S ratio gives the diameter of the measurement spot at a given distance. At 10:1, the spot is roughly 1/10 of the distance:
| Distance | Spot size — 62 MAX (10:1) | Spot size — 62 MAX+ (12:1) |
|---|---|---|
| 300 mm (12 in) | 38 mm (1.5 in) | 33 mm (1.4 in) |
| 600 mm (24 in) | 60 mm (2.4 in) | 50 mm (2 in) |
| 900 mm (36 in) | 100 mm (4 in) | 85 mm (3.4 in) |
Field rule: the target must be clearly larger than the measurement spot. For a fitting or a drain a few centimetres across, measure from 20–30 cm (8–12 in), not from a metre away. The laser marks the centre of the spot, not its extent.
Emissivity: the setting that makes or breaks accuracy
Emissivity (0 to 1) describes how well a surface emits infrared radiation. Matte and painted surfaces emit well (0.90–0.95); bare, shiny metals emit poorly and reflect their surroundings: the reading then comes out lower than reality — the Fluke manual flags this as a burn hazard (a scalding part that “reads” lukewarm). The 62 MAX adjusts from 0.10 to 1.00 with the EMS key.
Typical values (8–14 micron band):
| Surface | Typical emissivity |
|---|---|
| Paint, matte, any color (motor, cabinet, piping) | 0.93 |
| Aluminum “silver” paint (compressors, lines) | 0.40 — trap: treat it as a metal |
| Sticker label, opaque plastic, rubber (hose, belt) | 0.95 |
| Concrete (slab) | 0.93 |
| Steel, oxidized (tank, structure) | 0.80 |
| Cast iron (casing, pump, engine block) | 0.65 |
| Iron, rusted steel | 0.60 |
| Stainless steel (depends on finish) | 0.10–0.80 |
| Aluminum, oxidized (aged tube) | 0.25 |
| Polished metals (aluminum, copper, brass, cylinder rod) | 0.03–0.10 — unreliable |
| Electrical tape (PVC) | 0.95 — the field reference |
The tape method: on bare or shiny metal, stick a piece of electrical tape, let it reach the surface temperature, then measure the tape with EMS = 0.95. A dab of matte paint does the same job on a permanent measurement point.
📥 Download the Onyx M3 emissivity chart (PDF) — the full table (31 materials: the industrial service technician’s essentials — metals and their surface conditions, paints, belts, oil film, frost — plus general surfaces: concrete, brick, asphalt, sand, paper, skin), the tape method and the pitfalls, on one page to keep in the service truck.
The functions that earn their keep in the field
- SCAN / HOLD — trigger held: continuous measurement (SCAN); trigger released: the value freezes (HOLD) and the unit shuts itself off after a few seconds.
- MAX / MIN / AVG / DIFF — the select key cycles through the maximum, minimum, average and spread of the scanning session; MAX is the hot-spot hunting mode.
- High and low alarms (HiLo) — a crossed threshold triggers an alert: practical for sweeping a row of equipment against an alarm setpoint.
- Trigger lock (trig) — hands-free continuous measurement; the unit stops by itself after 10 minutes.
- Laser and backlight can be switched off (LAS / LitE menus) to save the battery — about 10 h of life with both on.
Best practices (and the pitfalls that ruin everything)
- Acclimate the instrument. After a marked change of environment (cold truck → compressor room), let the unit stabilize for 30 minutes before measuring.
- Never through glass — glass is opaque to infrared: you measure the pane, not the object behind it. An enclosure window skews the reading the same way.
- Steam, smoke, dust in the sight path: contaminated reading. Move, or wait for the path to clear.
- Low battery = wrong readings. Replace the battery as soon as the indicator shows.
- No explosive atmospheres: the instrument is not rated for areas with flammable gases or vapours, nor for saturated, wet environments.
- Lens cleaning: cotton swab barely moistened with water, case with soapy water — no solvents.
- Do not leave the unit on or near hot objects (the sensor drifts). Storage: −20 to 60 °C (−4 to 140 °F), battery removed for the long term.
What an infrared thermometer does not do
- Measure air or a gas (including compressed air) — surfaces only.
- Measure the dew point: that is the job of a dedicated hygrometric probe (see dew point).
- Replace a contact measurement when the surface is shiny and the tape method is impractical: the contact probe rules.
- See inside equipment: for a full thermal map, the thermal imaging camera (next installment in this series) is the right tool.
Going further
- Dew point in compressed air
- Water in the compressed air system — diagnostics
- Identifying an air compressor
- Worried about an abnormal temperature? Diagnose a specific problem
The Fluke 62 MAX is used here as an example; Onyx M3 is not affiliated with any of the manufacturers mentioned (Fluke, Testo, FLIR, Extech, Milwaukee, Klein Tools). Emissivity values are typical published values — the surface condition rules; verify with a contact measurement when in doubt.
Onyx M3 helps you interpret your readings and trace them back to the cause. Contact us.