Infrared vs Contact Thermometer, K vs J Thermocouple
Infrared vs contact: two different measurements
An infrared thermometer reads the thermal radiation coming off a surface and converts it to a temperature. It never touches the target, it answers in under a second, and it works on things you cannot or should not touch: a moving belt, a live busbar, a kiln wall, a pan of hot oil. What it reports is the average temperature of the surface spot it sees.
A contact thermometer puts a sensor, usually a thermocouple, in physical contact with the material. It is slower, because the probe has to reach the temperature of the material, but it can measure inside things: the core of a piece of meat, a liquid bath, the air in a duct, the gas in a furnace. With a good probe in good contact it is usually the more accurate of the two.
Choose infrared for fast surface scans, hot or moving targets and anything electrically live. Choose contact when you need the internal temperature, a reading you can defend in a record, or a measurement on a shiny metal surface. The models are on the infrared thermometers and thermocouple thermometers pages.
Emissivity: why shiny surfaces read low
Every surface radiates less than an ideal emitter at the same temperature. The ratio is its emissivity, from 0 to 1. Most organic materials, paint, rubber, water and oxidized metal are around 0.9 to 0.95, and the default setting of many infrared thermometers sits in this region. Polished or bright metals such as aluminium, stainless steel and copper can be 0.1 or lower, and they also reflect radiation from their surroundings. Point an infrared thermometer at a polished stainless pot and it may read tens of degrees low.
Three practical fixes:
- Put a piece of matte black tape or a spot of matte paint on the surface, wait for it to reach temperature and measure the tape.
- Measure the spot once with a contact probe, then adjust the emissivity setting until the infrared reading matches. The TM380 and TM550 adjust from 0.1 to 1.00, the TM500A series from 0.10 to 1.00.
- Skip infrared and use a contact surface probe.
Infrared also cannot see through glass or clear plastic at its working wavelength; it reads the glass. The TM500A, TM650A and TM750A list a spectral response of 8 to 14 μm. Steam, smoke and dust between the meter and the target also pull the reading off.
Distance-to-spot ratio
The D:S ratio says how large the measured spot is at a given distance. All five Vicimeter infrared models are 12:1: at 300mm the spot is about 25mm across, at 600mm about 50mm, at 1.2m about 100mm. The target should fill the spot completely, preferably with margin. If it does not, the meter averages the target with the background and the reading is wrong. On a small motor bearing housing, move closer instead of trusting the laser dot from across the room. The laser only marks the aim point; the measured area is the spot around it. The annular multi-point laser on the TM650A and TM750A outlines that area.
Surface vs internal temperature
An infrared reading on a chicken breast, a drum of oil or a transformer housing tells you the outer surface temperature. The inside can be quite different, especially while something is heating or cooling. Food safety checks, process liquids and furnace atmospheres need a probe inserted where the temperature matters. Infrared is still useful there as a fast screening step: scan, find the hot or cold spot, then confirm it with a contact probe. Note that this guide is about industrial and food-service measurement; body-temperature thermometers are a separate class of medical device.
K-type vs J-type thermocouples
A thermocouple is two dissimilar metal wires joined at the tip. The junction produces a small voltage that depends on temperature, and the meter converts it using the curve for that wire pair. The common types are defined in IEC 60584.
- Type K (nickel-chromium / nickel-aluminium): the general-purpose type. Wide range, about 41 μV/℃, works well in oxidizing atmospheres. The DM6803A+ measures type K from -200℃ to 1370℃.
- Type J (iron / copper-nickel): higher output, about 52 μV/℃, and suited to vacuum, inert or reducing atmospheres. The iron leg oxidizes in air at high temperature, so practical use in air is usually kept below about 750℃. The DM6803A+ reads type J from -200℃ to 1200℃.
Under IEC 60584-1, both K and J Class 2 wire are allowed ±2.5℃ or ±0.75% of reading, whichever is larger, so the probe tolerance often matters more than the meter's. The meter must be set to the same type as the probe. A J probe read on a K setting gives a wrong temperature with no warning. If your plant uses mixed sensor types, use a meter that accepts several.
Which Vicimeter model fits
| Model | Method | Range | Accuracy (basic) | Choose it for |
|---|---|---|---|---|
| TM380 / TM550 | Infrared, 12:1 | -50℃ to 380℃ / 550℃ | ±1.5℃ or ±1.5% | Kitchen, HVAC and maintenance spot checks |
| TM500A | Infrared, 12:1, IP54 | -30℃ to 500℃ | ±1.5℃ or ±1.5% (below 400℃) | Industrial use, 3 m drop test |
| TM750A / TM650A | Infrared, 12:1, IP54 | -20℃ to 750℃ / 650℃ | ±1℃ or ±1% (above 400℃) | Kilns, boilers, heat treatment |
| DM6902 | Contact, 1 input | up to 750℃ | ±(0.75%+1) | Simple direct readings |
| DM6801A+ | Contact, 1 input | -50℃ to 1300℃ | ±(0.2%+1) below 200℃ | Food and lab work, 0.1℃ below 200℃ |
| DM6802A+ | Contact, 2 inputs | -50℃ to 1300℃ | ±(0.4%+1) | Inlet/outlet difference, T1-T2 |
| DM6803A+ | Contact, 1 channel, 8 types | -200℃ to 1800℃ (by type) | ±(0.3%+1) on K and J | Alarm output, logging up to 99 h |
| DM6804A+ | Contact, 2 channels, 8 types | -200℃ to 1800℃ (by type) | ±(0.3%+1) on K and J | Two-point profiles, T1-T2, logging |
The TM380 and TM550 differ only in upper limit. The TM500A series adds IP54, a 3 m drop test and a tighter accuracy band above 400℃. For contact work, the DM6801A+ is the choice when tenths of a degree matter below 200℃, and the DM6802A+ when the temperature difference is the point. For J-type sensors or other types such as T, N, R, S and B, go to the DM6803A+ or DM6804A+, which accept eight thermocouple types and record 2500 readings manually or 6000 automatically.
Match the probe to the job: bead-type wire probes (TP-01 series) for air and tight spaces, needle and rod probes such as the TP-02 for liquids, food and soft materials, and surface probes such as TP-03 and TP-03H for pipes and plates. Confirm the probe's thermocouple type and rated range when ordering. All probes are listed on the temperature probes page.
FAQ
What is the difference between a K type and J type thermocouple?
Type K uses nickel-chromium and nickel-aluminium wires and covers a wider range, with good resistance to oxidation. Type J uses iron and copper-nickel, gives a higher output per degree, and is normally limited to about 750℃ in air because the iron oxidizes. They use different conversion curves, so the meter must be set to the probe type.
Infrared thermometer vs probe: which is more accurate?
A contact probe in good thermal contact is usually more accurate and measures internal temperature. An infrared thermometer is faster and safer for hot, moving or live targets, but its accuracy depends on emissivity, spot size and the surface. The TM380 is specified at ±1.5℃ or ±1.5%; the DM6801A+ at ±(0.2%+1) below 200℃, plus the probe's own tolerance.
Thermocouple vs thermometer: what is the difference?
A thermocouple is a sensor. A thermometer is the complete instrument that reads a sensor and displays temperature. The DM68 series meters are thermometers that read thermocouple probes. A thermistor is a different sensor, a resistor whose value changes with temperature, accurate over a narrower range and common in appliances and fridge thermometers.
Infrared thermometer vs thermal camera?
An infrared thermometer gives one number for one spot. A thermal camera measures thousands of spots and shows them as an image, which helps find a hot connection across a whole panel. Both obey the same emissivity and spot-size rules. For checking known points, a spot thermometer like the TM500A is enough.