Extech Thermometers

A thermometer is an instrument that reports the temperature of a substance or surface by converting a physical response — the voltage across a thermocouple junction, the resistance of a platinum element, or the infrared radiation leaving a surface — into a temperature reading. Extech's thermometer range covers all three sensing principles, from a pocket food probe to a twelve-channel datalogger writing directly to SD card.

The distinction that governs almost every purchase decision is contact versus non-contact. A contact thermometer — thermocouple, RTD or stem — must physically touch or be immersed in the target, and in return it reports the temperature of the target, including inside a liquid, a food product or a duct. A non-contact infrared thermometer reads only the outer skin of a surface, but does so instantly, from a safe distance, on live electrical gear, rotating machinery or a 900 °C kiln wall. Most sites need one of each.

Ecotao supplies Extech thermometers throughout South Africa, Namibia, Botswana, Zimbabwe, Mozambique, Lesotho and Eswatini. If you are unsure which model fits your application, the selection guidance below is written to answer that directly — or email sales@ecotao.co.za.

How to choose a thermometer

Work through these five questions in order. They eliminate most of the range within a minute.

1. Can you touch the target?

If the target is energised, moving, above roughly 250 °C, or physically out of reach, the decision is made for you: infrared. If you need the core temperature of something rather than its surface — a chicken breast, a bearing oil bath, air inside a duct — infrared cannot give it to you and you need a probe. Instruments such as the IR267 and VIR50 carry both a non-contact sensor and a Type-K input precisely so you do not have to carry two instruments.

2. What is the highest and lowest temperature you will ever measure?

Buy for the extreme, not the average. A mini infrared thermometer topping out at 332 °C is perfect for HVAC and electrical panels and useless at a furnace door. The Extech range spans roughly −200 °C on a Type-K input up to 1000 °C non-contact on the 42545, so the range is rarely the limitation — but choosing one notch too low is the most common regret.

3. Do you need a record, or just a reading?

This is the question most buyers underestimate. If anyone will ever ask you to prove a temperature — a food safety auditor, an insurer after a cold-chain loss, a client disputing a commissioning result — you need a datalogger, not a handheld. A handheld gives you a number you then have to write down; a logger gives you a timestamped file. The TH30, RHT30, SD200, SDL200, SDL500 and TM500 all produce Excel or PDF output.

4. How many points at once?

Single reading: EA11A or a stem thermometer. Differential between two points, such as flow and return on a chilled water line: EA10 or 421502-class dual input. Three to four points such as a cold room's corners: SD200 or SDL200. A full oven survey, a heat-treatment profile, or a whole distribution board: TM500 with twelve channels.

5. Does it need certification for food contact?

If the probe enters food destined for sale, an NSF certified instrument such as the TM55 avoids an argument with an environmental health practitioner that a generic probe will not.

Sensor technologies compared

Every temperature instrument on this page uses one of four sensing methods. The differences are not academic — they determine what the instrument can physically measure and how much you should trust the number.

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Infrared thermometers

An infrared thermometer collects the thermal radiation leaving a target through a lens, focuses it onto a detector, and converts the resulting signal into a surface temperature. Because nothing touches the target, the reading is available in well under a second and the operator can stay outside the arc-flash boundary of a live switchboard, clear of a rotating shaft, or several metres from a furnace door.

Two specifications decide whether a given infrared thermometer will work for your job, and neither is the temperature range. The first is distance-to-spot ratio, which sets how small a target you can measure from how far away. The second is emissivity adjustment, which determines whether the instrument can be corrected for shiny metal surfaces. Both are explained in detail further down this page.

Thermocouple & multi-channel thermometers

A thermocouple generates a small, predictable voltage where two dissimilar metal wires are joined; the instrument reads that voltage, compensates for the temperature at its own terminals, and converts the result to a temperature. The practical consequence is that the sensing element can be made tiny and cheap, put on the end of a long lead, and replaced when it is damaged — which is why thermocouples dominate industrial contact measurement despite being less accurate than platinum RTDs.

The instruments below differ mainly in channel count. Buy on that basis: one channel for spot checks, two for differentials across a heat exchanger or a chilled water circuit, and three or more where you are profiling a process over time rather than taking a single reading.

Stem, pocket & food thermometers

Stem thermometers put the sensor at the tip of a rigid stainless probe, which is inserted directly into what is being measured. For liquids, pastes, semi-solid foods and soft materials this is the only honest way to get a temperature, because the surface reading an infrared thermometer would give you can differ from the core by tens of degrees. The models below are sealed for washing down and sized to live in a pocket, since a food thermometer that lives in a drawer does not get used.

Temperature dataloggers

A datalogger differs from a handheld thermometer in one respect that matters more than any specification: it produces a record without anyone present. Left in a cold room, a vaccine fridge, a shipping container or a curing enclosure, it captures a timestamped series that can be handed to an auditor, an insurer or a client. Where a handheld reading is a claim, a logger file is evidence.

Choose between USB and SD-card logging on how the data will be retrieved. USB loggers plug directly into a PC and generate the report themselves; SD-card instruments write an Excel file you can pull without connecting anything, which suits equipment left permanently installed.

Emissivity and spot size: getting infrared readings right

Almost every incorrect infrared reading comes down to one of two errors, and both are avoidable.

Spot size — the target must fill the spot

Distance-to-spot ratio tells you how large the measured circle becomes as you move away from the target. If that circle is bigger than the object you are aiming at, the instrument averages the object with whatever is behind it, and a hot target will read low. The fix is either to step closer or to use higher-ratio optics.

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Read the table as a rule of thumb: to measure a 40 mm terminal lug from 2 metres away you need 50:1 optics. To measure the same lug with an 8:1 instrument you must be within about 300 mm of it — which may be inside an arc-flash boundary you should not be crossing.

Emissivity — how much radiation the surface actually emits

Emissivity is the fraction of thermal radiation a surface emits compared with a perfect emitter, on a scale of 0 to 1. Most non-metallic materials sit near 0.95, which is why fixed-emissivity instruments work well for general facilities work. Bare and polished metals sit far lower, emit very little of their own radiation, and instead reflect their surroundings — which is why an infrared thermometer set to 0.95 will read a polished aluminium busbar dramatically below its true temperature.

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Probes & accessories

The probe determines what a thermocouple thermometer can actually measure, and it is usually the cheapest part of the system to change. Bead wire probes respond fastest and suit air and surface work; immersion probes are sheathed for liquids; surface probes present a flat or sprung contact face to a solid; penetration probes are sharpened for solids and semi-solids; pipe clamps attach without holding. All of the following are Type-K and interchangeable across the Extech thermocouple instruments on this page.

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South African applications and verification

Altitude changes your boiling-point reference

The boiling-water check is the most common way of verifying a thermometer in the field, and in South Africa it is the most commonly misapplied. Water boils at 100 °C only at sea level. Atmospheric pressure falls with altitude, and boiling point falls with it — by roughly 1 °C for every 285 metres of elevation. A thermometer checked against boiling water on the Highveld and expected to read 100 °C will appear to be reading almost 6 °C low when it is in fact perfectly accurate.

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Cold chain and food safety under R638

Regulation R638 of 2018, made under the Foodstuffs, Cosmetics and Disinfectants Act, requires that potentially hazardous foodstuffs be held at or below 5 °C when refrigerated or at or above 65 °C when hot-held, and requires food premises to keep records demonstrating that they have done so. That second requirement is what drives the choice of instrument. A handheld thermometer gives a reading someone must transcribe; a datalogger produces a timestamped file that stands on its own. In practice a food business is usually best served by both — a TH30 or RHT30 logging the cold room continuously, and an NSF certified TM55 for the product spot checks that an inspector will ask to see performed.

Monitoring through load shedding

Scheduled outages create a specific measurement problem: the temperature excursion happens while nobody is watching, and the question afterwards is how far the product actually rose and for how long. That is a logging problem rather than a measurement one. Battery-powered USB loggers such as the TH30 and RHT30 keep recording through an outage and produce a report showing the excursion profile, which is what an insurer or a customer will want to see. Where several points matter at once — multiple cold rooms, a staging area, a vehicle bay — the SD200 logs three Type-K channels to SD card and runs on a mains adaptor with battery backup so it survives the switchover.

Other Southern African applications

In electrical maintenance, infrared inspection of distribution boards, transformers and motor terminations catches loose or corroded connections before they fail, and the 42510A or IR320 will do that work from outside the arc-flash boundary. In mining and heavy industry, the IR320's IP65 and drop rating survive conditions that destroy standard handhelds, while the 42545 covers furnace and smelter work at up to 1000 °C. In agriculture, grain and produce storage temperature is a spoilage indicator, and the SD200's multi-point logging maps a silo or store rather than sampling it. In HVAC commissioning, the SDL500's dew point calculation predicts where condensation will form on chilled water reticulation — a recurring problem in humid coastal installations in Durban and along the KwaZulu-Natal coast.

Discontinued models and current replacements

Several models that appeared in earlier Extech catalogues have now been withdrawn from production. If you are standardising on one of them, the table below shows the current equivalent. Where a replacement is listed by Extech we have said so; where the match is our own assessment based on specification, we have said that too.

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Full range comparison

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Frequently asked questions

What is the difference between an infrared thermometer and a thermocouple thermometer?

An infrared thermometer measures the thermal radiation leaving a surface and reports a surface temperature without touching anything. That makes it fast, safe on live or moving equipment, and completely dependent on knowing the emissivity of the surface. A thermocouple thermometer measures the small voltage generated where two dissimilar metals meet, so the probe must physically touch or be immersed in whatever is being measured.

Use infrared when the target is hot, energised, rotating or out of reach. Use a thermocouple when you need the temperature inside a liquid, a duct, a food product or a bearing housing rather than just the outside skin of it. The IR267 and VIR50 carry both sensor types in one instrument.

What does a 12:1 or 50:1 distance-to-spot ratio mean?

Distance-to-spot ratio describes how the measured circle grows as you step back. At 12:1 the instrument averages a spot one twelfth the size of the distance to the target, so at 2 metres it reads a circle of roughly 167 mm. At 50:1 the same 2 metre standoff reads a circle of only 40 mm.

The rule that matters in practice is that the target must completely fill the spot. If the spot is larger than the object, the reading blends the object with whatever is behind it, and a hot target reads low. High ratios such as the 50:1 on the 42545 exist so small hot components can be measured from a safe distance.

Why does my infrared thermometer read shiny metal incorrectly?

Polished and bare metals have very low emissivity, often between 0.05 and 0.20. They emit very little of their own thermal radiation and instead reflect the radiation of everything around them, so an instrument set to a typical 0.95 will read a polished aluminium busbar far below its true temperature.

There are two fixes. Use an instrument with adjustable emissivity — the 42510A, 42545, IR320, IR267 or VIR50 — and set the correct value. Or apply a patch of matt masking tape or flat black paint, let it reach thermal equilibrium, and measure that patch at about 0.95. For critical work on bare metal, a contact Type-K surface probe such as the 881602 is more reliable than any infrared reading.

How do I check a thermometer is still accurate without sending it away?

Two field checks catch most drift. The ice-point check is the more useful: fill a container with crushed ice, top it with clean water, stir, and insert the probe without touching the sides. It should read 0 °C within the instrument's stated accuracy, and this reference does not change with altitude.

The boiling-point check does change with altitude, which matters in South Africa — water boils near 100 °C at the coast but only around 94 °C on the Highveld. Use the altitude table above rather than assuming 100 °C. Neither check replaces traceable calibration where a quality system requires it.

Which thermocouple type should I use — J, K, E, T, R or S?

Type K is the default for general industrial work: roughly −200 to 1372 °C, with inexpensive and widely available probes. Type J suits older equipment and reducing atmospheres up to about 750 °C in practice. Type T is the choice for cold work and food, holding good accuracy down to −250 °C. Type E gives the highest output per degree, which helps when resolving small differences.

Types R and S are platinum-rhodium and are used above 1000 °C in furnaces and kilns where base-metal thermocouples degrade. The SDL200 and TM500 accept all six types, so one instrument covers a mixed site.

What temperature must refrigerated food be kept at in South Africa?

Regulation R638 of 2018 under the Foodstuffs, Cosmetics and Disinfectants Act requires potentially hazardous foodstuffs to be held at or below 5 °C when refrigerated, or at or above 65 °C when hot-held, and premises are required to keep records demonstrating it.

That record-keeping requirement is why an SD card or USB datalogger such as the TH30, RHT30 or SD200 is usually more useful to a food business than a handheld reading: it produces a timestamped file rather than a manual logbook entry. An NSF certified probe thermometer such as the TM55 covers the spot checks on the product itself.

Which Extech thermometer is best for monitoring cold chain through load shedding?

For unattended monitoring through scheduled outages you want a logger that runs on its own battery, timestamps every reading, and produces a file you can hand to an auditor or insurer. The TH30 records both an internal air channel and an external probe channel to 48,000 readings and exports directly to PDF or Excel when plugged into a PC — useful in a cold room where you want both air and product temperature. The RHT30 does the same for temperature and humidity.

Where several points matter at once, such as multiple cold rooms or a delivery staging area, the SD200 logs three Type-K channels to SD card and runs on an AC adaptor.

Do you still supply the Extech 42270, 42275, 421502, 392050 or IR100?

These models have been withdrawn from the Extech catalogue and are no longer in production. Extech lists the RHT30 as the replacement for the 42270 and 42275, and the EA10 as the replacement for the 421502. The IR100 has been removed from the range, with the IR400 the closest current equivalent. The 392050 penetration stem dial thermometer has no direct successor; the 39240 or the NSF certified TM55 cover the same penetration measurements digitally.

See the discontinued models table above, or email sales@ecotao.co.za if you need help matching a replacement to an existing installation.

Can an infrared thermometer measure body temperature?

Not reliably. The instruments on this page are industrial infrared thermometers, calibrated and specified for surface measurement across wide temperature ranges, not for the narrow band and tight accuracy that clinical measurement requires. Human skin also sits at roughly 0.98 emissivity, and readings are strongly affected by ambient conditions, perspiration and measurement site.

For human temperature screening, see our separate infrared body thermometer range, which covers purpose-built non-contact instruments for that application.

What is the difference between the SD200, SDL200 and TM500?

They are the same idea at three scales. The SD200 records three Type-K channels to SD card and is the sensible choice for monitoring a single space such as a cold room. The SDL200 adds a fourth thermocouple channel, support for all six thermocouple types, two Pt100 RTD channels and a differential T1−T2 display, which makes it the choice where a site has mixed sensor types or where you need the temperature difference directly. The TM500 records twelve channels simultaneously and exists for distribution work — oven surveys, heat-treatment profiling, mapping a large enclosure — where the pattern across many points is the result you are after.