Test & Measurement / Precision Meters / Thermometers
Temperature, Humidity & Dew Point Meters
Ecotao supplies the Extech range of temperature, relative humidity and dew point instruments across South Africa — from pocket hygrometer pens for spot checks through to dual-input psychrometers used for HVAC commissioning, coating inspection and mould remediation. This page explains what each measurement actually tells you, then helps you match an instrument to the job.
- Air temperature
- The temperature of the air itself, measured by a sensor shielded from radiant heat and direct sunlight. Sometimes called dry bulb temperature to distinguish it from wet bulb.
- Relative humidity (%RH)
- The amount of water vapour present in the air, expressed as a percentage of the maximum the air could hold at that same temperature. Because warm air holds more moisture than cold air, relative humidity rises as air cools even when no water is added — which is why it is a poor way to compare two rooms at different temperatures.
- Dew point
- The temperature to which air must be cooled, at constant pressure, before water vapour begins condensing out of it. Dew point reflects the absolute quantity of moisture in the air and does not change when the air is simply heated or cooled, which makes it the correct figure for predicting condensation, comparing conditions between spaces, and setting drying targets.
- Wet bulb temperature
- The lowest temperature air can reach through evaporative cooling alone. Traditionally measured by slinging a wetted wick thermometer through the air; modern electronic psychrometers calculate it directly. It is used in cooling tower performance work, heat stress assessment and drying calculations.
- Grains per pound (GPP) and grams per kilogram (g/kg)
- Absolute humidity — the actual mass of water vapour carried per unit mass of dry air. Restoration and drying contractors work in GPP because it shows directly whether a dehumidifier is removing moisture from a structure, which a falling relative humidity reading alone cannot confirm.
Why dew point decides whether a surface sweats
A surface condenses moisture when its own temperature drops to or below the dew point of the air touching it. The air temperature is irrelevant — only the gap between surface temperature and dew point matters.
Instruments on this page
Handheld psychrometers — dew point and wet bulb
Professional field instruments that derive dew point and wet bulb from measured humidity and temperature. Choose these when the reading feeds a decision: apply the coating or wait, keep the dehumidifier running or shut it down, pass or fail the commissioning test.
Precision Hygro-Thermometer
EXTECH RH490The most accurate instrument in this range at ±2 %RH, and the only one that reports absolute humidity in grains per pound and grams per kilogram. Restoration contractors use the GPP reading to prove a structure is actually drying rather than simply cooling, and the sub-30-second sensor response means you can walk a building taking readings room by room without waiting at each stop. The slim rubberised body is built for one-hand operation while the other hand holds a moisture meter.
- Humidity0 to 100 %RH, ±2 %RH
- Temperature−30 to 100 °C, ±1 °C
- Dew point−30 to 100 °C
- Wet bulb0 to 80 °C
- Absolute0 to 1120 GPP (0 to 160 g/kg)
- ResponseUnder 30 seconds
Dual Input Hygro-Thermometer Psychrometer
EXTECH RH350Built for refrigeration and HVAC commissioning, where the useful number is almost always a difference rather than an absolute. Two Type K probe inputs let the meter compute T1 minus T2 across a coil, air temperature minus T1, and T1 minus dew point, so superheat and condensation-margin checks come straight off the display instead of out of a calculator. The triple backlit LCD holds humidity and temperature permanently while you cycle the third line through dew point, wet bulb or probe temperature, and 99 stored readings with min, max and average recall mean a full commissioning sweep can be logged before you return to the van.
- Displays%RH + temp + dew point / wet bulb / Type K
- DifferentialsT1−T2, Air−T1, T1−dew point
- Probe inputs2 × Type K (both probes included)
- Memory99 records, min / max / average
- InterfaceBuilt-in RS-232
- Power4 × AAA
Hygro-Thermometer Psychrometer
EXTECH RH300The practical middle of the range: full dew point and wet bulb capability in a body small enough to live in a toolbag. Its distinguishing feature is a sensor cap that twists closed over the humidity element, which matters more than it sounds — the usual failure mode for field hygrometers is dust and knocks reaching an exposed sensor. Add the optional TP890 probe and it also reports the difference between air temperature and a surface, giving you a condensation margin without a second instrument.
- Humidity10 to 90 %RH
- Temperature−20 to 50 °C, 0.1° resolution
- Displays%RH + temp + dew point / wet bulb / probe
- DifferentialT1−T2 and T2−dew point (optional probe)
- ProtectionTwist-close sensor cap
- Power2 × AAA
Hygro-Thermometer + Infrared Thermometer
EXTECH RH101Solves the other half of the condensation problem. A psychrometer tells you the dew point of the air; the RH101 tells you how cold the wall, duct or pipe actually is, without touching it. The infrared sensor has a laser pointer and an 8:1 distance-to-spot ratio, so a ceiling void or a duct run three metres up can be checked from the floor, while the remote humidity sensor on a one-metre coiled cable reads air conditions at the same spot. Note that this model reports humidity, air temperature and surface temperature rather than calculating dew point itself — pair it with an RH300 or RH490, or use it as a fast survey tool to find the cold spots worth investigating.
- Humidity10 to 95 %RH, ±3.5 %
- Air temp−20 to 60 °C
- Infrared−50 to 500 °C, emissivity 0.95
- Optics8:1 distance to spot, laser pointer
- ProbeRemote sensor, 1 m coiled cable
- Power9 V battery
Wall and desk monitors — continuous watch with alarms
Fixed-position instruments for spaces where conditions need to be held rather than measured once: stores, server rooms, laboratories, archives, greenhouses and production areas. The two Humidity Alert models add a dew point line and a programmable threshold alarm.
Hygro-Thermometer Humidity Alert with Dew Point
EXTECH 445815The remote-probe version of the Humidity Alert, and the more useful of the pair when the place you need to measure is not the place you need to read. The sensor extends on a 457 mm cable so it can sit inside a duct, behind a wall panel or down in a cabinet while the triple LCD stays at eye level. Humidity, temperature and dew point display together, and an adjustable high and low threshold triggers both a beeper and a flashing LED — so a store room drifting into mould territory over a long weekend announces itself rather than waiting to be discovered. A rear calibration pot allows field trimming against salt reference bottles.
- Humidity10 to 99 %, ±4 %RH
- Temperature−10 to 60 °C, ±1 °C
- Dew pointDisplayed on third line
- AlarmAdjustable Hi/Lo, audible + visual
- ProbeRemote sensor, 457 mm cable
- MountingDesk stand + wall bracket included
Hygro-Thermometer Humidity Alert with Dew Point
EXTECH 445814Identical alarm logic and display to the 445815, but with the sensor built into the housing instead of on a cable — the right choice when the meter sits in the space it is monitoring. That covers most laboratory, archive and assembly-area installations, and it removes a cable that can be snagged or damaged. Humidity, temperature and dew point read simultaneously on the large triple LCD, the high and low alarm points are set by the user, and max/min memory with reset captures overnight and weekend extremes.
- Humidity10 to 99 %, ±4 %RH
- Temperature−10 to 60 °C, ±1 °C
- Dew pointDisplayed on third line
- AlarmAdjustable Hi/Lo, audible + visual
- SensorIntegral (no external probe)
- CalibrationRear adjustment pot
Big Digit Hygro-Thermometer with Remote Probe
EXTECH 445715A large-format monitor with the tightest humidity accuracy in the wall-mount group at ±4 %RH, and a remote sensor for reading a chamber, cabinet or duct from outside. The rear calibration adjustment pot is the feature that earns its place in quality-controlled environments: paired with optional saturated salt bottles you can verify and re-trim the reading on site rather than sending the unit away. Max and min with reset track the extremes between checks.
- Humidity10 to 99 %RH, ±4 %RH
- Temperature−10 to 60 °C, ±1 °C
- SensorRemote probe
- CalibrationRear pot, salt bottles optional
- Size109 × 99 × 20 mm, 169 g
- Power1 × AAA
Big Digit Hygro-Thermometer
EXTECH 445703Chosen for legibility rather than precision. The 25 mm digits on an 80 × 62 mm LCD are readable across a workshop floor or a greenhouse aisle, which makes this the sensible unit where staff need to check conditions in passing rather than walk up to a meter. Simultaneous humidity and temperature, max/min memory with reset, and a choice of desk stand or wall bracket.
- Humidity10 to 99 %RH, ±5 %RH
- Temperature−10 to 60 °C, ±1 °C
- Display25 mm digits, 80 × 62 mm LCD
- MemoryMax / min with reset
- MountingStand + wall bracket included
- Power1 × AAA
Hygro-Thermometer Clock
EXTECH 445702Three parameters on one wall: 12 or 24 hour time, temperature and relative humidity. Reception areas, meeting rooms and small clinics tend to want a clock anyway, and this puts the conditions reading somewhere people will actually look. Max and min with reset gives a simple record of the day's swing without any logging setup.
- Humidity10 to 85 %, ±6 %RH
- Temperature−10 to 60 °C, ±1 °C
- Clock12 / 24 hour
- MemoryMax / min with reset
- MountingTilt stand + wall bracket
- Power1 × AAA
Thermo-Hygrometer Clock with Outdoor Probe
FLUS FL-201WThe budget option in this group, and the only one here that reads a second, outdoor location through a wired probe. Alongside temperature and humidity it shows calendar, alarm clock and a comfort level icon, which makes it a reasonable fit for offices, libraries and homes where the requirement is awareness rather than compliance. Note that this is a Flus-branded instrument, not part of the Extech line — the Extech 445702 is the closest direct equivalent if brand consistency matters for your site.
- ReadsIndoor temp + %RH, outdoor temp
- DisplayLarge LCD, comfort level icon
- MemoryMax / min, temperature and humidity
- ClockCalendar + alarm
- Units°C / °F switchable
- BrandFlus (not Extech)
Pocket and mini instruments — spot checks
Small enough to carry permanently. Neither model calculates dew point; both are for quickly answering "what is it like in here?" before deciding whether a proper survey is needed.
Pocket Humidity/Temperature Pen
EXTECH 44550A pen-sized hygro-thermometer that clips into a shirt pocket alongside your torch, so it is with you on the walk-through rather than back in the vehicle. The dual LCD shows humidity and temperature at once, switchable between Celsius and Fahrenheit, and max/min memory with a clear key means you can leave it in a room for an hour and come back to the range. Runs for a long time on a single LR44 button cell.
- Humidity20 to 90 %RH, ±5 %RH
- Temperature−10 to 50 °C, ±1 °C
- DisplayDual LCD, simultaneous
- MemoryMax / min with clear function
- Units°C / °F switchable
- Power1 × LR44 button cell
Mini Hygro-Thermometer Monitor
EXTECH RHM15At 51 × 38 mm this fits where nothing else will — inside an instrument case, a humidor, a display cabinet or a specimen box — and simply sits there recording the highs and lows until you next open the lid. A built-in tilt stand angles it for reading, and it runs on one LR44 cell. Important caveat before ordering: the RHM15 displays temperature in Fahrenheit only. If you need Celsius in a comparably small package, choose the 44550 pen above.
- Humidity10 to 99 %RH, ±5 %RH
- Temperature14 to 140 °F (°F only), ±2 °F
- Size51 × 38 × 16 mm, 24 g
- MemoryMin / max with clear button
- StandBuilt-in tilt stand
- Power1 × LR44 button cell
Specification comparison
All twelve instruments side by side. Dew point capability is the first thing to check — four models calculate it, the rest report humidity and temperature only.
← Scroll to see full table →
| Model | Format | Humidity range | RH accuracy | Temperature range | Dew point | Wet bulb | Alarm | Best for |
|---|---|---|---|---|---|---|---|---|
| RH490 | Handheld | 0 to 100 %RH | ±2 %RH | −30 to 100 °C | Yes | Yes | — | Drying & restoration, GPP tracking |
| RH350 | Handheld | 10 to 95 %RH | ±3 %RH | −20 to 60 °C | Yes | Yes | — | HVAC / refrigeration commissioning |
| RH300 | Handheld | 10 to 90 %RH | ±3 %RH | −20 to 50 °C | Yes | Yes | — | General field survey work |
| RH101 | Handheld | 10 to 95 %RH | ±3.5 % | −20 to 60 °C air | — | — | — | Surface temperature survey (IR to 500 °C) |
| 445815 | Wall / desk | 10 to 99 % | ±4 %RH | −10 to 60 °C | Yes | — | Hi / Lo | Ducts, cabinets, concealed spaces |
| 445814 | Wall / desk | 10 to 99 % | ±4 %RH | −10 to 60 °C | Yes | — | Hi / Lo | Labs, archives, assembly areas |
| 445715 | Wall / desk | 10 to 99 %RH | ±4 %RH | −10 to 60 °C | — | — | — | QC areas needing field calibration |
| 445703 | Wall / desk | 10 to 99 %RH | ±5 %RH | −10 to 60 °C | — | — | — | Long-distance readability |
| 445702 | Wall / desk | 10 to 85 % | ±6 %RH | −10 to 60 °C | — | — | — | Reception and public areas |
| FL-201W | Wall / desk | Indoor %RH | — | Indoor + outdoor probe | — | — | Clock alarm | Indoor / outdoor comparison on a budget |
| 44550 | Pocket pen | 20 to 90 %RH | ±5 %RH | −10 to 50 °C | — | — | — | Carry-everywhere spot checks |
| RHM15 | Mini monitor | 10 to 99 %RH | ±5 %RH | 14 to 140 °F only | — | — | — | Cases, cabinets, enclosed spaces |
How to choose the right instrument
Work from the decision you need to make, not from the specification sheet. These five questions resolve most selections.
Do you need dew point at all?
You need it if you are deciding whether to coat a surface, whether condensation will form, or whether a drying job is finished. You do not need it if you are simply holding a room within a comfort or storage band — relative humidity alone is sufficient for that.
Dew point: RH490, RH350, RH300, 445814, 445815
Measure once, or monitor continuously?
A handheld instrument answers a question at a moment in time and moves on. A wall monitor holds position and tells you when conditions drift, which is what you want for a stored asset, a controlled process or an unoccupied space over a weekend.
Handheld: RH490, RH350, RH300, RH101 | Fixed: 445xxx series
Is the sensor reaching somewhere you cannot?
Ducts, chambers, cabinets, wall voids and cold rooms all need the sensing element separated from the display. A remote probe on a cable solves this without leaving an instrument inside where you cannot read it.
Remote probe: 445815, 445715, RH101, RH300 (optional TP890)
How accurate does the humidity reading need to be?
±5 %RH is fine for comfort monitoring and general awareness. ±4 %RH suits quality control and storage compliance. ±2 %RH is worth paying for when the number goes into a report, a dispute, or an insurance claim.
±2 %: RH490 | ±3 %: RH350, RH300 | ±4 %: 445814/15, 445715
Will the reading be audited?
If a client, auditor or regulator will review the figures, specify the NIST traceable variant at order stage and plan for annual recalibration. Instruments with a rear calibration pot can additionally be verified on site against saturated salt references between formal calibrations.
NIST variants available on most Extech models — ask when ordering
Do you need a logged record?
If you need a time-stamped history rather than a live reading or a min/max pair, this is a different class of instrument. Ecotao stocks USB and Bluetooth data loggers on a separate page, including units that export to CSV.
What these instruments are used for
Protective coating and painting inspection
Coating specifications almost universally require the substrate temperature to sit at least 3 °C above the dew point before application, because a film applied to a surface that is already condensing will lose adhesion and blister later. On a Cape Town or Durban site this constraint bites hardest in the early morning and late afternoon, when steel that has radiated heat overnight sits below the dew point of humid coastal air well after sunrise. An RH300 or RH490 gives the air dew point; an RH101 or a contact probe gives the steel temperature; the difference between them is the go or no-go.
Water damage restoration and structural drying
The trap in drying work is that relative humidity falls as a space warms, which can make a wet building look like it is drying when no moisture has actually left it. Absolute humidity in grains per pound does not have this problem: if the GPP figure inside is dropping relative to outside, the dehumidifiers are genuinely removing water. The RH490 is the instrument for this because it reads GPP and g/kg directly, letting a contractor demonstrate progress to a loss adjuster with a defensible number rather than an impression.
HVAC and refrigeration commissioning
Commissioning work is built on differences: air-on versus air-off across a coil, superheat at the compressor, the margin between duct surface temperature and dew point where insulation is marginal. The RH350 was designed around this, taking two Type K probes and computing T1−T2, Air−T1 and T1−dew point on the display so the technician is not transcribing four numbers into a calculator on a plant room floor. Its 99-record memory means an entire plant can be swept before the data is written up.
Mould prevention in coastal buildings
Along the Cape, KwaZulu-Natal and Eastern Cape coasts, ambient relative humidity commonly sits between 70 and 85 % through summer, which is above the threshold at which mould spores will germinate on a surface. Because the outdoor air cannot be changed, control comes down to keeping surfaces warm enough and air moving enough that local surface humidity stays lower. A 445814 or 445815 with a high-humidity alarm set around 65 % turns this from a periodic inspection task into an exception-based one — you find out when it matters, not at the next walk-through.
Static control in Highveld winters
The opposite problem applies inland. Johannesburg sits at roughly 1750 m and its winter air is exceptionally dry, with indoor relative humidity routinely falling below 25 %. Below about 30 %RH surfaces lose the thin conductive moisture layer that normally drains static charge, and electrostatic discharge starts damaging components in electronics assembly, causing misfeeds in printing, and creating ignition risk in powder handling. The same Humidity Alert monitors used against mould at the coast work here with the low threshold set instead, prompting humidification before losses occur.
Storage, cold chain and laboratory compliance
Pharmaceutical, food and laboratory storage areas are typically specified with both a temperature band and a humidity band, and the record of compliance matters as much as the condition itself. A fixed monitor with max/min memory covers day-to-day verification; where a time-stamped, exportable record is required for audit, a data logger is the correct instrument instead. Ecotao supplies both, and can advise on which the applicable standard actually calls for.
Greenhouses, seed stores and grain handling
Crop storage is governed by equilibrium moisture content, which is set by the relative humidity of the air around the grain rather than by the grain's own history. Holding store humidity below roughly 65 % keeps most cereals below the moisture content at which fungal spoilage begins. Under glass, the requirement inverts: propagation needs high humidity, but condensation dripping from the roof onto foliage spreads disease, so dew point relative to the glazing temperature is the number that matters.
Frequently asked questions
What is dew point and how is it different from relative humidity?
Dew point is the temperature to which air must be cooled, at constant pressure, before water vapour starts condensing out of it. Relative humidity is the ratio of the water vapour currently in the air to the maximum the air could hold at that temperature, expressed as a percentage. The key difference is that relative humidity changes as the air temperature changes even when no moisture is added or removed, while dew point stays fixed because it reflects the absolute amount of water vapour present. That makes dew point the more reliable figure when you need to predict condensation, compare conditions between two rooms at different temperatures, or decide whether a surface will sweat.
How do I know if condensation will form on a surface?
Condensation forms whenever a surface is at or below the dew point temperature of the air touching it. To check, measure the air dew point with a psychrometer such as the RH490 or RH300, then measure the surface temperature with a contact probe or an infrared thermometer such as the RH101. If the surface temperature is below the dew point, moisture is already condensing. Coating and painting specifications commonly require the substrate to be at least 3 °C above the dew point before application, so a meter that displays both readings removes the guesswork.
What is the difference between a hygrometer, a hygro-thermometer and a psychrometer?
A hygrometer measures relative humidity only. A hygro-thermometer, sometimes called a thermohygrometer, measures relative humidity and air temperature together, which is what most wall and desk monitors do. A psychrometer goes further and derives dew point and wet bulb temperature from the humidity and temperature readings. Traditional psychrometers used a wetted wick that had to be slung through the air; modern electronic instruments such as the RH300 and RH350 calculate wet bulb directly, so no slinging is required.
Which meter should I choose for HVAC commissioning?
HVAC commissioning generally needs dew point, wet bulb and temperature differential across a coil, so a dual-input psychrometer is the right class of instrument. The RH350 accepts two Type K probes and calculates T1−T2, Air−T1 and T1−dew point directly, covering superheat and subcooling work without manual arithmetic. If you only need single-point air readings with a differential against one external probe, the more compact RH300 is usually sufficient.
What humidity level prevents mould growth in a South African building?
Mould spores generally need sustained surface relative humidity above roughly 70 % to germinate, and most building guidance recommends holding indoor relative humidity between 40 and 60 %. In coastal areas such as Cape Town, Durban and Gqeberha, ambient relative humidity frequently sits in the 70 to 85 % band during summer, so the practical control point is usually surface temperature rather than air moisture. A humidity alert monitor such as the 445815 or 445814 lets you set a high threshold and be warned audibly and visually before conditions favour growth.
Why does static electricity become a problem on the Highveld in winter?
Highveld winters combine high altitude, around 1750 m in Johannesburg, with very dry continental air, and indoor relative humidity regularly falls below 25 %. Below roughly 30 %RH, surfaces lose the thin conductive moisture film that normally bleeds off static charge, so electrostatic discharge risk rises sharply in electronics assembly, printing and powder handling. Monitoring with a low humidity alarm, which the 445814 and 445815 both provide, gives an early warning to switch on humidification before damage occurs.
Do these meters read in Celsius?
All the handheld instruments on this page, including the RH490, RH350, RH300 and RH101, plus the 445814, 445815, 445715, 445703 and 445702 wall units and the 44550 pen, switch between Celsius and Fahrenheit. The one exception is the RHM15 mini monitor, which is a Fahrenheit-only display. If you need a Celsius reading in a comparably small format, the 44550 pocket pen is the direct alternative.
How do I calibrate a humidity meter?
Field verification of relative humidity is normally done with saturated salt solutions in a sealed calibration bottle, typically sodium chloride at about 75 %RH and magnesium chloride at about 33 %RH. The instrument is left in the sealed chamber until the reading stabilises, then adjusted to match. Models with a rear calibration adjustment pot, including the 445715, 445814 and 445815, can be trimmed in the field this way. For instruments used in regulated or audited environments, order the NIST traceable variant or arrange periodic laboratory calibration through Ecotao.
Can Ecotao supply these meters outside Cape Town?
Yes. Ecotao Enterprises operates from Plattekloof in Cape Town and delivers throughout South Africa, including Johannesburg, Pretoria, Durban, Gqeberha, Bloemfontein and East London, as well as to neighbouring countries such as Namibia, Botswana, Zimbabwe and Mozambique. Contact the head office on +27 21 911 5835 or email sales@ecotao.co.za for availability, lead time and delivery arrangements.
Related categories
Instruments that measure adjacent parameters, or the same parameters in a different format.
Not sure which meter fits the job?
Tell us what you are measuring and why, and we will recommend the right instrument — including whether you need NIST traceable calibration and whether a logger would serve you better than a meter. Ecotao supplies test and measurement equipment throughout South Africa and neighbouring countries from our Cape Town base.
Specifications are drawn from current manufacturer data and are subject to change without notice. Measurement ranges and accuracies are quoted at the manufacturer's stated reference conditions; performance outside those conditions may differ. Confirm current model availability, lead time and calibration options with Ecotao before ordering.