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Extech · Extech HT30 Heat Stress WBGT Meter | WBGT with or without sun, black globe temp, heat stress index, 0-100% RH, RS-232 interface, auto power off
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The Extech HT30 is a handheld WBGT meter with a black globe for direct sun exposure and an in/out function that switches between globe-corrected and un-corrected readings. Buy it if you need a portable WBGT number for outdoor work and want to see the difference direct sun makes. The listing does not publish accuracy, resolution, globe diameter or battery type, so treat the first readings as relative until you have established a routine. For a wider field of instruments, see our guide to heat stress meters for outdoor work.
The first hour: what the HT30 actually does when you switch it on
The HT30 is a WBGT meter. WBGT, wet bulb globe temperature, is an index that combines air temperature, humidity and radiant heat into one number used to judge heat stress on people. The listing gives you three things that matter in that first hour: a black globe temperature sensor, an in/out function, and a relative humidity range of 0 to 100 percent.
The black globe is the part that separates this instrument from a simple thermometer. It is a dark sphere that absorbs solar radiation, so its temperature rises above air temperature when the sun is out. The listing says the black globe temperature “monitors the effects of direct solar radiation on an exposed surface”. In practice, that means the meter can show you how much hotter the radiant load makes the environment, which is exactly the condition that catches outdoor workers out on a bright but breezy day.
The in/out function is the switch that decides whether that globe correction is included. The listing says it “displays the WGBT value with or without direct sun exposure”. So you get two readings from one instrument: an indoor or shaded WBGT and an outdoor WBGT that accounts for the globe. The first hour is mostly about learning which one you are looking at, because the difference between them is the whole reason to carry a globe meter rather than a hygrometer.
The listing does not publish the globe diameter, the WBGT range, the resolution or the accuracy. That is the first honest gap. You can still take readings, but you cannot state a tolerance for them from the published data alone.
Reading the black globe: why the in/out switch changes the number you report
WBGT is calculated from three inputs: dry bulb air temperature, natural wet bulb temperature and globe temperature. The HT30’s listing only names the black globe and the humidity range, so the other sensors are implied by the WBGT function rather than described. The in/out function is the practical control: indoors or in shade, the globe term is either omitted or treated differently; outdoors in sun, the globe term carries the solar load.
The mistake new owners make is to take one reading in the open, one in the shade, and report whichever is higher without saying which mode was used. That number is not comparable to anything. The useful routine is to decide the mode before you walk the site, note it, and keep it consistent across the shift. If you are monitoring a work-rest schedule, the mode is part of the record.
The listing does not say how the in/out function is implemented, whether it is a separate WBGT calculation or a display toggle. It also does not publish the globe’s response time. A black globe has thermal mass, so it does not settle instantly; the listing is silent on how long to wait. In the first hour, the safe habit is to let the globe sit in the location for several minutes before you trust the number, and to note that the manufacturer publishes no settling time.
For the wider context of how WBGT is used in industrial settings, our page on WBGT meters for industrial heat monitoring sets out the index and its limits.
What the listing publishes, and the numbers it leaves out
The published data is short. The HT30 has a black globe temperature sensor, an in/out function, a relative humidity range of 0 to 100 percent, auto power off with override, and a built-in RS-232 interface with optional PC software. That is the whole of it.
What is not published: WBGT range, WBGT accuracy, temperature range, temperature accuracy, globe diameter, resolution, response time, battery type, battery life, operating temperature range, dimensions, weight, and whether the RS-232 output streams live data or only stored readings. Each of these matters to a buyer. Accuracy decides whether the meter is suitable for a formal heat stress policy. Globe diameter affects how the radiant load is averaged. Battery type decides what you carry as a spare. The listing is silent on all of them.
The RS-232 interface is worth a note. It is a serial interface, and the listing says PC software is optional, which implies the cable and software are not necessarily in the box. The listing does not state what is included. Before you plan to log data to a computer, establish what the package contains; the listing does not publish a box contents list.
Auto power off with override is a small but real feature. Auto power off protects the battery, and the override lets you keep the meter awake during a long survey. The listing does not say how long the timeout is.
The maintenance nobody mentions: the globe, the humidity sensor and the battery
A black globe is a dusty object by design. It sits in the open, absorbs radiation and collects whatever is in the air. A layer of dust changes its absorptivity and therefore the reading. The listing does not publish a cleaning procedure, but the mechanism is simple: keep the globe clean and handle it as a sensor, not as a bumper. The same applies to the humidity sensor, which the listing gives as 0 to 100 percent RH but does not describe as replaceable or filter-protected.
Battery type and battery life are not stated in the listing bullets. That is a genuine inconvenience for anyone planning a full shift outdoors. The auto power off helps, but the override defeats it, so a long survey with the override engaged will draw down whatever cell is inside. Establish the battery type before the first field day.
The RS-232 port is another wear point. It is an exposed connector on a handheld instrument, and the listing does not mention a cover. If you use the PC software regularly, inspect the port for dirt and moisture.
Calibration is not mentioned in the listing at all. The listing does not publish a calibration interval, a calibration method, or whether the instrument is supplied with a certificate. For a meter used in a formal heat policy, that silence is the thing to resolve with the supplier before purchase. Our broader guide to environmental monitoring devices explains how calibration and traceability fit into instrument choice.
Where the HT30 stops being the right instrument
The HT30 is a handheld spot-check meter. It is not a logger, not an alarm, and not a personal heat stress monitor. The listing does not publish a logging interval, an alarm function, or a data storage capacity. If you need a continuous record of WBGT across a shift, this instrument as described does not provide it; the RS-232 interface suggests a connection to a PC, but the listing does not say whether it streams or dumps, and the software is optional.
It is also not a substitute for a formal heat stress assessment method. WBGT is an index, and the listing publishes no accuracy figure, so you cannot state how close the displayed number is to the true WBGT. For a workplace policy with legal weight, an instrument with a published accuracy and a calibration certificate is the appropriate tool. The HT30’s value is in quick, portable, relative readings that show the difference between shaded and sun-exposed conditions.
If your work is indoor air quality rather than heat, a dedicated CO2 or VOC meter is the right instrument; our page on CO2 versus VOC monitoring explains the split. If your concern is combustion gas, a WBGT meter tells you nothing; that is a different sensor entirely.
The HT30 is a reasonable first WBGT meter for a safety officer, a site supervisor or a grounds manager who needs to see how much radiant load the sun is adding. It is not the instrument for a research-grade heat study, and the listing’s silence on accuracy is the reason.
The first hour, and the first honest reading
The Extech HT30 gives you a black globe and an in/out switch in a handheld WBGT meter, which is enough to show how much direct sun changes heat stress on a site. The first hour should be spent learning which mode you are in and how long the globe takes to settle, because the listing does not publish a response time. The listing also does not publish accuracy, resolution, globe diameter or battery type, so the meter is best used for relative readings and screening rather than for a formal, traceable heat stress record. If you need a published accuracy figure, this is not the instrument the listing describes.
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Frequently Asked Questions
What does the in/out function actually change on the HT30?
The listing says it displays the WBGT value with or without direct sun exposure. In practice that means the outdoor mode includes the black globe's solar load and the indoor mode does not. The listing does not describe the implementation in more detail, so treat the switch as a mode selector and record which mode you used.
Does the HT30 measure air temperature and humidity as well as WBGT?
The listing publishes a relative humidity range of 0 to 100 percent and names the black globe temperature sensor. It does not publish a separate air temperature range or accuracy. WBGT is calculated from dry bulb, natural wet bulb and globe temperature, so the instrument must sense more than the globe, but the listing does not describe those sensors.
Can I log data from the HT30 to a computer?
The listing states a built-in RS-232 interface with optional PC software. It does not state whether the cable and software are included, whether the output streams live readings or stored data, or whether the meter stores readings internally. Confirm the package contents and the software's function before relying on it for logging.
What is the accuracy of the HT30's WBGT reading?
Not stated in the listing bullets. The listing publishes no accuracy figure for WBGT, temperature or humidity. Without a published accuracy you cannot state a tolerance for the reading, which is why the meter is better suited to relative comparisons than to a formal, traceable heat stress record.
Does the HT30 have an alarm or a data logging function?
The listing does not mention an alarm, a logging interval or internal memory. It mentions auto power off with override and an RS-232 interface. If you need an alarm or continuous logging, the listing as published does not describe those functions.
Is the HT30 suitable for a workplace heat stress policy?
The listing publishes no accuracy, no calibration information and no certificate. A policy that requires traceable measurements needs an instrument with published accuracy and a calibration route. The HT30 as described is a portable screening meter, not a reference instrument.
Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.
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