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JEDEW · JEDEW 2 Pack Hygrometer Thermometer Digital Humidity Meter Indoor/Outdoor Humidity Monitor Reptile Thermo-Hygrometer for Greenhouse Humidors Terrarium Jars, Fahrenheit (℉) /Celsius(℃) (2)
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A pair of small vented thermo hygrometers reading 0 to 70 C and 10 to 99 percent relative humidity, sampling every 10 seconds, specified to plus or minus 1 F on temperature and plus or minus 3 percent RH on humidity, each running from an LR44 button cell with a spare supplied. The question worth asking about a 2 pack is what that humidity tolerance permits. Two units that both conform may legitimately read six percentage points apart in the same air, and comparing them against each other is the first thing most buyers do.
Two units, three percent each, and six points of legitimate disagreement
Put both units of the pack on the same shelf and they will probably not agree. That is not a fault, and the specification says so if you read it as a specification rather than as a reassurance.
Plus or minus 3 percent RH means a conforming unit may read three percentage points either side of the true value. Two conforming units can therefore sit at opposite ends of that allowance. In air genuinely at 69 percent RH, one unit reading 66 and the other reading 72 are both inside specification and neither is broken.
This is worth stating plainly because a 2 pack invites exactly the comparison that makes it look faulty. People buy pairs, set them side by side, see a gap and conclude one is wrong. The gap is the tolerance.
There is a constructive use for the same fact. Leave both units in one place for an hour and note the difference, and you have measured the offset between them. From then on the pair is more useful than either unit alone, because a change you see on both is real and a change you see on one is not. A salt saturation check against a known humidity, which our guide on calibrating a hygrometer describes, turns that relative figure into an absolute one.
Ten second sampling that is remembered by nothing
The second bullet states that the sensor measures every 10 seconds. That is a sampling interval and a good one for a device of this kind, since a display lagging the room by minutes is worse than no display at all.
What the bullets do not describe is memory. There is no mention of a minimum and maximum record, no logging, no history and no connectivity. The display shows the current reading and nothing else.
Set that against what the listing says the instrument is for. Greenhouses, humidors, terrariums, warehouses, instrument workshops and wine storage all share one characteristic: you are not standing in front of them most of the time. The question you want answered is what happened while you were away, and an instrument that samples diligently every 10 seconds and keeps none of it cannot answer it.
A minimum and maximum memory is a cheap feature and its absence is the single biggest functional gap here. If the reason you are buying is to find out whether humidity went out of band overnight, this is not the instrument for the job, and the comparison to make is against the digital hygrometers that hold at least a daily extreme.
Twenty four vents buy response time and give up protection
The same bullet credits 24 sensitive vents, paired with the 10 second sampling, for updated readings. The physics is right. A humidity sensor can only read the air that reaches it, and a sensor behind a nearly sealed case responds to the room with a long lag, which is why cheap monitors in solid housings seem to ignore a window being opened.
The cost of a well vented case is that nothing sits between the sensing element and the environment. No ingress protection rating appears anywhere in the five bullets, and the applications named include aquariums, reptile tanks, hermit crab habitats and greenhouses, which are splash prone and sometimes condensing places.
Two practical consequences follow. Keep the unit well clear of the waterline and away from any misting or direct spray, because a vented case admits water as readily as it admits air. And expect dust to reach the sensor in a workshop or a warehouse, which is one of the ways a humidity sensor drifts over time.
A high humidity environment is also where a 10 to 99 percent RH range earns its keep. A reptile enclosure or a greenhouse can sit above 90 percent for long stretches, and the published range covers it, which is more than some monitors in this class manage.
The conversions are correct, which is not a given
Small credit where it is due. The working temperature range is printed as 0 to 70 C with 32 to 158 F alongside it, and those are the same two temperatures: 0 C is 32 F exactly and 70 C is 158 F exactly. The humidity range and the sampling interval are both stated as single unambiguous figures.
The one place the units slip is the tolerance. Temperature accuracy is given as plus or minus 1 F and never in Celsius, so a Celsius user has to convert: plus or minus 1 F is about plus or minus 0.56 C, which is roughly half a degree. That is a reasonable figure for an instrument of this type, and reading the display in Celsius does not make the instrument better than its Fahrenheit tolerance.
The battery claim is modest in a way that suggests somebody thought about it. One LR44 cell, up to 10 months of working life, and a second cell in the box as a backup. None of that is implausible for a small display, a low power sensor and a 10 second sample interval. Bear in mind that LR44 is an alkaline chemistry and alkaline cells lose capacity as the temperature falls, so an unheated greenhouse or a cellar will give you less than a living room does.
The bullets as a table, including the empty rows
| Specification | In the listing |
|---|---|
| Quantity in the pack | Two units |
| Temperature range | 0 to 70 C, 32 to 158 F |
| Temperature accuracy | Plus or minus 1 F, about 0.56 C |
| Humidity range | 10 to 99 percent RH |
| Humidity accuracy | Plus or minus 3 percent RH |
| Sampling interval | Every 10 seconds |
| Ventilation | 24 vents |
| Unit switching | Celsius and Fahrenheit |
| Power | One LR44 button cell, one spare supplied |
| Battery life | Up to 10 months per cell |
| Display resolution | Not stated in the listing bullets |
| Minimum and maximum memory | Not stated in the listing bullets |
| Logging or application link | Not stated in the listing bullets |
| Response time to a step change | Not stated in the listing bullets |
| Range over which the accuracy applies | Not stated in the listing bullets |
| Ingress protection rating | Not stated in the listing bullets |
| Stand, magnet or mounting | Not stated in the listing bullets |
| Physical size | Not stated in the listing bullets |
Two of those blanks deserve a word. The accuracy figures are given without saying over what part of the range they apply, which matters because humidity sensors of this class are usually at their worst towards the extremes, and the extremes are where a greenhouse and a reptile enclosure operate. And a response time is a different thing from a sampling interval: the interval says how often the reading refreshes, the response time says how long the sensing element takes to equilibrate with air that has changed. A 10 second refresh on a sensor that takes two minutes to settle still takes two minutes to settle.
Where a pair of these earns a place
Buy a pack like this to put a number where you currently have a guess, in places where being within a few points is enough: a propagator, a garage, a store cupboard, a terrarium you check daily, a spare room you suspect is damp. Two units means two rooms, or one room and one reference outdoors, and the vented case and 10 second sampling mean the readings track reality instead of trailing it.
Do not buy them to hold a tight band. A cigar humidor target sits in a window only a handful of points wide, and a tolerance of three points either way uses most of that up before the humidity has moved, so a pair of these will tell you when something has gone badly wrong rather than keep you inside a band. For that the instruments worth looking at are the ones with a calibration offset you can set and a minimum and maximum record, which we cover in our roundup of hygrometers for cigar humidors. And do not buy them for anything that needs a history, because nothing in this listing keeps one.
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Frequently Asked Questions
Will the two units in the pack read the same?
Nothing in the specification says they should. Plus or minus 3 percent RH means each unit may sit three points either side of the true humidity and still conform, so two conforming units can differ by six points. In air at 69 percent RH, one could show 66 and the other 72 and neither would be faulty. That is the most useful thing to understand about a 2 pack, because the first thing most people do is put both in the same place and compare them.
Is 3 percent RH good enough for a cigar humidor?
It is enough to tell you whether you are in the right region and not enough to hold a tight band. The usual target for a humidor sits in the high sixties with a few points either side treated as acceptable, and a tolerance of three points consumes most of that allowance before the humidity has moved at all. The honest use is as an indicator that something has drifted badly, with a salt test to establish each unit offset if you want to hold the band properly.
Does it keep any history?
Nothing in the bullets describes memory, a minimum and maximum record or logging of any kind. The listing states that the sensor measures every 10 seconds, which is a sampling interval rather than a record, so what you get is the current value. For a greenhouse, an instrument case or a wine cellar the question you usually want answered is what happened overnight, and a display showing only now cannot answer it. That is the gap to weigh against the convenience of a pair of cheap glanceable units.
What are the 24 vents for, and what do they cost?
They let air reach the sensing element quickly, which is why the listing pairs them with the 10 second sampling claim. A sensor behind a sealed case lags the room by minutes. The cost is that a vented case offers the sensor no protection, and no ingress rating appears anywhere in the bullets. The named applications include aquariums, terrariums and hermit crab habitats, which are humid and splash prone, so keep the unit above the waterline and out of direct spray.
Why is the temperature tolerance given in Fahrenheit only?
Because the specification was written that way, and it leaves Celsius users converting. Plus or minus 1 F is about plus or minus 0.56 C, so a Celsius reader should think of it as roughly half a degree. The range itself is given in both units and converts correctly, with 0 to 70 C matching 32 to 158 F exactly, so the conversions in this listing have been done properly even where the tolerance appears only once.
How long do the batteries last?
The listing states one LR44 button cell with a working time of up to 10 months, and a second cell supplied as a backup. That is a modest and plausible claim for a small display, a low power sensor and a 10 second sample interval, and shipping a spare is a sensible touch. LR44 is an alkaline chemistry, and alkaline cells lose capacity as the temperature falls, so expect less in an unheated greenhouse or a cellar than in a living room.
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