Zoo Med Analog Humidity Gauge
Part no.
TH-21
Warranty
6 months
Zoo Med Analog Humidity Gauge
Humidity is the parameter most keepers guess at. The Zoo Med analog humidity gauge (Zoo Med item TH-21) is a dial hygrometer reading the full 0 to 100% relative humidity range inside a terrarium. It runs on nothing, so it cannot go flat between checks, and it puts a number on the one husbandry variable that is invisible until the animal has a problem.
Temperature announces itself — you can feel a hot spot with the back of your hand. Humidity does not. By the time it shows, it shows as a stuck shed, a retained eye cap, or the early wheeze of a respiratory infection, and by then it has been wrong for weeks.
Key features
- Full 0 to 100% relative humidity scale — covers a bone-dry desert setup and a saturated rainforest vivarium on the same dial, so one gauge suits whatever you keep next.
- Unpowered — no battery to die quietly while you keep glancing at a frozen number.
- Dial readout — you read the trend, not just the figure. A needle sitting where it sat yesterday is information a digit does not give you as quickly.
- Repositionable — humidity is not uniform inside an enclosure, so the gauge needs to move while you work out where the honest reading is.
- Pairs with a thermometer — humidity and temperature are read together or not at all; warm air holds more moisture, so the same water content reads differently at each end of the gradient.
Specifications
| Zoo Med item number | TH-21 |
|---|---|
| Our SKU | ZM14 |
| Barcode | 097612300215 |
| Measures | Relative humidity (water vapour saturation of the air) |
| Range | 0 to 100% RH |
| Display | Analog dial |
| Power | None — no battery |
| Manufacturer warranty | 6 months |
Reading it honestly
A hygrometer reads the air touching it, at the moment you look. Mist the enclosure and the needle will spike to something impressive within a minute, then fall away over the next hour. Neither figure on its own is your husbandry humidity.
What matters is the reading between mistings, taken at the height and place the animal actually spends its time. A gauge stuck high on the glass beside a ventilation panel is reading the room, not the enclosure.
Zoo Med does not publish an accuracy tolerance for the TH-21, so treat it as an indicative instrument. That is not a criticism — it is what a dial hygrometer is for. You are watching a trend across weeks, and a trend does not need laboratory precision to be useful.
What the reading is telling you
Species requirements differ far too much for one number to be right — a central bearded dragon and a green tree python do not want the same air. Check the published requirement for your species, then use the symptoms below to sanity-check the gauge against the animal.
| What you are seeing | What it usually means | Where to look first |
|---|---|---|
| Retained shed, stuck eye caps, shed coming off in flakes | Air is drier than the species needs, at least during the shed cycle | Add a humid hide, increase water surface area, reduce cross-ventilation |
| Constant condensation on the glass, substrate staying wet | Too wet with too little air movement | Increase ventilation before you reduce misting — stagnant damp is the real problem |
| Needle spikes after misting then crashes within an hour | Nothing is holding moisture in the enclosure | Substrate depth and type, and a larger water bowl on the cool end |
| Needle barely moves at all, ever | Gauge is mounted in dead air or against cold glass | Move it to animal height, away from the vents and out of direct lamp light |
| Reading drops every winter | Household heating drying the room the enclosure sits in | Seasonal, not a fault — expect to adjust misting between June and August |
Suitable for, and not suitable for
Suitable for: glass and PVC terrariums, bioactive vivariums, gecko and tree frog enclosures, python tubs, tortoise tables, hermit crab crabitats, and invertebrate setups — anywhere a permanent unpowered humidity read is wanted.
Not suitable for: logging humidity while you are away, controlling a fogger or mister, submerged use, or as the sole instrument where a species has a narrow and critical humidity requirement. It also will not measure substrate moisture, which for burrowing species matters as much as air humidity and has to be judged by hand.
Which Zoo Med gauge do I need?
| Gauge | Measures | Reads out | Choose it when |
|---|---|---|---|
| Analog Humidity Gauge (TH-21) | Humidity | Analog dial, no battery | Humidity is the parameter you are chasing, not heat |
| Analog Reptile Thermometer (TH-20) | Temperature | Analog dial, no battery | You want a permanent unpowered temperature read at one point |
| Creatures Dual Hygrometer & Thermometer | Temperature and humidity | Single unit | You want both parameters from one mounting point in a small enclosure |
| Digital Min/Max Thermometer | Temperature, with recorded highs and lows | Digital, battery | You need the overnight low or daytime peak you were not there to see |
| Creatures Digital Thermometer | Temperature | Digital, battery | You want a numeric read rather than a dial |
Care and maintenance
Keep the face clear. Condensation on the inside of the lens in a high-humidity vivarium is normal and clears as the enclosure dries; substrate dust on the outside is what actually stops people reading it.
Wipe with a dry cloth only. Solvents and glass cleaners craze the lens and can get into the sensing element.
Expect a dial hygrometer to drift over years rather than months. If the needle disagrees badly with a second instrument, replace it rather than trying to recalibrate it.
Goes well with
- Zoo Med Analog Reptile Thermometer — the matching dial for temperature; the two readings only mean something together.
- Zoo Med Repti-Fogger — what you reach for when the gauge says the enclosure will not hold moisture between mistings.
- Zoo Med Repti Rock Water Dish — a larger water surface on the cool end is the simplest way to lift a low reading.
- Zoo Med ReptiSafe Water Conditioner — for treating the tap water going into the bowl and the mister.
- Zoo Med ReptiCare Ceramic Heat Emitter — worth knowing that a ceramic emitter dries an enclosure faster than most keepers expect.
Warranty and delivery
Zoo Med publishes a 6 month warranty on this gauge. That warranty sits in addition to your rights under the Australian Consumer Law and does not limit them.
Standard freight. Small, light, no special handling.
Common questions
What range does the Zoo Med analog humidity gauge read?
The Zoo Med TH-21 analog humidity gauge reads the full 0 to 100% relative humidity scale, which covers everything from an arid desert terrarium to a saturated rainforest vivarium. It measures relative humidity — how close the air is to holding all the water vapour it can at that temperature.
Where should I mount it?
At the height the animal spends its time, on the cool half of the enclosure, away from ventilation panels and out of direct lamp light. A gauge mounted high near a vent reads the room air, and a gauge sitting under a basking lamp is warmed by the lamp and reads artificially dry.
How accurate is it?
Zoo Med does not publish an accuracy tolerance for the TH-21, so it should be read as an indicative instrument rather than a calibrated one. For its actual job — spotting that an enclosure has drifted drier or wetter than it was last month — that is enough. If a species has a narrow critical requirement, verify against a second gauge.
Why does the reading jump after I mist?
Because it is measuring the air correctly. Misting saturates the air briefly and the needle follows it up, then falls as the moisture settles onto surfaces and vents away. The number worth recording is the one between mistings, not the peak.
Do I need one if I already have a thermometer?
Yes, if you keep anything other than a true desert species. Warm air holds more moisture than cool air, so the same amount of water in an enclosure produces different relative humidity at each end of the temperature gradient. A thermometer tells you nothing about whether an animal can shed properly.
