The alert says 82 degrees. That is either completely fine or the worst news of your afternoon, and the number by itself cannot tell you which — because a rig sitting at 82 and holding is a different animal from a rig that was at 74 twenty minutes ago.
Two things turn that single reading into something you can act on. How fast it is moving, and what the humidity beside it is doing. Neither is a metric anything broadcasts, and both are available from a sensor you already own.
The pet safety page is where the stakes and the placement live, and the air conditioner article is how to tell whose fault a stoppage is. This is the narrower thing: reading the rise.
The ladder is the rate instrument
There is no rate-of-rise rule. Rules fire when a reading crosses a fixed line, and no line describes a slope. So the substitute is deliberate: set two thresholds with a known gap between them, and time how long the rig takes to get from one to the other.
An example pair with pets aboard: warn at 80°F, critical at 88. Eight degrees apart, so the interval between the two messages is a rate you can read off your own phone without any arithmetic beyond subtraction. Under twenty minutes between them is roughly eight degrees in twenty minutes, which on a summer afternoon means no air conditioning and full sun. Two hours between them means the compressor is running and losing, which is a completely different afternoon.
The gap is the instrument, so choose it deliberately. Too close together and both alerts arrive at once and tell you nothing. Too far apart and the second one is academic. Eight degrees is a useful default because it is wide enough to time and narrow enough that the critical rung still lands while there is something to do about it.
Two honest limits. Nothing fires on “rising fast but still under 80” — the first rung always has to be crossed before you learn anything, which is why the readings in the next section matter. And a sunbeam landing on the sensor can do eight degrees in twenty minutes entirely by itself, which is what a second sensor somewhere shaded is for.
Humidity is the cheapest air-conditioner sensor you own
This is the one people do not know, and it costs nothing because the sensor already reports it: a running compressor dries the air. That makes the humidity line beside the temperature line the best indication of whether the air conditioner is actually working that you can get without a power monitor.
Temperature over your warning line with humidity climbing means the compressor is not running. Temperature over the line with humidity falling means it is running and losing — sun load beating a unit that is still doing its job. The first is a phone call about a broken air conditioner. The second is shade, an awning, or a lower ceiling next time.
Set it as a rule at 65 per cent warn and 75 critical, and treat those as a reason to go and look at the temperature line rather than as an alarm on their own. Humidity alerts on plenty of harmless afternoons — a shower, a kettle, cooking, rain with a vent cracked, a wet dog. Cheap capacitive sensors also drift and read a few points apart from each other, and the Jinou‘s humidity in particular is read from a stable byte position rather than checked against a reference, so it is a trend and not a hard line.
Humidity also changes what the temperature means
Dogs cool by panting, and panting stops working as the air gets wetter. Eighty-five degrees at seventy per cent humidity is not the same eighty-five degrees as a dry day, and the difference is large enough to matter.
There is no heat-index rule, so the arithmetic goes into the threshold. Temperature and humidity are two separate metrics and nothing combines them — the app will never tell you what a reading “feels like”. On a muggy day, lower the temperature ceiling instead: 80°F rather than 85, set the night before, changed back when the weather does. That is a five-second edit doing a job no rule can.
What is safe for your particular animal is a question for your vet and not for us — breed, age, weight and coat all move it, and a flat-faced dog is in trouble at readings a spaniel would shrug off. The pet page links the veterinary guidance.
The combinations
The ladder gives a rate, humidity gives a cause, and the pedestal gives a head start. The cookbook pattern, with a clock attached.
- Pedestal monitor drops at 13:02, warn at 13:31, critical at 13:50 — eight degrees in nineteen minutes, and twenty-nine minutes of head start you would not have had from a thermometer. That is the case for a shore power monitor in one line.
- Warn and critical inside twenty minutes, pedestal reading a healthy 120 volts — the air conditioner itself failed and the rig is baking. Do not wait for the second rung to phone somebody; the first one was the call.
- Warn fired, no critical two hours later — cycling and losing rather than dead. Shade, awning, or a lower ceiling tomorrow.
- Temperature up and humidity up together — the compressor is not running. The strongest reading you get without a power device.
- Temperature up, humidity down — running and losing. Not a service call.
- Humidity over its line with the temperature comfortably under its ceiling — somebody showered, or it is raining with a vent open. Not a heat problem.
- Inside within about three degrees and a few per cent of an outdoor reference on a hot day — the rig is simply at ambient. The air conditioner is off, or a door is open.
That last one wants a sealed sensor outside, in the shade under the rig or in a vented bay. A RuuviTag is the one we would use, because the enclosure is sealed and it holds its calibration. It carries no alert rule of its own — it exists so that “inside equals outside” becomes visible at a glance.
What none of this tells you
When the rig will become dangerous. The ladder measures a rise that has already started; it does not predict one. Nothing alerts on a fast rise before the first rung is crossed, and no reading anywhere knows how the animal in the rig is actually coping.
And if the hub is on a pedestal-fed outlet, an outage takes the whole apparatus with it — no readings, no logging, and an offline alert as the last thing you hear all afternoon. On 12 volts the same outage arrives as a shore power alert from a hub that is still watching the cabin warm up, which is the entire difference.
What to do about it
Set two temperature thresholds about eight degrees apart rather than one, and turn on the humidity metric that the same sensor is already reporting. Neither costs anything and between them they turn one number into a rate and a cause.
Then lower both rungs on a humid day, because nothing will do that arithmetic for you.
And the part no threshold reaches: the head start is only worth what somebody does with it. If the honest answer to “who goes when the first rung lands” is nobody, the missing piece is a person with a key, not another sensor.
Where to get one
Everything above runs on sensors bought for other reasons — the second threshold and the humidity metric are free. The only purchase this page argues for is a sealed one to put outside. The compatibility database lists which sensors report humidity at all, which is worth checking before buying the cheap one.
RuuviTag
The one to buy when the reading has to be right. Open published protocol, sealed enclosure, and it holds calibration in a way the cheap sensors do not.
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Govee H5075
The small white puck a lot of RVers already own. It broadcasts continuously, so the hub reads it without pairing and without touching the Govee app.
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Hughes Power Watchdog (30 amp)
For a 30 amp rig, which is most travel trailers and smaller fifth wheels. Reads voltage, current, power, energy used, line frequency and error codes over Bluetooth, so a pedestal disconnect reaches your phone instead of only its own screen.
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