External air temperature reading inaccurate

Discussion in 'Prime Main Forum (2017-Current)' started by jb in NE, Jul 19, 2019.

  1. jb in NE

    jb in NE Senior Member

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    On several occasions recently (frequently), I note that on my Prime when in EV the ambient (outdoor air) temperature does not report correctly. A typical case - park car in garage at about 82F, on a 96F day. Leave the garage in EV mode, and VERY slowly the ambient air temperature creeps up to eventually match the actual temperature (as noted on a weather app). By slowly I mean 10 miles of driving.

    I suspect that this is due to the location of the temperature sensor. If it is under the hood and the air intake flaps are closed (as you would expect them to be without the ICE running), this could explain it.

    Has anybody else noticed this on a Prime?

    Here is the second half of a drive today (the Hybrid Assistant crashed and didn't start when I left the garage, I caught it when the car was sitting while I was making a short stop).

    upload_2019-7-19_17-2-39.jpeg

    When I returned home, the dash display was showing 100F ambient, shortly after I pulled into the garage

    2019-07-19 16.23.46.jpg
     
    #1 jb in NE, Jul 19, 2019
    Last edited: Jul 19, 2019
  2. Marine Ray

    Marine Ray Senior Member

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  3. bisco

    bisco cookie crumbler

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    i've had the same on all three of my prius, none with flaps. somedays, i can be driving for over half an hour, and it will read in the 80's on a 94 degree day.
     
  4. eskudo12791

    eskudo12791 Junior Member

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  5. bisco

    bisco cookie crumbler

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    but the article says it will read hotter than actual temp on hot days(n)
     
  6. jb in NE

    jb in NE Senior Member

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    Thanks. I should have checked before posting a new similar thread.
     
  7. sam spade 2

    sam spade 2 Senior Member

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    Depends on the actual situation but most of our travel these days is on BLACK asphalt and if the sun is shining the temp above the road surface can be 20 degrees hotter than the "official" temp......or more.

    Every car I've ever owned with an outside temp display can't be trusted.
    It IS useful in the winter time though to note when it gets around 35F and you can expect icing on the road.
     
  8. jb in NE

    jb in NE Senior Member

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    My last VW was quite accurate, as is our Subaru Outback.
     
  9. Trollbait

    Trollbait It's a D&D thing

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    None of my cars have had an accurate thermometer.
    For an ICE car. An EV doesn't throw off as much waste heat to skew the sensor.
     
  10. a_gray_prius

    a_gray_prius Rare Non-Old-Blowhard Priuschat Member

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    How does the reported outdoor air temp compare to the IAT (Intake Air temperature)? It wasn't on your chart.
     
  11. sam spade 2

    sam spade 2 Senior Member

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    IF......the sensor is properly located AND the vehicle is moving.....the "waste heat" shouldn't make any difference.
    But in ALL cases, the road makes "waste heat" which is impossible not to measure.
     
  12. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    None of my cars (including an Outback) have ever read accurately in the first 10-15 minutes of operation.

    All of them were accurate outside of that time range. I never really paid attention to whether the errors were consistently high or low.
     
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  13. ETC(SS)

    ETC(SS) The other One Percenter.....

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    Outside air temps are measured with a simple, cheap thermistor.
    They’re VERY accurate.
    They’re NOT very responsive, IIRC.

    Plus...how often are they being polled?
    How often is the display in the cabin bring updated?

    Do what everybody else does.
    Look at your phone.
    Those temperatures use (mostly) ASOS and NWS data which also use thermistors, but since they’re already outside then thermal inertia works both ways and you get a more or less accurate reading, depending on how far you are from the sensor.
     
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  14. fuzzy1

    fuzzy1 Senior Member

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    How does a weather app know the temperature inside a particular garage with any reasonable degree of accuracy?

    In my environment, I'm not the least bit surprised to see that much actual thermometer difference between my garage and the nearest major official weather station that would be reporting to the apps.
     
  15. ETC(SS)

    ETC(SS) The other One Percenter.....

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    IIRC my phone displays the local ASOS (airport) data.
    Rest assured, it’s “fairly” (very) accurate but air temperatures are going to vary quite a bit even in the same small town because the source of much of the Earth’s heat is pretty far away - and that heat is being inequality distributed.
     
  16. jb in NE

    jb in NE Senior Member

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    Inside the garage, I have a thermometer installed that feeds through my home automation system to my phone (and I can read the display, which is right in front of the car). The weather app shows me the external temperature only, not the garage. Thus, I know the temperature in the garage is 82F and the temperature outdoors is 95F (Wunderground feed from a nearby neighbor).
     
  17. jb in NE

    jb in NE Senior Member

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    Hybrid Assistant didn't record this parameter. I don't know if this is because the ICE was not running or if that just isn't one of the parameters that software records.
     
  18. Trollbait

    Trollbait It's a D&D thing

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    Which is what I actually meant by my cars not being accurate. Beyond that time, I just use the display to see shifts in temperature.
    The car's measurement of fuel consumed is also accurate. The computer knows fuel flow rates and time injectors are open. So it knows how much is used, even if it doesn't display it;).

    Cars are very poor platforms for measuring out side temperatures.
    From How to Measure Temperature Correctly | WeatherWorks
    1. Place the thermometer 5 feet above the ground (+/- 1 ft.). A thermometer too low will pick up excess heat from the ground and a thermometer too high will likely have too cool of a temperature due to natural cooling aloft. 5 ft. is just right.

    2. The thermometer must be placed in the shade. If you put your thermometer in full sunlight, direct radiation from the sun is going to result in a temperature higher than what it should be.

    3. Have good air flow for your thermometer. This keeps air circulating around the thermometer, maintaining a balance with the surrounding environment. Therefore, it is important to make sure there are no obstructions blocking your thermometer such as trees or buildings. The more open, the better.

    4. Place the thermometer over a grassy or dirt surface. Concrete and pavement attract much more heat than grass. That is why cities are often warmer compared to suburbs. It is recommended to keep the thermometer at least 100 ft. from any paved or concrete surfaces to prevent an erroneously high temperature measurement.

    [​IMG]
    5. Keep the thermometer covered. When precipitation falls, you do not want your thermometer to get wet as that could permanently damage it. A Stevenson screen is a great place to store thermometers and other instruments as they provide cover as well as adequate ventilation. If you can’t get one, a simple solar radiation shield is adequate.

    I'll add to 5 that a wet thermometer is actually measuring the temperature of the water, which could vary from the air.

    A parked car's thermometer is violating three of those rules, depending on where the thermistor is mounted.

    A Scangauge does, and it is a basic variable the ECM is monitoring, so the data is readily available for any other gauge type application. The Scangauge would display an 'ambient' temperature when the engine wasn't a running.

    IIRC, the air intake temperature was always higher than the air's. The senor is part of the airflow sensor, which is mounted downstream of the engine air filter box, and can be sitting over the engine itself.
     
  19. bisco

    bisco cookie crumbler

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    the only time i see a difference in asphalt temp is on a busy highway, and the later in the day, the hotter. it comes down fairly quickly after getting off.

    i don't notice much difference on sunny side roads.
     
  20. jb in NE

    jb in NE Senior Member

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    This may be the case, but I'm using Hybrid Assistant software to see the trip data. If it doesn't record this parameter, it won't appear in the Hybrid Assistant reports.
     
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