External Temperature Gauge Inaccurate

Discussion in 'Electronics' started by Max Shine, Dec 12, 2016.

  1. Max Shine

    Max Shine Hardcore MB Enthusiast

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    The external temperature indicator on my 2012 W212 today was reading 5c when infact it was 2c, according to the weather on the radio (which prompted the query) and on checking various weather apps and online, they too said it was 2c.

    Is there a calibration required or faulty sensor?
     
  2. grober

    grober MB Club Veteran

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    Have a look at the STEVENSON SCREEN entry in wikipedia
    https://en.wikipedia.org/wiki/Stevenson_screen
    This is used to shield temperature measuring instruments from the direct effects of wind and solar radiation and is the basis for many metrological measurements taken at weather stations round the UK. I can only suggest that the discrepancy may be due to solar radiation [ even on an overcast winter's day it has an effect witnessed by temperature changes from night to day] Not sure where your outside temperature gauge is located but that might explain it--- that and a likely tolerance of at least +/- 1 degree if not more
    Puting it another way-- your car temperature sensor [ within its accuracy limits] and the reported temperature are both correct- they are simply measuring different temperatures in their particular location.
    ps I have no idea about how accurate Mercedes outside temperature gauge is when its working properly but gave them the benefit of the doubt.
     
    Last edited: Dec 12, 2016
  3. Larkone

    Larkone Hardcore MB Enthusiast

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    Any weather report is only going to give you the average for your area. It cannot take into account local conditions which can vary a lot. A recent trip I took to my local garage gave me temps from 0C to -4C depending if I was on a rise or in a dip. Best test is to get a known accurate thermometer and test it against that.
     
  4. ernie

    ernie Hardcore MB Enthusiast

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    Our current car gives outside temp. readings that are consistent with those published for the area and also agrees with my own outside thermometer.

    Out of the other 4 Mercs we have had, one of them was +2 on our own thermometer, another was +3, the third was +1 and the final one gave the same reading.

    However, it has to be said that all 5 cars could take a bit of time to settle whether in the sun or even during frosty conditions.

    Our local dealership changed the sensor on the +3 car as he said that was out of spec., but +1 was within the range expected.

    We didn't have the +2 car long enough to get it checked!

    My main concern would be that it should give adequate warning of temperatures approaching freezing and therefore black-ice. Incidentally, why Merc can't give an audible warning at low temperatures as VW do, is beyond me!


    Ernie
     
  5. Larkone

    Larkone Hardcore MB Enthusiast

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    Ice can form at 4C and that would depend on local conditions. Personally I am cautious as soon as temps drop to 5/6C because a dip in the road could harbour temps that could cause ice. Based on what you said about your previous vehicles then you have answered your own question - back to the stealer.
     
  6. OP
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    Max Shine

    Max Shine Hardcore MB Enthusiast

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    Given the local radio gave the temperature of the exact location I was at during the time and this temperature was also the same on 2 weather apps and also what the Met Office were reporting. Goes without saying that the temperature of 2c from numerous sources and all being the same was a good reflection of the actual temperature, not the 5c my car was reading after 45min journey
     
    Last edited: Dec 12, 2016
  7. Ted

    Ted MB Club Veteran

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    On my A208, the external temp always read a fee degrees warm.
    Found that it had 'fallen out' of the hole that it was meant to be in.
    Putting it back in place sorted the problem.
    In mine it was a sort of rubber bung in the front spoiler - low down and out if sight unless you got right down.
     
  8. OP
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    Max Shine

    Max Shine Hardcore MB Enthusiast

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    That would be the next question; where the sensor is!
     
  9. Yugguy

    Yugguy Hardcore MB Enthusiast

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    The temperatutre on tv/radio is for a wide area, there can be a vast difference in localised temperature.
     
  10. KillerHERTZ

    KillerHERTZ Administrator Staff Member

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    It will be sticking out of one of the front vents, it will be a black needle.
    Highly likely that it has been dislodged and is getting some heat from the engine area.
     
  11. whitenemesis

    whitenemesis MB Club Veteran

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    Be careful quoting temps from various sources as the majority get the information from the same place, the met office.
    When did you take note of the car temperature? When stopped after your journey? There's like to be heat soak coming from the engine. WIS states somewhere the temperature is only accurate when the car is moving, forward.
     
  12. BTB 500

    BTB 500 MB Club Veteran

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    You always get a higher reading when the car has been standing warm for a while ... the sensor is only a few cm from the engine compartment so heak soak is inevitable. The reading drops back down when you drive off.

    I would echo what others have already said ... I had readings of between -1 and -4 last week on a 5 mile drive along a single stretch of A road. Regardless of forecast the temperature varies a lot depending on how high/sheltered/busy/etc. a particular location is. Also how high above the ground you take the temperature reading.
     
  13. Larkone

    Larkone Hardcore MB Enthusiast

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    and we are in the UK so the only accurate forecast is 'there will be various types of weather in all sorts of places'
     
  14. OP
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    Max Shine

    Max Shine Hardcore MB Enthusiast

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    Temperature was whilst driving
     
  15. OP
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    Max Shine

    Max Shine Hardcore MB Enthusiast

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    The local radio station quoted the temperature of Tollbar Island, Coventry (basically a massive roundabout where the A45 and A46 converge) where they have a weather station. I was at Tollbar Island at the same time, had been travelling for 45 minutes or so and was not stationary.

    Negating the debate of other factors discussed, I would presume there is a sensor fault, or indeed it has dislodged. I do not subscribe to the inaccuracy of radio, Met Office, websites etc being incorrect.
     
  16. RichardF

    RichardF Hardcore MB Enthusiast

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    A lot of traffic and congestion around Tollbar Island roadworks, while the underpass is being constructed. Is the meteorological station actually on the island or the adjacent airport? And is it at road level or higher up measuring air temp? Did they give the time the temp was taken?

    I can get a 3 degree variation in my back garden.

    Not sure whether your car sensor is accurate or not, but the methodology, repeatability and accuracy of measurement is flawed. Get a cheap infrared thermometer and check, it'll be hours of fun. :D

    ETA don't forget your outside temp gauge is damped to stop it flicking up and down all the time.
     
    Last edited: Dec 12, 2016
  17. OP
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    Max Shine

    Max Shine Hardcore MB Enthusiast

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    Thanks everyone for the advice. I have located the sensor which points upwards just in front of the radiator (Don't know if this is correct). When its dry and daylight, Ill take a proper look at it. Just the first car I have had where there is a notable discrepancy.
     

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