[Ugly Mongrel]

At the RAAF base at Amberly (west of Brisbane) the outside temperature at 2:30pm was 47C. It was a stinking hot day and it isn't even the summer months yet

When I got in, I started my car and put the air conditioner on and made a quick phone call before I got moving. In the few minutes I was on the phone, the outside temp gauge dropped from 47 to 41. When I got out on the open road, the temp stayed at 41.

Can anyone explain that 6 degree drop in a couple of minutes while the car sat idling??

[Dazzler]

I noticed that with the rental i30 in Qld recently.. it was 39 when we got in in Bundaberg by the time I took a photo it was 37 and I think it went down to about 32 before stabilizing..

I assumed the sensor (behind the grill apparently) must have had direct sun on it or something 

[Ugly Mongrel]

I noticed that with the rental i30 in Qld recently.. it was 39 when we got in in Bundaberg by the time I took a photo it was 37 and I think it went down to about 32 before stabilizing..

I assumed the sensor (behind the grill apparently) must have had direct sun on it or something 

I don't think the sun on the sensor was an issue, Dazzler. I didn't move the car during the time it dropped 6 degrees, just started the motor, turned on the aircon and sat in the car.

[rustynutz]

Maybe with the engine running the fan was blowing or drawing air past the sensor....

[Ugly Mongrel]

Maybe with the engine running the fan was blowing or drawing air past the sensor....

Exactly what I thought rustynutz, it's just I hadn't noticed it before, but then, 47C is not a temp I've seen too often.

[eye30]

I think this has been asked but where is the sensor located as if it is by the fan any air being drawn in could cause the sensor to reduce its reading (as per rustynutz)
 But again wouldn't it draw in hot air?

[diablo]

I've noticed this a couple of times recently. When I start the car in the drive (which is sheltered by a wall and large shrubs) then it has tended to read around 15C. After driving for a minute or two the temp displayed has come down to 13 or 12, which is rather more accurate.

Can't help with an explanation though.

p.s. I thought the fan was electronically controlled and only came on when the water temp rose somewhat - or am I imagining that ?

[Lakes]

This issue with the outside temp, has been there with both my i30's. if they did not act like that i would start to worry
But this morning i'll lift the hood b4 driving off & with motor runing & see if thermo fan is on or off at start up.
years ago i bought a temp gauge from dick smith, it came with two sensors one for inside one for outside, dirrections said mount outside one out of direct sunlight, i think i mounted it  on one of the doors near the hinge. worked well.

[Phil №❶]

This is actually a complicated issue, as there are many contributing factors to be considered.

1. If you were to look using an infra red camera, you would see all the possible heat sources surrounding the sensor. They typically would be - Radiation from the road - Radiation from the radiator / condenser, if they had been used.

2. We know that heat rises, so convective heat from a hot road surface will permeate into the metal of the car & sensor.

3. Finally, the movement of air cased by the thermo electric fan will cause a temperature drop due to "wind chill factor". This is further accentuated by the humidity at the time. A more dramatic effect occurs as the car is driving as the wind shear over the sensor becomes more extreme. This proves that the sensor is obviously quite sensitive to temperature change, which is what we want.

Having driven motorcycles in the past, I have encountered significant temperature changes even on 30 Deg C days by driving into valleys. This is an important factor for those living in colder climates where ice on the road is an issue. So after all that, I disregard the temp display until the car is moving forward at road speed.   

[Ugly Mongrel]

Thanks for the replys everyone.

[eye30]

Thanks for the replys everyone.

How is it now.

Have you been able to replicate?

[Lakes]

i checked & on start up in morning thermo fan does not activate, so that could not effect temp sensor. but would expect if temps were up in 40c the fan would soon activate on idle just not till motor warms fully. diesels take longer to warm up very thick heavy block & more efficent

[Phil №❶]

Even in cold weather, the fan should come on at start up if the A/C is on and dehumidifying is required. This used to happen when making various adjustments and I found it annoying. It is possible to cancel this function & so I have. Now I select when A/C is on or not at all times. 

[diablo]

Even in cold weather, the fan should come on at start up if the A/C is on and dehumidifying is required. This used to happen when making various adjustments and I found it annoying. It is possible to cancel this function & so I have. Now I select when A/C is on or not at all times. 

I'd expect the aircon fan to come on if the dehumidifier is on, but not the radiator fan. 

To be efficient the engine needs to get up to running temperature as fast as possible and I wouldn't expect the aircon setting to make any difference to that.

[Phil №❶]

Either way, there is movement of air and the sensor is obviously, quite sensitive.

[FatBoy]

I've been thinking about this for a couple of days.  I get the same thing at work (must be Defence sites!!).  What I can think of is the engine bay area is hotter than outside when the car is doing nothing but sitting in the sun (a lot like inside the car).  eg. The air "outside" was 41, but under the bonnet/hood/engine bay it was 47.  When you put the air conditioner on, the fan that sits next to the a/c condenser comes on straight away (this gets hot very quickly as you are compressing a gas).  This fan drew air from outside the engine bay, so the temp then registers the air temp that was outside.

Basically, I don't think I've added anything extra that hasn't been said before.  So I've just wasted five minutes.  Oh well. 


FatBoy

[2i30s]

I've been thinking about this for a couple of days.  I get the same thing at work (must be Defence sites!!).  What I can think of is the engine bay area is hotter than outside when the car is doing nothing but sitting in the sun (a lot like inside the car).  eg. The air "outside" was 41, but under the bonnet/hood/engine bay it was 47.  When you put the air conditioner on, the fan that sits next to the a/c condenser comes on straight away (this gets hot very quickly as you are compressing a gas).  This fan drew air from outside the engine bay, so the temp then registers the air temp that was outside.

Basically, I don't think I've added anything extra that hasn't been said before.  So I've just wasted five minutes.  Oh well. 


FatBoy

so basically your saying that all defence sites are full of hot air slim.    but to be serious mate,i can get in my car after work and the temp says 40c but a few kms down the road its only 30c,it must be the Temp of the concrete/bitumen  the cars parked on. 

[FatBoy]

I have been accused of being full of other things!!  Agree with the temp raise though, it is air being trapped in the engine bay until air flow (either fan or car movement) causes the outside air to flow past the sensor.  I'll ask the guys at work (I actually do work with Rocket Scientists, sometimes they are so smart they forget how to dress!!)

FatBoy

[2i30s]

I have been accused of being full of other things!!  Agree with the temp raise though, it is air being trapped in the engine bay until air flow (either fan or car movement) causes the outside air to flow past the sensor.  I'll ask the guys at work (I actually do work with Rocket Scientists, sometimes they are so smart they forget how to dress!!)

FatBoy

they forget their white coats? 

[Dazzler]

So I've just wasted five minutes.  Oh well. 

  So you are not a very fast Typist then ...

[2i30s]

So I've just wasted five minutes.  Oh well. 

  So you are not a very fast Typist then ...

a one finger man like myself. 

[eye30]

So I've just wasted five minutes.  Oh well. 

  So you are not a very fast Typist then ...

a one finger man like myself. 

 
Thinking out load:
I wonder if he knows what that means here in the UK?

[FatBoy]

So I've just wasted five minutes.  Oh well. 

  So you are not a very fast Typist then ...

Oh, I can type fast!!  I just think very, very, very.....  Where is my beer?   

[2i30s]

So I've just wasted five minutes.  Oh well. 

  So you are not a very fast Typist then ...

Oh, I can type fast!!  I just think very, very, very.....  Where is my beer?   

[2i30s]

So I've just wasted five minutes.  Oh well. 

  So you are not a very fast Typist then ...

a one finger man like myself. 

 
Thinking out load:
I wonder if he knows what that means here in the UK?

i should have worded that a bit better. 

[FatBoy]

Okay, I finally got around to checking with the boffins at work (yes, they actually are Rocket Scientists) about why the temp would do the things that it has been doing.  They agreed with what we have all been saying, but in different ways.  The air in the engine bay is being heated in some way, either by the road below, or the sun on the bonnet.  Once the fan for the aircon starts up, it draws "free air" from outside of the car into the engine bay and therefore past the sensor, hence a more accurate reading for the actual outside air temperature.

The temperatures that you see on the TV each night is the "shade" temperature, taken from a special box that has air flowing around it all the time.


FatBoy

PS. I still want to know where my beer is!!!

[Phil №❶]

So, we must ALLl be as smart as rocket scientists then. 

[Dazzler]

 

So, we must ALLl be as smart as rocket scientists then. 

[FatBoy]

So, we must ALLl be as smart as rocket scientists then. 

They are all smart, some of them even drive i30s!!  (So I suppose that means that some of them are as smart as us)  Some of them are super intelligent, but I wouldn't try to talk to them, they get scared by conversations.

[mjt57]

I decided to check mine against a couple of different instruments at work. The ambient is around 23.5 degs and is measured in 3 different locations around the site.

I took a Raytek handheld temperature measuring device down to the carpark. I checked the temp of a few surfaces including the road, a car's door, and under the left guard of the i30.

Roadside and surface temps were all around the 23-24 deg mark. However, under the left wheel arch it read 27 degs, the same as what the Outside Temp display is reading.

I started the car and drive forward about 20 metres and backed back into my parking spot. The temp didn't change.

So, I'm guessing that under bonnet temp is affecting the reading and that it would only start to get more accurate as the car is moving at higher speeds.

I would've thought that maybe placing the sensor further away from the engine bay, say, under the front or rear bumper, would've given a more accurate reading.

I'll wait til near morning when the engine/bay has cooled right down and I'll check it again. I'll post the results here.