Outside temperature sensor.

Hi to all & A merry xmas,
We had a scary moment last week coming off the snake pass going down to Glossop. Just going down past the golf course I took my foot off the pedal & the van kept going..ice on the road & I don't know how I got round the corner without piling it up ! Not good.
So I decided to fit an external thermometer From Maplins part no N76GQ [You must be registered and logged in to see this link.]

The point is, where is the best place to mount the probe? The unit comes with 1.5 metres of cable I'm probably going to put it somewhere as close to the road as possible , has anyone else fitted something similar to their vans?

As it was, we stayed at Burrs country park for the weekend & had a wander round Bury on the Saturday, Then watched the sex pistols experience at Hark to Towler..Riot!

Tim

Tim - horrible feeling isn't it? I hit black ice in a dip in an otherwise clear road recently and that sudden feeling of no resistance at the steering wheel is most unpleasant. Feet off the pedals, just fingertips on the wheel, and hope you're not going in a ridiculous direction when you get to the other side...

All of the installations that I've looked at or done myself have it mounted behind the front bumper, so that it's low but not so low that it gets covered in road salt, and also so that there is a body panel between it and the radiator and other hot engine bits.

WOW!! Lucky lucky eh? There must have been a bit of skill involved though!

We got an "el cheapo" one from Lidl late last year and so far all I have done is run the external probe up through the skylight in the toilet/shower and have the read-out on the work-surface above the fridge.

The low temperature alarm is set for plus three degrees. The one time it triggered when we were on the move Helen heard it (she sits in the back when we are rolling) and even though the probe is two metres up in the air I assumed that ice was possible and drove accordingly.

Looking at the one you have bought I think the key would be to make sure that you can see it easily (because it doesn't seem to have an alarm) and if possible to fit the probe as low down as possible (a "ground frost" happens just where the name implies) in a protected position (as recommended by mojo) and in front of the engine (to stop any warm air affecting the reading).

Hope this helps.

Best regards,

I have one of these:
[You must be registered and logged in to see this link.]
and the probe is in the void between the inner and outer drivers side wings where it won't get any windchill but will still feel the cold.

Frank

boxerman wrote:I have one of these:
[You must be registered and logged in to see this link.]
and the probe is in the void between the inner and outer drivers side wings where it won't get any windchill but will still feel the cold.

Frank

Cheers for that Frank, much easier to see than the one I got from Maplins...Especially when its dark in the cab. I'll buy one & let you know
how I get on with it.
Merry Xmas.

Tim.

boxerman wrote:where it won't get any windchill but will still feel the cold.

BTW Frank - you don't need to worry about wind chill for a temperature probe. Wind chill only applies to skin, not to inanimate objects!

In fact having it in free airflow will make it reach the correct surrounding air temperature quicker - but does make it more vulnerable to road spray.

I've fitted several ice warning devices /outside temperature gauges to vehicles over the years and the ones that came with instructions always said to mount the sensor out of the airstream to get an accurate reading.

Frank

boxerman wrote:I've fitted several ice warning devices /outside temperature gauges to vehicles over the years and the ones that came with instructions always said to mount the sensor out of the airstream to get an accurate reading.

It wouldn't be the first time that people who write instructions have included things that are "common knowledge" even when they are not factually correct!

Fact is that we experience "wind chill" because of a combination of two things - an internal heat source and the constant evaporation of moisture through the skin. A temperature sensor has neither of those, so it cannot get colder than ambient by placing it in a flow of air that is at ambient temperature. In fact in extreme conditions the opposite can happen, and air friction can cause it to get warmer than ambient, though you have to be going seriously fast for that effect to become measurable!

However, did I mention that placing it out of the airflow ~does~ prevent or reduce the effect of road spray and crud accumulating on it?

Sorry! not convinced, you do it your way, I'll do it mine.

Frank

The sensor can be in the air flow, but the key thing is that the sensor must be kept dry. If it is damp then evaporation will depress the temperature of the sensor below the ambient temperature in a manner identical to the effect of evaporation on human skin. If there is air flow over it then the rate of evaporation will be increased accordingly.

This is the principle of the wet and dry bulb hydrometer or psychrometer. In this instrument the temperature of a wet bulb thermometer (which has a wet wick over its bulb) is depressed by an amount proportional to the relative humidity: the dryer the air the greater the evaporation on the wet bulb and the greater the depression. The more accurate instruments such as the Assman aspirated hygrometer generate a greater wet bulb depression by passing air over the wet bulb at a higher, constant velocity. In other words, as far as the sensor is concerned, if it's wet, and the relative humidity is less than 100%, it will read low of ambient. If it's wet and got air going over it, then the sensor will read significantly lower than ambient. If it's dry, in all cases it will read the correct ambient temperature, whether or not it is ventilated.

How did we used to keep milk cool when camping before the days of fridges? Stick the milk bottle in a bowl, put and inch of water in the bowl and wrap a tea towel around the bottle such that it wicks water from the bowl. Stick it in the shade but where the wind can pass over it. Result: cool milk!

Bartfarst

-mojo- wrote:

boxerman wrote:I've fitted several ice warning devices /outside temperature gauges to vehicles over the years and the ones that came with instructions always said to mount the sensor out of the airstream to get an accurate reading.

However, did I mention that placing it out of the airflow ~does~ prevent or reduce the effect of road spray and crud accumulating on it?

Hi there,

Play nicely now; because you are both correct!

1) If a temperature probe gets wet it will reflect the temperature of whatever is hitting it.

2) If there is a wind blowing across a wetted probe the water will evaporate if the humidity is less than 100% and the process of evaporation will cause the temperature of the probe to drop.

With regard to "humidity" my good lady has had washing out on the line for two days now! Despite there being a slight breeze it hasn't dried because the humidity has been between 90% and 95% here since yesterday morning!

Best regards,

Ian

boxerman wrote:Sorry! not convinced, you do it your way, I'll do it mine.

Well in fairness I'm not disagreeing with the way you suggest. I'd choose to keep it out of the airflow as well!

And yes, I agree with both of the last two posts about evaporation if it's wet, though I have a strong suspicion (not borne out by any theoretical or experimental evidence, I should add) that in a situation where the oncoming air, the water in it and the probe are all roughly at the same temperature, the additional effects of the air flow will be minimal on evaporative cooling.

But we are in agreement anyway - out of direct airflow behind the bumper or in a well ventilated body cavity or similar.

dogseal wrote:.......So I decided to fit an external thermometer From Maplins part no N76GQ [You must be registered and logged in to see this link.]

The point is, where is the best place to mount the probe? The unit comes with 1.5 metres of cable I'm probably going to put it somewhere as close to the road as possible , has anyone else fitted something similar to their vans?

Having owned a fairly sophisticated Weather Station a number of years i've always been sceptical about the readings given from vehicle outside temp sensors. Chaps I used to work with would often come in to work on a winter morning stating, "blimey it's minus 15c this morning", whilst another would have -8c or -3c. In reality -1 or 2c would be closer to a true reading!

Whilst in Bulgaria this year I spotted a radio controlled clock wireless Weather Station complete with outdoor temp sensor on sale at just four quid....which I snapped up pronto! Bought primarily for the van but like you....where to bung the outside sensor?!!

For starters it has to be well away from the engine and also covered so that it avoids false readings. About the only place I have is inside the back box, which being white does at least reflect some sun. Not perfect......but it has to be somewhere like that.

Some mount the sensor behind the door mirror - in the casing - I, like Frank mount mine between the inner and outer wings. Didn't know about wind chill not affecting inanimate objects - learn something every day - s'pose it's similar to the workshop floor "feeling" colder than the polystyreen box in the same workshop....................

Just a rider to all this.
If ice is about listen to the noise from the wheels, if you are on ice the noise will disappear. Then is the time to keep calm.

Regards Mike

Hi All,
Before we went to Italy I decided that we should have a fridge thermometer and as the one I wanted had some reviews which said some people had received faulty ones I decided as they were cheap, £2 99 each I would have two, just in case, they both worked (and tempratures roughly matched to within 1 degree) so I was left with a spare one.
This thread came up and as we intend to visit UK in February I thought my spare fridge thermometer will do that job, sensor under the front wing in a plastic bag to prevent crud and water, and the display stuck to the top of the dashboard with double sided tape. Job done, now all I have to do is fit the batteries before we go and bingo.

Brian2

Brian,

The "KISS Principle Rules" eh!

Best regards,

Ian

I have a similar Maplin inside/outside temp and clock. The sensor fits on a sticky pad near the hinge on the passenger door frame, still plenty of room to close the door. It works well here but its my second try. First time I fitted it too close to the bulkhead where it picked up some heat from the engine until I had driven for a mile or so.

Hi Dutto,
Never heard of the kiss principle but I assume it is something to do with making use of existing items in stock.

Brian2

murph wrote: Hi Dutto,
Never heard of the kiss principle but I assume it is something to do with making use of existing items in stock.

Brian2

Brian,

It stands for:

Keep
It
Simple
Stupid!

... and reminds people not to over-engineer anything!

Can't get much simpler than:

"..... sensor under the front wing in a plastic bag to prevent crud and water, and the display stuck to the top of the dashboard with double sided tape ..."

Best regards,

Ian

Hi Dutto ,
10 min, job done.


Brian2

Hi All

-mojo- wrote:
I agree with both of the last two posts about evaporation if it's wet, though I have a strong suspicion (not borne out by any theoretical or experimental evidence, I should add) that in a situation where the oncoming air, the water in it and the probe are all roughly at the same temperature, the additional effects of the air flow will be minimal on evaporative cooling.

I wasn’t sure about that but you’re right - and here’s that experimental evidence:

I’ve got two sensors in still air. One has a piece of tissue attached so I can add a few drops of water to make it wet without the water running off, while the other remains dry.

A 3rd one is in the jar of water to prove that the water I’m adding is at the same temperature as the sensor.

The idea is measure both sensors dry, then to add water a see what happens. After that I’ll expose them both to wind (well in this case a desk fan) and again see what happens. Initially all three sensors agree to within about 0.1ºC



I was quite surprised by the results, the temperature of the wet sensor dropped by over six degrees in still air and only by another one degree when the fan was turned on (results below).



The bumpy bits in the middle are me trying to get the maximum airflow from the fan.

Strangely the fan had a small effect on the dry sensor. I’m not sure what caused it but I suspect it’s to do with moving air from different parts of the room that may be at different temperatures or perhaps something to do with the humidity.

Anyway the moral of the story is that keeping the sensor out of the airflow is preferable but not all that important. Keeping it dry is important.

Brod.

Nice bench load of gear you've got to play with!

As you say, a bit surprising to see the temperature of both wet and dry go ~up~ as you adjusted the airflow around them - I guess you're right and the fan was disturbing the natural layering of warmer air higher in the room, and mixing it more evenly - which should make the air at desk level slightly warmer.

Dutto wrote:................

1) If a temperature probe gets wet it will reflect the temperature of whatever is hitting it.

2) If there is a wind blowing across a wetted probe the water will evaporate if the humidity is less than 100% and the process of evaporation will cause the temperature of the probe to drop.

..................

Brod,

Thanks for that.

My second wife always said "I can forgive anyone for being wrong but I find it very difficult to forgive you when you are right!"

From the experiment and the kit displayed I presume that you had some time on your hands!

Best regards,

Ian

Hello Ian

Dutto wrote: From the experiment and the kit displayed I presume that you had some time on your hands!

It came about because – sadly - I still have to work for a living. It just so happens that I work in an R&D/design dept where we design this sort of kit so there’s loads of it literally lying around (the controller in the pic is one of our products).
I quite frequently get involved with work experience students and thinking of his thread, it struck me that it would be a good little project for them to set up and do the test in the R&D lab.
Actually it didn’t take long. Five minutes or so to setup and a few minutes wetting the sensor and adjusting the fan. What took the time was waiting for the water to come up to temperature. I set it up about 8:30 in the morning and the water didn’t get up to temperature until about 4:00PM!

Brod.