Outside temperature sensor.

brodco wrote: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).

That explains the rather well-equipped work bench. I did wonder how you happened to have 600+ quids worth of recorder/controller at your disposal... Looks to be a very nice piece of kit if you have the budget for that type of professional equipment.

Hi Brod,
It is rare for us ill equiped members to recieve reliable information like this on a very useful subject, which could help to prevent accidents. I did guess at the possible results which is why I put my sensor in a plastic bag to prevent it getting wet but is this really sufficient or will water evaporating from thre plastic bag have the same effect as water on the bare sensor, the bag I have used is in fact only a corner cut off a much larger bag so it only really provides protection. I always remember our Chemistry master at school, Wally Collister, telling us evaporation causes cooling, and your experimient has proved it to all and sundry. Six to eight degrees out would mean the sensor was usless.

Brian2

Brod,

I really appreciate your practical approach to this problem. I'm also somewhat relieved that it confirms what I went on about at length earlier on in this thread! FYI most of my experience in this field stems from my collection of Casella temperature and humidity instruments which, I think, are still in the loft: I may dust them off and have a play when I get the Christmas tree out later. There goes Saturday!

I also deal with tempertaures in my work, generally very low ones associated with liquid gases, and I will take a look at the Eurotherm nanodac which looks particularly useful. Thanks for bringing this to our attention.

Brian2, if the sensor is in contact with the outside of the (wet) plastic bag then I reckon you'll still get some depression due to evaporation.

Bartfarst

Hi Brian(2)

murph wrote:Six to eight degrees out would mean the sensor was usless.

I was quite suppressed how dramatic the effect was. I think these results prove that this is a real effect and could cause problems. Hopefully armed with this information anyone fitting an outside sensor will be able to make a better decision as to how to install it rather than having to guess.

That said, I think that in practice the error would be much smaller. Here are a couple of reasons:

The sensors I used were quite big and the casing made of stainless steel. If the sensor is smaller possibly cased in plastic I’d expect the error to be much less. The other thing is that at around zero degrees the rate of evaporation will probably be slower anyway.

If the sensor is getting wet it probably means that it’s raining or has been and the humidity is high which also means less evaporation.

From then safety point of view (which what we’re really interested after all) the errors are in the right direction so if anything the system will warn you of ice early.

I don't think anyone should be put off fitting a warning system because it's not 100% accurate. It's still well worth doing.

Brod.

murph wrote:Hi Brod,
It is rare for us ill equiped members to recieve reliable information like this on a very useful subject, which could help to prevent accidents. I did guess at the possible results which is why I put my sensor in a plastic bag to prevent it getting wet but is this really sufficient or will water evaporating from thre plastic bag have the same effect as water on the bare sensor, the bag I have used is in fact only a corner cut off a much larger bag so it only really provides protection. I always remember our Chemistry master at school, Wally Collister, telling us evaporation causes cooling, and your experimient has proved it to all and sundry. Six to eight degrees out would mean the sensor was usless.

Brian2

And that's exactly how a petrol engine vaporises the air/fuel into a gaseous state before it is burned and why it is essential for all you petrol (carb or single point injection) owners ensure the heating duct from the exhaust manifold to the air filter isn't missing or torn - even in plus freezing conditions iceballs can appear in the carb/injector and literally STOP the engine! Taking heat away to vaporise the mixture causes the water in the air to freeze - it usually happens when decelerating with a closed throttle.

dandywarhol wrote:even in plus freezing conditions iceballs can appear in the carb/injector and literally STOP the engine! Taking heat away to vaporise the mixture causes the water in the air to freeze - it usually happens when decelerating with a closed throttle.

This is exactly what happened to my little rear-engined Fiat in stop-go traffic a few years ago. The temperature was around 3 degrees and it was quite foggy - so the carb gradually iced up and it eventually stalled. I was a bit embarrassed when the AA man turned up and it started immediately - but he then explained to me what had happened...

The problem with the temperature sensor is that what you're dealing with is actually a fairly complex system with several inter-related factors. So, for example, if you wrap it in plastic to reduce inaccuracies due to evaporation, you reduce airflow around the sensor itself, and that will reduce how quickly it responds to temperature changes. You don't want this, because ambient temperatures can change fast where there are deep dips in the road or (obviously) in hilly areas.

One other point relating to the need to keep it dry or not is whether evaporative cooling is a significant factor at the temperature range where we need it to be effective - at, or slightly above, freezing. I'm fairly sure that the effects of evaporative cooling drop away to near insignificance once water has frozen, which suggests that at the very least the effect must be self-limiting at temperatures just above freezing. Sublimation cooling can take place below freezing, but I ~think~ it's a much smaller effect than evaporative cooling (for water, not so for some other materials).

So although Brodco's results show a drop of maybe 6 degrees for a wet probe at room temperature and unknown humidity, I suspect (but don't have the fantastic experimental setup that Brodco does, to confirm or refute my suspicions) that he would see a much smaller effect with an ambient of, say, 2 degrees C and the same level of humidity.

The one good thing about the probe getting wet is that it ~should~ always make you err on the side of caution, as it should almost always make ambient appear to be colder than it really is.

Hi All.

-mojo- wrote:
So although Brodco's results show a drop of maybe 6 degrees for a wet probe at room temperature and unknown humidity, I suspect (but don't have the fantastic experimental setup that Brodco does, to confirm or refute my suspicions) that he would see a much smaller effect with an ambient of, say, 2 degrees C and the same level of humidity.
The one good thing about the probe getting wet is that it ~should~ always make you err on the side of caution, as it should almost always make ambient appear to be colder than it really is.

Agreed (we must have posted at almost the same time) .

brodco wrote: I think that in practice the error would be much smaller. Here are a couple of reasons:
The sensors I used were quite big and the casing made of stainless steel. If the sensor is smaller possibly cased in plastic I’d expect the error to be much less. The other thing is that at around zero degrees the rate of evaporation will probably be slower anyway.

If the sensor is getting wet it probably means that it’s raining or has been and the humidity is high. That means little or no evaporation.

I suspect the RH was quite low. Actually if you’re keen you could calculate the relative humidity from the wet and dry sensor temperatures (technically you need the pressure as well but it doesn’t make a lot of difference in practice).

If this thread has shown us anything it's that there is measuring temperature is more tricky than it may seem at first. I think an understanding of the issues makes it easier to make a good decision as to where and how to mount the sensor.

Brod.

brodco wrote:The other thing is that at around zero degrees the rate of evaporation will probably be slower anyway.

Apologies - I read your reply, but missed this for some reason - so I wasn't saying anything you hadn't already pointed out! Oh well, at least I didn't contradict what I hadn't noticed you had said...

Hi All,
It would seem that the consensus of opinion is that at near freezing tempratures, the rate of evaporation is so low, as to make very little difference to the accuracy of the sensor, am I correct.


Brian2

Hi Brian.

murph wrote: Hi All,
It would seem that the consensus of opinion is that at near freezing tempratures, the rate of evaporation is so low, as to make very little difference to the accuracy of the sensor, am I correct. Brian2

An un-characteristically simple answer!
Yep! (might try and prove it one day)

Brod

brodco wrote:Hi Brian.

murph wrote: Hi All,
It would seem that the consensus of opinion is that at near freezing tempratures, the rate of evaporation is so low, as to make very little difference to the accuracy of the sensor, am I correct. Brian2

An un-characteristically simple answer!
Yep! (might try and prove it one day)

Brod

Whoa there!

When any frost is forecast the Council chuck a lot of salt on to the roads and therefore any liquid hitting the sensor will be a "salt brine"!

Salt brine has a much lower freezing point than pure water (which is why salt is chucked on to the roads) and it will probably evaporate at a different rate.

Come on Brod. Experiment two please; this time with brine at near zero degrees celsius and (if possible) with varying RH values.

Best regards and

Ian

Hi All,
Should have expected somthing like this from Dutto, but surely the sensor at that temprature will be similar to that of the road surface.

Brian2

Oh eck! Now we’re getting involved (love it).

To address Brian's question first.

murph wrote: Should have expected somthing like this from Dutto, but surely the sensor at that temprature will be similar to that of the road surface. Brian2

Sadly not. The discrepancy depends on the weather. If it’s cold and clear the road surface cools by radiation and as heat loss by radiation can be greater than heat gain by conduction (from the air). So the road surface can get colder than the surrounding air. Ice can form when the air temperature is a little above freezing. I’d be careful when the sensor read four or five degrees (but only on clear windless nights).

Dutto wrote:
When any frost is forecast the Council chuck a lot of salt on to the roads and therefore any liquid hitting the sensor will be a "salt brine"!

Yep!

Dutto wrote:Salt brine has a much lower freezing point than pure water (which is why salt is chucked on to the roads) and it will probably evaporate at a different rate.

Not sure.
I’m on dangerous ground here because I’m no chemist but is suspect that the evaporation rate will be the same just leaving a stronger salt solution. I'm quite ready to be corrected on that though (any Chemists on the forum that can comment?).

Dutto wrote:Come on Brod. Experiment two please; this time with brine at near zero degrees celsius and (if possible) with varying RH values.

Actually we do have an environmental chamber so I could do just that but I'd have to run it passed the boss first.
Unfortunately it’s in use most of the time and my employer thinks product testing is more important than anything I may like to try (unreasonable I know but there it is).
But! There’s nothing to stop me putting an extra bit of kit in when we’re doing some low temperature testing. Can’t give a time scale though.


Brod.

murph wrote:Hi All,
Should have expected somthing like this from Dutto, but surely the sensor at that temprature will be similar to that of the road surface.

Brian2

Moi??

Thing is, at zero degrees water will not evaporate because it will freeze whereas brine MAY still be evaporating! (The temperature gauge should show zero or less than zero though so it is just an academic question!)

I once did a 360 degrees on ice in a Ford Capri. I partly blame myself because the ice was under 3" of snow and I should have been going a lot slower anyway!

Best regards and Merry Christmas Brian,

Ian

Dutto wrote:
Thing is, at zero degrees water will not evaporate because it will freeze whereas brine MAY still be evaporating! (The temperature gauge should show zero or less than zero though so it is just an academic question!)

Humm!

Can brine evaporate? Water evaporates leaving the salt and water can still be liquid down to about -40 degrees C. Not sure about the physics of super cooled water (bet that last statement attracts a few comments) .

Chemists seem to be like policemen, never there when you need them.

Need to think about it, can't do it now though - got to go and do some last minute shopping.

Brod.

brodco wrote:

Dutto wrote:
Thing is, at zero degrees water will not evaporate because it will freeze whereas brine MAY still be evaporating! (The temperature gauge should show zero or less than zero though so it is just an academic question!)

Humm!

Can brine evaporate? Water evaporates leaving the salt and water can still be liquid down to about -40 degrees C. Not sure about the physics of super cooled water (bet that last statement attracts a few comments) .

Chemists seem to be like policemen, never there when you need them.

Need to think about it, can't do it now though - got to go and do some last minute shopping.

Brod.

Brod,

Me too!! Love a good discussion.

The water part from brine can evaporate (indeed even ice "evaporates" but because it goes from a solid to a gas without passing through the liquid phase it is called "subliming").

But, (there is always a "but" isn't there ) as the water evaporates from the brine the percentage of salt left behind in the brine increases; so the temperature at which the brine will freeze should keep falling shouldn't it?

Come on!! I dare you to bring up the term "eutectic"!

Best regards,

Ian

boxerman wrote:I have one of these:
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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

Easy to fit, wired it up to the same circuit as my reversing camera. I had to extend the probe cable by 400mm to fit the probe. Checked it with some ice cubes in a plastic bag placed against the probe. All works fine so thanks for everones input..very interesting.

Happy new year to all.

Tim

Hi Dutto,
In my welding days Eutectic was a make of welding rods nothing to do with ice. I still have some.

Brian2

murph wrote:Hi Dutto,
In my welding days Eutectic was a make of welding rods nothing to do with ice. I still have some.

Brian2

Brian,

It's all related. It's the point where a mixture of two or more liquids turns into a solid. The "eutectic" welding rods sold on the basis that the weld was stronger because when the liquid steel from the plate/pipe etc mixed with the liquid steel from the rod the mixture wold set at the same time and therefore be stronger. A 'non-eutectic" weld example is brazing.

With regard to water mixtures (from memory and maybe up to 5 degrees adrift):

o Water freezes at 0 degrees.

o Monoethylene glycol freezes at -15 degrees.

o A 55% Water / 45% MEG mixture freezes at -40 degrees. The -40 degrees point is the eutectic.

Best regards and

Ian

Sorry about the late reply! (wasn't allowed on the forum over Christmas).

Dutto wrote:Come on!! I dare you to bring up the term "eutectic"!

Technically I can’t bring it up because:
a) You’ve already done so!
b) I've never come across the term so had to look it up (I told you I was no chemist).

brodco wrote:I’m on dangerous ground here because I’m no chemist but is suspect that the evaporation rate will be the same just leaving a stronger salt solution.

Been doing a bit of reading up and come to the conclusion that I disagree with myself! The evaporation rate does change when the water is a solvent (which it always is to some extent except for very pure water).

Dutto wrote:But, (there is always a "but" isn't there ) as the water evaporates from the brine the percentage of salt left behind in the brine increases; so the temperature at which the brine will freeze should keep falling shouldn't it?

I sense a bit of fishing – you already know the answer don’t you?

I tried to find a phase diagram specific to brine but I only found some links to publications that needed to be paid for. Looking at the “generalized” phase diagram from the Wikipedia article, and assuming the one for brine is a similar shape (and that the Wikipedia article is accurate) the answer would seem to be NO! (right/wrong?)

Interesting because it suggests that there is an optimum deposition rate for salt spreading above which it’s ineffective.

Brod.

murph wrote:Hi Dutto,
In my welding days Eutectic was a make of welding rods nothing to do with ice. I still have some.

Brian2

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PB

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

But, (there is always a "but" isn't there ) as the water evaporates from the brine the percentage of salt left behind in the brine increases; so the temperature at which the brine will freeze should keep falling shouldn't it?

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

Brod,

I honestly don't know!!

Evaporation of the water will increase the concentration of salt which may change the freezing point or alternatively it may precipitate salt crystals to keep the liquid in equilibrium at the reduced temperature.

Fascinating eh?

Just the kind of conundrum to think about as your vehicle is waltzing through its second 360 degree spin and heading sideways towards the grass verge!!

Best regards and


Ian