Basics 1

[xabproject]

Dear members,
Need help on some basics. I do not know whether the question is too silly or too profound.
Question: The metal temperature is generally 'more' or 'less' than the surrounding ambient temperature (dry bulb).If the statement is right (it is most often seen to be so),then when is the metal temperature same / nearly same as the ambient temperature?
Thanks

 

[MortenA]

I think metal feels cold because when you touch it you get a MUCH better heat transfer than with ambient air.

Wind chill or solar heating may make all surfaces much colder or hotter than ambient temperature so im assuming that you mean metal objects indoor or i shadow/shelter.

Best regards

Morten

 

[MortenA]

just though of another funny think when "gauging" temperature "yourself". Think about a cup of coffee and a gals of e.g. milk at "room temperature" If you drink the coffee it will seem cold and if you drink the milk it will seem warm - funny isnt it

Best regards

Morten

 

[xabproject]

Dear MortenA,
Your first reply has helped clear my first level (!)misunderstanding.
Thanks.
Rewording my question:
Let us say, I measure the temperature of ambient air by using an ordinary 'Mercury in Glass bulb thermometer' left hanging "free" in a Air Conditioned Room (let the recorded temp be T1). Then I measure the temperature of a metallic part using the same thermometer by inserting the bulb into a closely fitting hole in the metallic part filled with grease so that the bulb is fully in 'contact'(say temp T2). What will be relationship between T1 and T2?

Purpose: All this is to apply the correct correction factor to a "very low" resistance measurement exercise.
Thanks.

 

[25362]

Radiation and other modes of heat transfer are affected by the geometry, colour, position (up, down, inclined), type of paint or coating, temperature, chemical composition, air turbulence, etc.

The ulterior purpose of your query could be what happens to the metal surface concerning corrosiveness or electric properties, who knows. Anyway, one shouldn't forget that atmospheric air contains, apart from humidity, also acids, salts, and a plethora of traces of other chemicals.

In regard to moisture even when the metal temperature asymptotically nears that of the surrounding air's room (DB) temperature, impinging water molecules would be adsorbed forming a monomolecular vapour layer on non-porous surfaces, and penetrate to a depth on porous surfaces. The amount of molecules reaching the metal surface depends on the RH, air turbulence and the exposure time. Equilibrium takes some finite time to occur but eventually it is reached.

Would this be the issue of your interest ?

Nil homini certum est. Nothing is certain for man. Ovid

 

[iainuts]

Assuming
- The metal is in thermal equilibrium with the air in the room.
- There is no significant heat transfer such as suggested by MortonA and 25362, such as could happen if sun was shining on the metal directly, or it might be night time and the metal may be next to a window radiating heat to a cold atmosphere.
- Assuming there is no significant chemical reactions going on, just normal daily rust.
- If the room is reasonably well insulated, and there are no large temperature disparities within the room.
- If this is a fairly "normal" room in a house....

You should find the temperature of the metal is equal to the temperature of the air.

Note also, that given this situation, even by blowing air on the metal, the temperature will not change. "Wind Chill" does not apply in this case. "Wind Chill" may apply to people because we're warm, and cold air blowing cools us. But if we're the same temperature as the wind, there will be no heat transfer.

Dave.

 

[IRstuff]

The question is how you are going to change out the sensor without disturbing the thermal equilibrium of the room.

Depending on what level of matching you're trying to achieve, you might be better off with TWO calibrated platinum PRT's. They could be matched to less than 5 mK. This way, you avoid problems with equilibrium and instrument drift.

TTFN

 

[Montemayor]

I agree with iainuts' succinct summary. For all practical purposes, the temperature of the metal and the air are the same at equilibrium - which is inevitable, if allowed.

However, his explanation of the "wind chill" factor effect is not complete. While it is true that "cold air blowing cools us", the real dynamics involved is that the relatively dry, cold air is evaporating water moisture ("sweat) on our skin and causing a cooling effect by the evaporation of the same water. This is the same effect that takes place in an atmospheric cooling water tower.

In this example, wind chill does not contribute any heat transfer effect - if the metal is dry and devoid of any vaporizable liquid. In other words, if the air is colder than the metal, there will be initial heat transfered from the metal to the air; but if the metal is dry, the temperatures will eventually equate at equilibrium and heat transfer will cease during that duration.

Art Montemayor
Spring, TX

 

[MintJulep]

I think Ianiut's assumtion of thermal equalibrium invalidates the original question. The question seems to deal with non-equalibrium conditions. I think what xab is trying to get at is the rate of temperature change of a metal object is different from the rate of temperature change of the air around it due to factors such as specific heat, mass, conductivity, convection coeficients etc.

So, if you start with the metal object in thermal equilibrium with the air around it and then reduce the temperature of the air, the temperature change in the metal will lag the temperature change of the air because in general, metal has a higher specific heat than air.

If you reduce the air temperature and then hold it at a constant temperature, eventually the temperature of the metal will reach the temperature of the air.

If you then raise the air temperature, the temperature of the metal will also start to rise, but at a slower rate. etc.

So, to answer the question asked, which was when are the metal and the air at the same temperature:

1. (equlibrium case) When the air temperature has been constant for a sufficiently long time for the heat transfer between the metal and the air to reach zero.

or

2. (transient case) At some time after the slope of the air temperature vs time function changes sign, if the sign of the new slope remains sufficiently long enough for the air-temperature vs time function to cross the metal-temperature vs time function.

Oh, by the way, in case 2 the temperature within the metal will never be uniform.

 

[xabproject]

Dear responders,
Thank you very much. Each one of you have helped me renew my look at the problem for better. I hope you will all be in agreement if I take the metal temperture same as the room ambient temperature considering that the 'equilibrium is reached' as the equipment is in the 'steadily' air conditioned room for over a week as of today/now.
Thanks.

 

[IRstuff]

Again, it depends on the level of accuracy. "steady" A/C even in a well controlled system is +/- 2°C.

An office A/C is may be up to +/-5°C

TTFN

 

[imok2]

IRstuff, IN well designed direct digital conroled air conditioning systems I believe it's possible to get +-1*F

 

[quark]

In my opinion MintJulep and IRStuff are straight to the point. Thermal inertia of metals play a key role.

As Mintjulep rightly said, the time and constant temperature of air primarily govern thermal equilibrium. +/-2 deg.C as noted by IRStuff is quite a huge range as far as thermal equilibrium is concerned.

I disagree considering same temperature for air and metal, if it makes any significant sense to your process.

Regards,

 

[25362]

This thread is a good example of Seneca's saying: Docendo discitur We learn by teaching.

 

[Productmanager]

metal temperature is the same as ambient in air 100% relative humidity and no air movement