Temp sensor and its placement to know the temperature of the 'New York' today

Thread Starter

Willen

Joined Nov 13, 2015
294
Lets say it is 12 noon now. if I placed a sensor in my yard or in a street, exposed to the Sun light, it shows 38°C. If I placed a sensor to inside a building then it shows 27°C. If I placed inside the front room, it shows 33°C and if I placed inside the back room it shows just 22°C. What is the temperature of the surrounding place (I said 'New York' for example) now? And where to place the sensor to get valid reading?

I am planning to make simple analog thermometer using LM35A.
 

Thread Starter

Willen

Joined Nov 13, 2015
294
Hi,
Hm...Stevenson Screen!
Now I am focusing on the sensor component- to get accurate temperature reading the sensor's case should be a better heat conductor (I think). LM35A case has similar black epoxy as other TO-92 transistors. Is it a nice heat conductor or won't it block the surrounding temperature to reach to the chip inside?
 

Alec_t

Joined Sep 17, 2013
10,918
Atmospheric temperature measurements aren't ususally measured on a second-by-second basis, so the LM35 case will be fine.
 

GopherT

Joined Nov 23, 2012
8,012
Hi,
Hm...Stevenson Screen!
Now I am focusing on the sensor component- to get accurate temperature reading the sensor's case should be a better heat conductor (I think). LM35A case has similar black epoxy as other TO-92 transistors. Is it a nice heat conductor or won't it block the surrounding temperature to reach to the chip inside?
A TO-92 has a simple thermal time constant. Essentially a logarithmic decay of current temp to new temperature, similar to charging / discharging a capacitor through a resistor. In the is case, thermal conductivity of the To-92 package limits the rate of heat transfer and the mass and materials determine the heat capacity of the part. The conductor leads also contribute to the thermal conductivity into the sensor point in the part.

You can try to remove the plastic package to reduce the time constant. And, increase thermal conductivity of the leads into the package by attaching small "radiators" to each but that would also add to the heat capacity of the part so everything is a trade-off.

As you search, you may find that thermocouples have the shortest time constant but will certainly be more difficult to interface with the controller than an LM35.
 

GopherT

Joined Nov 23, 2012
8,012
Are you paying attention? You need a screen.
You can build your own using plastic plates or saucers.

View attachment 101126

http://aaybee.com.au/StevensonScreen.html
http://kalium-projects.blogspot.ca/2015/04/weather-station-part-i-sensor-shield.html

Choose white saucers.
Are you paying attention, chips?

He understands what a Stevenson screen is (or anything else that makes shade). He asked about the LM35 and seems concerned with the thermal response rate.

Not keeping up with the conversation is understandable but accusing someone of not paying attention is unforgivable if you are the one not paying attention.
 

AnalogKid

Joined Aug 1, 2013
8,312
Solid state temperature sensors like the LM35 get a significant amount of thermal energy through the leads, whereas something like platinum thermistor gets most of its energy through the case. When measuring air temperature, it is important to remember that air is a reasonably good insulator. The sensor is reporting the temperature of the air within 1 inch. In still air this can be a significant source of error.

ak
 
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