Author Topic: Correct positioning of thermometer?  (Read 851 times)

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Offline wcndave

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Correct positioning of thermometer?
« on: August 28, 2018, 03:08:13 PM »
So, in the olden days with mercury thermometers, I put one in a Stevenson screen that I had built for a school project.

However, I don't really have the choice/luxury of that any more, so was looking at what position for my 433 sensor will be best.

My 5n1 is on a pole, in the sun, with two solar panels, and the aspirator inside running during the day.
My outdoor sensor is on a North facing stone windowsill, with it's back against the side of the window recess (which is 20cm), so the sun never shines on it.
My lightning detector is also on another North facing windowsill, same situation.

I have the following readings from the sensors:

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The yellow line is the 5n1 and the blue and green are the other two.

As you can see, the 5n1 reports colder at night, and hotter in the day.

I guess the shelter of the back wall may have an effect at night, although am surprised it's that much.

The 5n1 during the day get's much hotter, as it has the sun on it.

Here in Italy, the sun can be very hot indeed, and with a screen it would be the same.  It can be 15 deg out, and feel like 30+ in the sun, too hot to sit outside.

So my questions are, which one would be my most accurate outdoor reading now? And, what options do I have to be more consistent.

If the 5n1 aspirated and outside is correct, then so be it, however I thought ideally you'd take readings from the shade.

I think the rising sun heats the stone windowsill, and reflection, radiation and conduction cause them to get hotter.
So, out the back, I can mount them on the outside wall under the eaves, where they will never, ever, see the sun.

However, they will then be potentially more sheltered...

This screenshot actually shows a drop out the back, so that must occur when the sun moves round, even though the sensor is in the shade.

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Any thoughts on correct positioning, or whether the 5n1 in the sun is in fact the correct readings to be taking?

Offline nincehelser

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Re: Correct positioning of thermometer?
« Reply #1 on: August 28, 2018, 05:39:29 PM »
Here are the CWOP guidelines:

Temperature / Humidity Sensor
Should be placed on a patch of level ground, over a surface representative of the area.
 [Avoid rock/concrete and dark-colored surfaces, roofs, or irrigated lawns].
Should be mounted in a ventilated radiation shield.
Height between 4 1 and 6 7 above ground (1.25 2.0 m)
Place sensors at a horizontal distance of 2 times the height of the nearest object (tree,
structure, etc). [Example: 40 feet away from a 20 foot tall tree].
Keep away from other sources of heat such as chimneys, air vents, air conditioners, etc.

Keeping it a horizontal distance 2 times the height of the nearest object (tree, structure) makes it really tough to find shade.

I guess that means they prefer it be in the open sun with a shield than in the shade?