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On the performance of a Stevenson Screen

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HansR
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On the performance of a Stevenson Screen

Post by HansR »

I describe the building of a (not too small) Stevenson Screen here.
In this thread I will make observations regarding the use of this screen (which may be valid for other screens as well).

My first observation is based on the charts screenshot you see below (Weerkast === Stevenson Screen). The Temperature is the minute value of the main sensor and Temp Weerkast is the temperature measurement inside the Stevenson Screen.

The (interesting) observation is that the Temp inside the Stevenson Screen is lower when rising and higher when going down compared to the main Temperature measurement. This cannot be a device measurement error : that should have a constant deviation. So I explain this by the screen itself absorbing and giving off heat and because of the mass of the screen it is lagging behind. On a 'normal' descent rate this appears to be around 0.3 °C but on the steep rise in the morning it apparently can be up to 1.5°C, quickly catching up when the rise slows down.

I currently don't have the time to analyse the effect on the average, but will do (much) later.
Will take some top and bottom planks out to open up and have more vertical circulation and see what that effect will be.

(A man with one thermometer knows what temperature it is; A man with two thermometers is never quite sure. :))
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    Re: On the performance of a Stevenson Screen

    Post by freddie »

    I think more horizontal movement within the screen would be a good move too - the "beehive" panels would restrict a lot of air flow.

    Have you considered aspirating during daylight hours? Perhaps with a solar-powered fan?
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    Re: On the performance of a Stevenson Screen

    Post by HansR »

    freddie wrote: Sun 08 May 2022 9:29 am I think more horizontal movement within the screen would be a good move too - the "beehive" panels would restrict a lot of air flow.
    Yes, I am thinking of drilling extra (large) holes in the beehive panels to accomplish just that.
    But nevertheless, I think that that will not compensate for the heat content of the box radiating out (or in). We'll see.
    freddie wrote: Sun 08 May 2022 9:29 am Have you considered aspirating during daylight hours? Perhaps with a solar-powered fan?
    I have not but I might make that an add-on next year. Thanks, good idea. :clap:
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    Re: On the performance of a Stevenson Screen

    Post by HansR »

    So I have two more observations (see screenshot below):
    1. from roughly 1400h - 1800h I removed the bottom and the top at the T/H sensor. This showed an immediate increase of the temperature of approximately 1°C. I explain this from the heating of the soil by the sun directly under the screen which makes the air rise up. Normally this heat from below will be deflected by the white of the outside of the bottom.
    2. This morning it was fresh (freezing point at the soil surface, 1°C at +10 cm) with a rapid temperature increase when the sun came up. The screen was longer in the shadow than the WS80 and this resulted in an extreme difference of 2.5 - 3.0 °C which it had not caught up at 1000h. The slope (speed of temperature rise) seems the same for both thermometers with the screen maybe slightly slower.
    My conclusions at the moment:
    • Again, the argument of improving airflow in the screen seems obvious.
    • Don't take a temperature from a Stevenson Screen for granted, there is some work to be done to get the correct air temperature
    • But don't take the temperature measured from the WS80 for granted either. It seems to me the device gets warmer than the air temperature when direct sun hits the small device radiation protection.
    The man with two thermometer observation above seems correct :o
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      Re: On the performance of a Stevenson Screen

      Post by Dador »

      @HansR

      For meteorology, the daily average is more important than the extremes. So what are the average daily temperatures of the compared screens?
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      Re: On the performance of a Stevenson Screen

      Post by HansR »

      @Dador:
      Quickly done through a spreadsheet from 8/5 10h00 - 9/5 10h00: main T/H sensor: 8.695 °C and the Stevenson Screen T/H: 8.260 °C
      I think that is too big a difference.
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