Aspirated air temperature sensor for a Polytunnel

In the UK air temperature is normally measured in a passively cooled Stevenson screen. The louvred design of the screen allows air to flow around the thermometer. The trouble with a polytunnel is there is no wind at all, as a result the sun heats the sensor up and without airflow you don’t know by how much.

By running a computer fan driven off a solar panel I can move enough air past the sensor to exchange the heated air from the sun shining on the sensor. For the sensor I use the standard Chinese supplied DS18B20 encapsulated in a stainless steel tube

Dallas DS18B20 epoxied in a stainless tube housing, from a Chinese Ebay supplier
Dallas DS18B20 epoxied in a stainless tube housing, from a Chinese Ebay supplier

The sensor is housed in a 6cm piece of white plastic waste pipe

sensor mounter in centre of white waste pipe
sensor mounted in centre of white waste pipe

The fan is mounted at the top of the pipe, designed to pull in air from below; this way the sensor is not heated by air passing the fan motor, and the airflow works with the natural tendency of warm air to rise. I’ve tried to keep the airflow as unimpeded as possible.

side view - the flange for the fan is made from a piece of wood glued to the pipe
side view – the flange for the fan is made from a piece of wood glued to the pipe

Looking at the results there is a difference of a few degrees

the difference opens up a few degrees at high temperature

between the aspirated sensor and another sensor mounted on the outside of the plastic tube. They track at low temperatures but not when the sun is shining – the difference here is about 6 degrees, even in March, before the vernal equinox. It is remarkable just how much the air temperature swings – 27 degrees on a couple of days which still have hazy sun.

Sensor mounted in polytunnel
Sensor mounted in polytunnel

Weatherproofing the sensor is easier in a polytunnel because as well as the wind not blowing, it also doesn’t rain. I can use a cheaper indoor solar panel, the one I used is a 12V 1.5W unit, Maplin L58BF bought on sale for about £6, not the £20 they seem to be charging for it. even £6 is a little dear! I extracted the flashing blue LED and series diode to maximise the power available to the motor. This also charges the battery of the temperature sensor dual unit, which reports back to the collecting station using Ciseco’s XRF every 10 minutes.

Solar panel schematic
Solar panel schematic

The computer fan was a 12V brushless unit but I run it at about 7V, we’re not after blowing a gale through the tube. It will start at 5V. The Zener is there to limit overcharging of the 4.8V NiMH battery pack in the electronics to about 4mA. It only reports every 10mins so this is enough. The 1N4148 diode stops the battery discharging back through the fan and solar panel in the night. I should really measure what the leakage current of that Zener is 😉

I used a PIC 16F628A driving a Ciseco XRF to send the temperature data from two sensors back. Nowadays I would use the Ciseco RFu which includes an Arduino and low-power standby mods to make this cheaper.

Other implementations

This is a nice weatherproof design – I can’t work out if I missed a trick with using just one plastic tube rather than a coaxial design. Lots more ideas here.

Postscript (July 20 2015)

five months of data
five months of data

This rig works reasonably well; if power were available I’d run the fan all the time in daylight for a more rigorous result on summer cloudy days. The biggest problem in a polytunnel is that they are shockingly dusty places, and you have to sponge the dust of off the solar panel every month or so.

2 thoughts on “Aspirated air temperature sensor for a Polytunnel”

  1. Hi Richard
    I really like this idea and was just wondering if you would consider sending me the data you are generating, please? I have a polytunnel I built myself and have yet to find a detailed source for temperature data over a prolonged period in the UK. I am in the South East but any data would be extremely helpful to plan crops and see what time of the year I am unlikely to get frost in the tunnel.
    Kind regards

  2. Hi Jonathan,

    I’d added a postscript to this with the data collected by this sensor so far. From other sensors the risk of frost in East Anglia over the last year (July 2014-July 2015) was limited to two periods one late December and the other mid-January, the air temperature minimum was -1.9C and the lowest soil temperature (~5cm depth) was 0.6C. It was a mild winter, though.

    Operational things you should bear in mind is we have people who open the polytunnels in the morning in the spring and I believe they are left open in the summer months. Unfortunately I don’t really have any stable long-term data because these sensors haven’t been out that long and they’re usually deployed for specific target – as it’s on a community farm people sometimes knock the sensors off what they’re looking at.

    You may also be interested in the germination temperature experiment

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