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Osbourne One-Nil

Measuring Ground Frosts

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With a suspected ground frost here last night, I'd like to be able to say for sure whether there has been one, or not. I can't justify the expense of getting an add-on for my Vantage Pro 2, but think I can stretch to a new max/min thermometer.

Would it suffice to attach this to a post and simply place the bulb of the thermometer at ground level, and does it have to be grass, or could it be in an open area of a border?

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With a suspected ground frost here last night, I'd like to be able to say for sure whether there has been one, or not. I can't justify the expense of getting an add-on for my Vantage Pro 2, but think I can stretch to a new max/min thermometer.

Would it suffice to attach this to a post and simply place the bulb of the thermometer at ground level, and does it have to be grass, or could it be in an open area of a border?

The thermometer needs to be lying almost horizontally, supported by a couple of Y shaped pegs, above a short grass surface, OON, with the bulb end of the thermometer very slightly lower than the other end and almost touching the grass.

It also needs to be completely unsheltered by anything above it. I once did a few experiments with two thermometers, both of which were out in the open but one of which had a shield of 2" chicken wire suspended about a foot above it.

The thermometer with the chicken wire above it regularly read higher than the other one and sometimes by several degrees C.

If you attach the thermometer to a post, even with the bulb touching the grass tips, the post will greatly affect your readings. If you place it over bare soil the errors will be even greater as grass blades radiate heat much more effectively than soil, particularly if the soil is damp or wet.

Ideally you need a sheathed minimum thermometer but these are mightily expensive ( £95 including [email protected] )

An ordinary minimum thermometer will do the job but you'll need to take it out of the sun during the day as they rapidly become inaccurate due to slight evaporation of the spirit in hot sunshine.

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What's this "hot sunshine" you talk of?

I have emailed a firm asking for the price of this - http://www.fairmountweather.com/products_bottom.php?cat=4&pid=30 - but how long it would last in the lawn with three kids and a Labrador is open to debate. So, I've just been to Pigneys -it's a marvellous shop; think fork candles in the Two Ronnies and you've got it right (for example, they sell the washers for they type of salt cellars you have at school). I bought myself a cheap max/min thermometer, and have carefully removed it out of the case and separated the thermometers. I could calibrate it with my VP2 one evening, and perhaps simply hold the scale against it if and when I suspect a ground frost has occurred. Or I could simply keep it mounted in the scale and place it as you suggest. If I did that, the bulb would be totally exposed at least.

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The thermometer from Fairmount is exactly what you need, kids and Labrador notwithstanding. Perhaps you could erect a steel security fence around it, topped with razor wire. I'd be very interested to know the price of it as I've just bought a spare one from Munro at £95 all in.

I'll have a browse around their web site to see what else they've got.

The thermometer you've bought will certainly give a rough idea of whether or not there's been a ground frost. The main problem I've found with relatively cheap thermometers is that you can calibrate them quite well at a given temperature but they won't hold the calibration at a significantly higher or lower temperature.

What's the price of a grass min' add on for the Vantage Pro you've already got?

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Well, I'm not entirely sure I know how much it would cost to use my VP2. I currently have a wireless sensor suite and a separate anemometer transmitter. I did wonder if I could plug a probe into one of those, but I think I'd need to get a separate leaf and soil temperature station (£264) in addition to the probe (£54). Perhaps you're thinking you might get it for me for Christmas?

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Well, I'm not entirely sure I know how much it would cost to use my VP2. I currently have a wireless sensor suite and a separate anemometer transmitter. I did wonder if I could plug a probe into one of those, but I think I'd need to get a separate leaf and soil temperature station (£264) in addition to the probe (£54). Perhaps you're thinking you might get it for me for Christmas?

Strangely, the thought hadn't entered my head.

Is it not possible to mount the probe in a similar fashion to a standard grass min' thermometer and plug it in to the existing system?

Forgive me if I've made a ridiculous suggestion, I only use traditional equipment ( old skool for our younger members ) and am not at all familiar with automatic stations and how they operate.

However I could do with learning as lately I've been toying with the idea of buying one as a back up to my current set up.

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I'd have thought it would be possible to do as you suggest, but then, how would Davis make an additional £300+ if they allowed that?

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I'd have thought it would be possible to do as you suggest, but then, how would Davis make an additional £300+ if they allowed that?

Ah yes, the dynamics of commercial enterprise. I hadn't allowed for that.

It's not possible then to buy the sensor from Davis without all the peripheral stuff? Are other Davis temperature sensors compatible with the VP?

I have an old Davis Weather Wizard mounted on a mast on the roof and I seem to remember buying the temperature sensor and mini screen as separate items.

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There is a separate temperature station that Davis provides, which wouldn't require anything additional, but then it's £225 on its own. I'll give Skyview a ring or drop Davis an email, just in case I'm missing something obvious, but I don't think I am.

I still think a sheathed thermometer would be best, and I do have a completely secure area of grass; the en-suite garden for the rabbits, which could securely home it. It's not the most exposed part of the garden, but it's better than nothing. Another option would be to stop obsessing about ground frosts, but that wouldn't be me at all.

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Another option would be to stop obsessing about ground frosts, but that wouldn't be me at all.

No, me neither.

While we're on the subject of the V.P, I see from the photographs of the sensor suite online that the raingauge is mounted on a post, is it possible to remove it from the post and set in in the ground, as a standard raingauge, but still connect it to the rest of the system?

If it's mounted on a post it will undoubtedly under-record, although judging from some of your rainfall totals in recent years you may find that hard to believe.

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You can't separate the rain sensor from the temperature gauge as it is one complete item, but you could supplement the system with an additional temperature sensor and place that where you wanted. It does start to get even more expensive though.

I wouldn't be surprised if my rain gauge under-records because it's in the rain shadow of some big sycamores, although well outside their drip line.

post-717-037079600 1283001289_thumb.jpg

I also wouldn't be surprised if my temperatures read a little too high (if anything) during the winter, as the sensor is only 6m from the house, but at the right height and above gravel, so hopefully it's about right.

post-717-023934700 1283001441_thumb.jpg

Similarly, my wind speeds are going to be moderated by the big trees, although the anemometer does have some height to it.

post-717-012354600 1283001513_thumb.jpg post-717-031145800 1283001528_thumb.jpg

Now, this is where I wondered about any grass thermometer. It is close to the house (about 3m), but on a cold calm night, I don't think it should affect it too much, and the plants are deciduous, and can be cut back anyway. At least it's protected. What do you reckon?

post-717-021279200 1283001607_thumb.jpg post-717-066913300 1283001666_thumb.jpg

It's difficult to get it perfect...as I tell my wife whenever she attempts to cook.

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Now, this is where I wondered about any grass thermometer. It is close to the house (about 3m), but on a cold calm night, I don't think it should affect it too much, and the plants are deciduous, and can be cut back anyway. At least it's protected. What do you reckon?

post-717-021279200 1283001607_thumb.jpg post-717-066913300 1283001666_thumb.jpg

It's difficult to get it perfect...as I tell my wife whenever she attempts to cook.

You'll find it will read rather high, compared to the same thermometer in the middle of an open lawn, due to the proximity of the fences, rabbit hutch etc but it's certainly better than in a border or on a post.

It'll be the marginal ground frost situations you'll miss out on, the ones where it would have fallen to less than 1c below freezing on the open lawn.

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Then it wouldn't be good enough. The kids can play on the rabbits' grass, and my lawn shall be devoted to ground frost detection. That, to me, seems a perfectly reasonable compromise.

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Then it wouldn't be good enough. The kids can play on the rabbits' grass, and my lawn shall be devoted to ground frost detection. That, to me, seems a perfectly reasonable compromise.

Absolutely! How can a few moments of children's play be compared to the pursuit of scientific excellence?

A philosophy to which my own children would bear bitter testament.

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You need a proper thermometer. The Davis isn't accurate enough as it only record in 0.5C steps.

I do have my Davis recording grass min but it's only a very rough guide. Got it connected to a Leaf Soil moisture station.

The fox and it's cubs like playing with the grass min which can be a bit of a problem at times.

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Having given this subject a bit more thought, as the grass min' thermometer only needs to be exposed at night, usually from 1800 until 0900 the next morning, would it not be possible to put the thermometer in the middle of the lawn at night and take it in the next morning?

That way you'd get the required readings with as much accuracy as possible and the kids and dog would be able to play without fear of breaking the thermometer.

Of course this relies on the kids and dog not playing after 1800 which in winter is probably not an issue but could well be in summer, although you could put the thermometer out later, when they were in bed.

I'm assuming, given that you have fluffy rabbits, that your kids are not teenagers who are out kicking a football until such a time that you've lost the inclination to drag yourself from the t.v to put out the thermometer.

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You need a proper thermometer. The Davis isn't accurate enough as it only record in 0.5C steps.

I agree I probably need a proper thermometer, but my Davis records in 0.1C intervals. Surely yours does too?

Does this look like the right sort of thing? Why would it have to be horizontal? Is it to reduce any sheltering from above, or is it so the minimum needle doesn't simply slide down the tube?

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I agree I probably need a proper thermometer, but my Davis records in 0.1C intervals. Surely yours does too?

Does this look like the right sort of thing? Why would it have to be horizontal? Is it to reduce any sheltering from above, or is it so the minimum needle doesn't simply slide down the tube?

It's actually supposed to be 5 degrees from horizontal with the bulb end lower. This is to reduce any gravitational effect on the indice as the spirit expands and contracts within the tube but is not so steep that the indice moves under its own voilition.

The idea of having the thermometer horizontal is so that the bulb presents the maximum surface area to the sky, if it were vertical the length of the thermometer tube would affect the amount of outgoing radiation and therefore affect the reading. You wouldn't believe how little shielding is required for a major effect on the reading.

The link you posted didn't work for me by the way.

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http://www.metcheck....hermometer.html

You might not want to look, as it's £15 cheaper than yours.

I assume it's still a traditional needle which records the lowest temperature reached, and if so, how do you re-set it?

That's exaactly what you need. To re-set it you simply tip the thermometer bulb end up until the indice sinks back to the column of spirit.

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I put out a wireless sensor (around £10) when a GF is possible, or an especially severe frost is on the cards. THis works well and it can go back on its post in the daytime. I do fancy a proper grass min thermometer though.

OON- I will get a VP one day, how do you find the seperate solar anemometer is it reliable, and where did you get that impresssive mast from?

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My impressive mast is the talk of the Eden Valley. It's just an aerial mast I got from the local TV shop with the only additional costs being some expensive therapy after being so high up a ladder, and a new pair of pants.

It's been up for three years now, I think, and I've never had to change its battery yet. It also transmits unfailingly to the console inside, which is on the opposite side of the house.

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My impressive mast is the talk of the Eden Valley. It's just an aerial mast I got from the local TV shop with the only additional costs being some expensive therapy after being so high up a ladder, and a new pair of pants.

It's been up for three years now, I think, and I've never had to change its battery yet. It also transmits unfailingly to the console inside, which is on the opposite side of the house.

Thanks. It is a useful mast ! I would have to get someone to put mine on the roof - I would at last record gales then.

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Received the thermometer, got rid of the bubbles, and placed it outside last night for the first time. I reckon it read a minimum of 3.7C (depending on how accurately I can read it) compared to a minimum air temp of 5.9C.

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