Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_android_vs_ios_winners.thumb.jpg.803608cf7eedd5cfb31eedc3e3f357e9.jpg

ollypenrice

Concrete and heat: how to measure?

Recommended Posts

I use concrete bases for our scopes but I can see the reasoning behind objecting that they would be heat stores. They seem OK to me but I've decided I should look into this. So how do I do it? I buy some thermometers and I put them...where? How about at scope height above the concrete and at the same height above natural land nearby? Or should I put them lower, just above ground?

I'll post the results because it strikes me as important.

Olly

Share this post


Link to post
Share on other sites

You could always use foil covered insulating mat to help stop them warming up during the day...

Or how about covering them in astroturf...

Peter...

Share this post


Link to post
Share on other sites

Olly,

Microclimate monitoring is my thing, or at least it was until I got out of instrumentation and fully into the IT side.

Ideal setup? A control which would be three monitors in and over undisturbed land. One buried to 10cm and disturb the soil as little as you can, one on the surface and one at objective height. Do the same in and over the concrete pad. Monitors buried are not important as the surface and objective height ones though, they just give an indication of daytime heating.

As soon as incoming radiation ends, just before sunset, you'll then get meaningful numbers.

Concrete releases it's heat nearly as fast as undisturbed earth (from what I recall) but has a much larger capacity. So you should see temps on the surface drop faster from the undisturbed site than from the concrete. It'll also cool faster at objective height.

Basic aim is for the delta T to change slowly over time. If it changes quickly, that will mean currents passing in front of the objective and heating of the OTA assuming it doesn't have almost perfect emissivity.

All this of course on totally stable and windless evenings.

Let us know how you get on :(

Cheers

Ian

P.s. some of our 1D rigs would measure temps. At 10cm intervals from surface to 6m. Very interesting output from those.

Share this post


Link to post
Share on other sites

Thanks Ian, I won't be digging (fed up with it!!!) but the two heights thing is a good idea. So I should set up the thermometers after sundown?

Around here the land consists of a mix of clay soil and limestone boulders (you wouldn't build a house on it. Ahem...) I wonder if the thermal mass is really that different from concrete. To be seen...

Olly

Share this post


Link to post
Share on other sites

I used to do it with thermocouples but thats a different story

I would drill a small hole 1 to 2" fill with water allow to stabilise and monitor the difference from air temp just above and at mean telescope height

Steve

Share this post


Link to post
Share on other sites

Olly,

Setup just at sunset is probably the best way if using normal thermometers. Keep them cool beforehand as they have their own heat storage themselves :(

Clay and boulders? Boulders in the sub surface too? Not that much different to concrete as far as heat storage and release is concerned then. But at least it isn't heavy peat and constantly damp :)

One of the best subsurfaces is sand for cool down. Not sure if you have much of that.

If you get stuck for accurate thermometers, look for ones called iButton sensors. Not much bigger than a watch battery but can be programmed for data logging with synced time codes and run many hours on their own. I use them all the time, even have a couple in my big frac.

I used to monitor temps and humidities over runways for military aircraft as this would dictate the amount of engine power and lift the wings would get when fully fuelled and armed in air defence roles. We always used to say in Suffolk that if the Russians wanted to invade, to do so in high summer when our jets could only get airborne on lower fuel/lesser weapons fit out. :)

If you get stuck, just yell.

But one of the best things you can do is cover a concrete pad during the day with highly reflective material to reduce the heat build up. After that a surface with massive surface area, e.g. AstroTurf as Peter suggests with a sub base 10-15cms of dry coarse sand will give you quick cool down but only in the immediate area of course, which is only good if you are aiming close to the zenith. And even that relies on a well mixed boundary layer (up around 400m) and a jet stream a good 300km away.

I.e. you cannot win, but you can help. :evil6:

Cheers

Ian

Share this post


Link to post
Share on other sites

As an alternative to iButtons you can get the same thing in TO92 case like transistors - these are much cheaper and what I use as digital temperature sensors. You can connect any number using just two wires, wiring them in parallel - preferably in a string like fairy lights. The string can then be connected to a computer using a USB to "1-wire" adapter. Display/logging is then done in software.

Share this post


Link to post
Share on other sites

Thanks for this. It will be interesting to see what happens. Alas our soil is about as sand-free as it gets. When I say sub-surface boulders I should maybe say large rocks, sometimes a metre and a half long, for instance. I come across the horrible things whenever I dig a new base or lower a floor, as in our old stable. I named two of the worst ones Pluto and Charon. You just have to smash them with a lump hammer.

You're not alone, Ian, in having temp guages in your scope. One of my guests has a watercooled mirror with a sensor actually inside the glass, plus a couple in the tube.

Regarding National Security the French, too, have a blind spot. It's called lunch time!

Olly

Share this post


Link to post
Share on other sites

Sorry Gina , do you mean serial,like fairy lights?

what is tha addressing schema and calibration like?

I USE LM35Z sensors which are analogue but calibrated and attached to aPic easily read.

Mike

Share this post


Link to post
Share on other sites

I use the TO92 ones, too. Not sure what software you'd need on the PC, but I imaging Windows-compatible stuff is easily available.

James

Share this post


Link to post
Share on other sites
Sorry Gina , do you mean serial,like fairy lights?

what is tha addressing schema and calibration like?

I USE LM35Z sensors which are analogue but calibrated and attached to aPic easily read.

Mike

The sensors Gina is talking about use the "1-wire" interface. Many of them can share a single ground and single power/data line as they identify themselves individually. You can just connect them up in parallel across the wires wherever you need them just as you might do with fairly lights.

James

Share this post


Link to post
Share on other sites
Sorry Gina , do you mean serial,like fairy lights?

what is tha addressing schema and calibration like?

I USE LM35Z sensors which are analogue but calibrated and attached to aPic easily read.

Mike

Being an "analog" sort of person myself, I've used quite a lot of LM35s.

If you like them, have a look on http://www.microchipdirect.com (That's the PIC maker's retail website). They are selling their equivalent, the MCP9700 quite cheap - and since they reduced their shipping costs from "scandalous" to merely "high" :( it's viable to buy PICs and peripheral devices direct from them. They also have an MCP9843 I2C device that works out at < £1 each, for the "digital" people.

Also, if you have an Arduino, the ds18b20 is available from far-eastern sources for a reasonable price, too.

Edited by pete_l

Share this post


Link to post
Share on other sites

I got my DS18B20s from a UK site at 20p each for 10 in quantity I think. I must check if the company are still in business. They sell (or used to sell) a small range of 1-wire stuff.

Share this post


Link to post
Share on other sites

It's a couple of years since I last looked at 1-wire and I'm a bit rusty on the subject. I want to get back into it as I want to monitor temperatures and control devices for my DSLR cooling project. I also would like to get back to my 1-wire weather station.

I've been searching my bookmarks and Google for the information and this is what I've come up with...

Main support site for 1-wire :- http://www.1wire.org/

Suppliers :-

Software :-

Edited by Gina

Share this post


Link to post
Share on other sites

when you set your telescope out to cool, spray about 10L of water on the ground/concrete where you will be setting up. It will be plenty cooled off after about a half hour.

Share this post


Link to post
Share on other sites

The telescopes are there all the time. I don't want to be offering Telescopes a la vapeur!!! (Steamed telescopes...) However, if you are just talking about a concrete base you probably have a point.

Olly

Share this post


Link to post
Share on other sites

Just throwing it out there, but a good jet wash (ok scrub) then an IR reflective coat of paint, wouldn't matter too much if it got a little dirt on either..

http://www.ntt-at.com/products_e/surfcool/

HTH

Edited by Glen
Edit i to'd when i should of too'd

Share this post


Link to post
Share on other sites

I use an infrared thermometer for my outdoor pizza oven. I think it's the perfect device for measuring concrete surface temps.

Share this post


Link to post
Share on other sites

IR thermos require a surface of sorts as they detect reflected or emitted IR from said surface. But we are not really interested in surface temps as they don't affect the seeing, they are just the heat source.

We are interested in the difference between the *air temperature* above the surface and the *air temperature* at the objective. This difference, the deltaT, is a direct measure of the local turbulence you'll see at the eyepiece.

Bottom line, you are looking through about 80km of atmosphere (the air) plus any suspended particulates (smoke, ash, insects, haze etc). It doesn't matter what that mix is with height, we want it to be consistent. Constantly changing seeing especially is hard to image through, trans less though.

Cheers

Ian

Edited by iwatkins
Speling

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.