New dew system

matt · #1 26-03-2006, 05:16 PM

Guys. Would really appreciate some advice on this.

As we're heading into the cooler nights it's becoming increasingly obvious I'm in need of more than a simple dewshield to combat the dew problem with my SCT.

I recently bought a 12V 17aH power pack which has 2 12V outlets and is already powering my EQ6 and Synscan.

I'm thinking of plugging a Kendrick Model VI dew controller into the second outlet to primarily power an AstroZap heater for the CP on the 9.25.

I'm not too concerned about heaters for the finder and ep's at this stage.

Based on that, how much time should I get out of a fully charged battery with very moderate use of go-to and setting the controller on low-med?

Also, can you think of a better way to set myself up as far as my power needs go?

Your help much appreciated.

vespine · #2 27-03-2006, 08:20 AM

Quote:

Originally Posted by matt

Based on that, how much time should I get out of a fully charged battery with very moderate use of go-to and setting the controller on low-med?

That's a tough one, unless someone has specific experience with exactly this set up and viewing habits it would be hard to guess, even roughly. My guess tho is that the motors of the go-to would be the thing that use the most power so the less you use that the longer it will last.
Another thing you can do is calculate how much power the heater uses, I've googled it but can't find what the wattage of it is, do you have the specs? Looking at other home made dew heaters it looks like you may need as little as 1 - 4 watts, say it's 3 watts, 3w/12v = 0.25 amps per hour. Which very roughly means your battery will theoretically run a 3 watt heater for 68 hours

Now, that's 'theoretical' which doesn't account for imperfections like discharge rates, internal battery resistance and other "inefficiencies", in reality it will probably be considerably shorter then that, around 80% for lead acid batteries, which is still over 50 hours.
On the other hand, small electric motors are more like in the order of 30 watts and there's probably 2 in a go-to right? So that's using a lot more power, does the go-to use power while it's "not going to"? If it has a display and stuff it probably is but that would be quite low too I guess.
Over all, I think that set up sounds pretty good really, anything else I could imagine would start getting more complicated and much heavier. My guess is that the battery should last at least 15 hours with the type of usage you describe, but ultimately you'd have to try it, could be a bit less, could be a lot more. That sounds like a couple of nights viewing between charges. Lead acid batteries, unlike some others, are generally not adversely affected by frequent recharging so you can charge it up every night after use if you want.

sheeny (Al) · #3 27-03-2006, 08:21 AM

G'Day Matt,

Not sure how long you'll get out of the power pack, but if time becomes a problem, of course you can always go to a second battery pack. I'm using a 7AH Celestron power tank to drive my scope, and a 24AH jump starter pack to run my laptop and dew heater. I think the lappy is ther major load on that one!

I find that after about a 6 hour session, the 24AH pack can take over 24 hours to charge! I haven't driven the power pack down till the recharge warning light comes on yet in a session.

9.25 dew heater should draw what... say 1.5A flat out? My 8" one draws 1.1A flat out.

Al.

Roger Davis · #4 27-03-2006, 08:31 AM

A Kendrick 10" heater draws 1.9A as max. Since the current is intermittently drawn and if you have cracked the power on early in the evening then you may suck as much as 1.4A.

vespine · #5 27-03-2006, 08:44 AM

Quote:

Originally Posted by Roger Davis

Since the current is intermittently drawn and if you have cracked the power on early in the evening then you may suck as much as 1.4A.

Oh jeez, I guess I really under shot it with the 3 watt guess, 1.4A means it's about 17 watts, so , divide my time estimates by 5

Although looking back, 15 hours was probably too conservative an estimate too, it may still be around 10 hours....

matt · #6 28-03-2006, 05:22 AM

So ... sounds like the 17Ah battery should do the trick ... for the Eq6 and Synscan plus Kendrick dew system for a session of about 8-10 hours, you reckon???

I guess I won't truly know until I plug it all in out in the field and switch everything on ... and stand back and go "awwwwwwwwwwwww"!!!!!

vespine · #7 28-03-2006, 07:09 AM

Ok, having a quick last look, if the heater runs at 1.4A the battery will last almost 10 hours, that's full charge in good conditions and JUST the heater. So it really depends on how much that EQ5 and Synscan uses. If it uses anything close to 1.4A then you'll have to halve the time to 5 hours. I guess, looking at it again, if you DO sometimes stay out a long time and use the go to a couple of times an hour, you might be pushing it to last the whole session... But it's definitely still worth giving a go.

matt · #8 28-03-2006, 07:22 AM

Likewise ... I may not have to run the heater for the entire session???

Cheers

sheeny (Al) · #9 28-03-2006, 11:34 AM

Quote:

Originally Posted by matt

Likewise ... I may not have to run the heater for the entire session???

Cheers

Even if you do run it all session, you don't have to run it on full heat, so using a Kenrick controller (or other PWM controller) you can extend the life of your battery.

For example last night I set up about sun set with the dew heaters on about 25%. After twilight when thing started to cool quickly, I cranked it up to about 75% but a couple of hours later when it looked like the dew just wasn't going to happen, I dropped it down to 25% or so again. If the dew hits hard and heavy, you might need to run it flat out. It's prudent to run the heater before you get dew forming... its far easier to keep it away than to evaporate it afterwards.

If you are interested I had a dewpoint table somewhere. If you know your relative humidity and can get a forecast of the expected minimum temp overnight, the table will tell you if you'll get dew or not and, with some experience, will give some indication of how heavy the dew will be. Had I been deciplined/organised enough to use it last night I might have known I wouldn't need the dew heater

...

Al.

matt · #10 28-03-2006, 12:26 PM

Yep. Would appreciate that dewpoint table. Thanks Al

GrampianStars (Rob) · #11 28-03-2006, 12:37 PM

Hey Guys
I only run the DH when needed
I keep a mini dryer on hand
just in case of sudden temp differentials
to warm the corrector
then turn on the DH
no warm current eddies all night

sheeny (Al) · #12 29-03-2006, 05:11 PM

Quote:

Originally Posted by matt

Yep. Would appreciate that dewpoint table. Thanks Al

I knew you'd say that! I went looking for it after my last post, but I can't find it at home at the moment... I'll have a look at work tomorrow.

Al.

sheeny (Al) · #13 29-03-2006, 05:16 PM

Quote:

Originally Posted by GrampianStars

Hey Guys
I only run the DH when needed
I keep a mini dryer on hand
just in case of sudden temp differentials
to warm the corrector
then turn on the DH
no warm current eddies all night

hmmm good point. Since I have a 12V hair dryer as well, when I dig out the dewpoint table for Matt, I could adopt your approach on low humidity nights, and revert to my own on high humidity nights... not that it's a biggy!

Al.

sheeny (Al) · #14 29-03-2006, 06:09 PM

Matt,

This isn't the table that I have, but found this on the web.

http://mvddaq.desy.de/~mvddaq/SC/Tdew.html

This has more data points than the table I have (somewhere), but doesn't extend below 0 degrees C for the dew point. I expect this table is calculated from the formulae for condensation rather than reflecting the real conditions which include frost (hoar frost particularly) when the "dewpoint" is below zero...

To use this chart:
Measured temp (dry bulb) is down the left.
Relative humidity (rh) is across the top.
The corresponding dew point temp is in the table.

So if you know the temp and the rh at the same time (have a weather station?) you can find the dew point temp to expect that night. Why? the relative humidity changes with temperature because warm air can absorb more water vapour. The actual proportion of water vapour in the air (absolute humidity) changes very slowly, so we can assume it doesn't change if you check the temp and rh the arvo before observing. So if you know the forecast minimum temperature, you can tell whether it will be dewy or not.

If the forecast minimum temp is less than the dewpoint temp, you will get dew. The lower the forecast minimum is below the dew point, the heavier he dew will be.

If you don't know the forecast minimum temp, you will probably get a pretty good feel just from the rh. High rh, high probability of dew. Low rh, low probability of dew.

Hope this helps. I'll still try to find the other table, though, since it includes dew point temps below zero degrees C i.e. frost.

Al.

Striker (Tony) · #15 29-03-2006, 06:29 PM

Sounds like a lot of drama just to know when to operate a dew heater.

Buy another 17AH battery pack and use that to run the dew heater...their only around $40

My Orion dew heater drains approx 1.7amps per hour.

Also you would be better of using a seperate power source for a dew heater as it may affect the operation of your mount.

matt · #16 29-03-2006, 06:41 PM

No dramas, Tony. Just a question that's been out there and the boys are just being helpful and offering more info than my little brain will probably be able to process

If you like to run multiple power tanks, then good. I'm just keen to find out whether it's possible to take care of all my needs off just the one. If not, I'll get another one. Simple.

The more info, the better. Especially when I've only just encountered the SCT/dew monster. I'm still all a bit new to this, mate. Don't kill off the debate just yet. I'm finding it all useful

Striker (Tony) · #17 29-03-2006, 06:46 PM

Thats cool Matt...my offering is just be very careful running a high drain heater of your mount power supply.

Keep debating.

sheeny (Al) · #18 30-03-2006, 08:18 AM

OK Matt,

Here's the Dew Point Table I promised. I've also included a graph of the same data - the waviness in the lines of the graph is just rounding error from the data. In reality they should be straight, but we're only talking 1 degree resolution here!

Striker I also appreciate your very practical advice (that's why I use two power tanks) but I'm happy to explore the science of the whole situation. You never know, someone might learn a little bit that helps them to make a wise decision somewhere down the track. It's all just little pieces of the big jigsaw!

Al.

matt · #19 30-03-2006, 08:28 AM

Onya Al.

More tables, charts and printed How-To pages to slide into my AstroNerd folder/bible

sheeny (Al) · #20 30-03-2006, 05:04 PM

Let me guess... you've found an astrobabe who's really impressed by the tables and graphs and printed how to pages in your astronerd folder?

I know how it is!!!!

Al.