Nichrome dew heater questions - Page 3

erick (Eric) · #41 02-09-2007, 10:50 PM

Actually Ken, I did read the earlier posts a few days ago (or whenever it was). I was very surprised with the big difference between 1 metre and 1.3 metre. Maybe someone else can explain that, but it's odd since it should be a linear relationship?

Still, the 15V supply won't be a major problem, you'll just need to add a suitable dropping resister for your fan. We can work that out - but you sure need a multimeter working on the job! I'm hoping to come up Friday night - it's sort of 50:50 at the moment. I'll bring my meter, some leads, and we'll check things out - if it can wait that long? (Remind me later in the week!)

ballaratdragons (Ken) · #42 02-09-2007, 10:57 PM

Yeah Eric, the heat loss over only 300mm was a wierd one.

As for bringing a multi-meter up on Friday, I'll probably have the whole thing built and running by tomorrow afternoon

I will put a 'Pot' in the fan line, and re-measure the Nichrome at 15 volts. It will be slightly different to the 13.5 volt supply.

I'll work something out

EDIT: I might as well put 'Pots' on all the lines while I'm at it. I have some

Doug · #43 02-09-2007, 11:33 PM

Quote:

But if I use twice the length of Nichrome, it won't even get slightly warmish. Heat is almost undetectable at 1300mm.

Looks like I'll have to use the 3.5 amp power supply, but then I have the other problem. It's lowest voltage is 15v.

Ken why is that a problem?
I'd respectfully suggest that you should repeat your earlier experiment.
If 1m of wire was piping hot, 1.3m would have to be better than just barely warm. You might have had a poor connection for the second test.

I gather Ballarat is somewhat cooler than cold at night, so the wattage of a heater will need to be somewhat more gutsy that what is needed up here in Sydney. The Dew heater I use on the LX 200 is about 28Watts, (at 12v) and it is sometimes necessary to crank the voltage up to about 18v to keep the dew away. You wont fight dew with a 12v 1Amp supply; 12 Watts is way too lite IMO. (Though an 80 or 100mm scope will need less wattage that a larger 12" scope.) As for Pulse circuits and potentiometers, pass transistors and a few other 'name dropping' devices, I believe that you will find that as long as you keep the wattage rating down around the 18-20 watt mark you should have no problems. Why?
Because (assuming good thermal contact) your scope bodies will act as massive heat sinks and so the thermal gradient across the OTA will be negligible, leaving the lens cell barely heated above ambient temperature.
which is more or less what you need. Dew point is of course what you are fighting, but there are still practical limits to the wattage you apply to a dew heater; a fan to augment the dew heater should not be overlooked as a possibility IMO.
Doug

ballaratdragons (Ken) · #44 02-09-2007, 11:40 PM

Doug, sorry to frustrate you too, but what the hell is wattage?

I've already been confused about amps and volts, now watts!

The test on the length was carried out and measured several times, so it wasn't a bad contact

ballaratdragons (Ken) · #45 03-09-2007, 02:34 AM

Too late!

2 Heater strips made and working perfectly

I measured with the 15v power supply, cut the Nichrome to length, and encased it all inside cloth tape with Felt as a heat barrier on one side to stop heat from escaping into the outside air.

I decided to wrap the heaters around the outside of the dew-shields. It was too much hassle trying to wire them inside.

Tomorrow night I should have it all wired up and fitted

I'll let you know how successful (or not) it is.

Thanks everyone for your help

montewilson (Monte) · #46 03-09-2007, 06:33 AM

Wattage (Watts) = Voltage (Volts) x Amperage (Amps)

Watts may also be written at E (Energy)

W or E = VA

PS are you heating the scope or the dewshield? It is important to get the heat into the scope body. If not you will have the driest dewshield in the land and a dewed up telescope.

avandonk · #47 03-09-2007, 08:18 AM

How dew inhibiting heaters work is quite simple. The infra red radiation from the heated dew shield onto the optic surface should only just counteract the infra red radiation of your lens or corrector plate into the clear night sky which has a temperature of about 3 degrees Kelvin (-270C)!
The loss of heat into the sky is called supercooling. That is why a frost can occur even when the air temperatue is about 2 or 3 degrees.

A modest wattage is all that is needed about 10W for a 100mm and 20W+ for a 200mm etc. So for 12V about one amp for a 100mm and two amp for a 200mm. For Ballarat a bit more might be needed on really cold nights ie -5C.

Bert

wasyoungonce (Brendan) · #48 03-09-2007, 10:26 AM

I found different types of nichrome wire..different resistances and gauges at Wiltronics on ebay

http://stores.ebay.com.au/wiltronics...QQftidZ2QQtZkm

They come in rolls but at least you can get the resistance and length configuration that suits your needs.

ballaratdragons (Ken) · #49 03-09-2007, 12:41 PM

Quote:

Originally Posted by montewilson

PS are you heating the scope or the dewshield? It is important to get the heat into the scope body. If not you will have the driest dewshield in the land and a dewed up telescope.

The ED80 dew shield sits directly on top of the lens cell. That's where I'll be placing it. The heat will have to seep through the dew sheild, then slowly work its way through the cell, then to the lens.

It may take 15 minutes or so to soak in.

The guidescope is easy as it has no cell. The lens it directly up against the OTA.

Doug · #50 03-09-2007, 07:51 PM

Ken what a watt is can be a bit confusing I admit. But apart from the mathematical relationship Monte (I think) gave you, Here is a practical (I hope) explanation.
The Watt is a measure of power. It is the Watt what does all the work.
The Ampere, the Volt of the Ohmic resistance do nothing on their own.
If a certain wattage is needed to do a thing, then that is what must be provided. Within reasonable limits it doesn't matter what the voltage is, it doesn't matter what the current is, it doesn't matter what the Ohmic resistance is.
Those reasonable limits would mean that a very High voltage is not desirable because of safety, insulation/ arc over problems.
In the same way, very High currents are to be avoided because of the risk of overheating the wires used to carry the stuff, and the need for larger cross sectioned conductors of higher conductivity.
Have you heard of 66KV +++ transmission lines? Those Extra High Voltage Transmission lines are feeding a City, even a State. Imagine the power that these lines are called upon to carry.
If that power were to be supplied using 240V, refer to Monte's formulas,,,Current = Power/Voltage, then to supply the megawatts needed by a city, the current is going to be very high and the cross sectional area of the conductors will need to be impracticably large to avoid severe losses. However at 66KV and higher, the same power can be delivered at considerably less current, hence smaller /lighter cables.
Ken if you are interested, you might like to look at this link:
http://www.powercor.com.au/docs/education/sheet_f.pdf

So to come back to the point, power (Watts) is what is going to move your objective temperature from ambient temperature to some value above ambient.
12v at 1 amp and 12 ohms will give 12watts. 240V at 0.05 Amp and 4.8K ohm will also give you 12Watts. 1volt at 12 Amps and 0.083 ohms will give you 12 Watts. So theoretically the same power can be delivered by various combinations of voltage and current, but in practical terms we must choose wisely what we will use. Anyways I've probably prattled on needlessly, it is a big subject to tuck under your belt overnight, however you might gain much understanding by studying the material at the link I provided.

cheers,
Doug

ballaratdragons (Ken) · #51 03-09-2007, 07:55 PM

Thanks Doug. A bit of reading might help