Dew Point calculation

[baggywrinkle]

I will place this here and in the observation section as it covers both areas.

The link below will take you to a piece of free software that can be put on your desktop as well as iPhone or Android.

Though it was primary written for calculation of moisture in industrial and natural gas the first page might prove useful for helping to predict or warn of those conditions where the dreaded dew condenses onto your 'scope.

http://michell.com/uk/support/sware-downloads.htm

Go to humidity calculator.

Once installed use the first button and drop down the Rh calculation page.

This will ask for two inputs, ambient condition and the %Rh at the time. You might need to get the %Rh from the local met bureau or if you have one a reasonable hygrometer. Be aware the very low cost ones (sub £20) are likely to be only +/-10 to 15% accurate.

Enter the two numbers and press the calculate button.

The dew point will then appear.

This will be the temperature that will be the onset point for condensation to occur.

E.g Enter 15 C as ambient temp and 55% Rh. The dewpoint of +6.03 Deg C will appear. That means is the temperature at which condensate will occur.

Remember the ambient could also be the body temperature of your warm scope taken from indoors to outdoors, if outdoors is cooler and hits the dp you will get dew forming on the warm scope as there is an almost step change in temperature. The same can happen in reverse. Cold scope taken into a warm indoors. The answer to that is to upset the Mrs and turn off all the heating... some things are worth saving....

I know it is pointing out the obvious but of the scope is set up before there are sudden temp changes then the risk of formation of dew is much reduced as the scope gets a chance to equilibriate

[baggywrinkle]

Just to go with the calculator in my previous post is the above image which I hope will explain the relationship between temperatures and dew point.

If the temperature stays to the right of the saturation curve then everything is happy, the water vapour remains in a gaseous form. But as soon as you cross the line (in any direction) then you get dew formation.

The Y axis is shown as pressure. This is the partial pressure of water vapour. Water vapour along with the larger components such as N2, O2, CO2, CO etc all have individual partial pressures which when combined make up the atmospheric pressure. N2 make up 78%, roughly 780mB of a 1 bar atmsophere of air. O2 about 21% (at sea level) or 210mB and so on.

Depending on the temperature of the air it can hold a maximum certain amount of water vapour. Warm air hold more than cold.

A lot of the time we hear the term Rh which is a ratio of the actual water vapour present to the maximum that can be held at a defined temperature and pressure. 100% = condensing conditions, 50% = a comforable level <20% very dry conditions.

Rh is affected by both temperature and pressure. So you can have two days with the same temperture, but on one day it feels humid on another dryer. That is the influence of ambient pressure of the day.

The lenses in your eye pieces and refractors and the mirrors in your Newtonian, Dobbson's and SCT are of course glass and glass is a poor conductor of heat. It tales time to equilibriate. If you take a warm scope into the cold outdoors or a cold scope to the warm indoors you will cross the line and dew will form.

Traceheating works by moving the temperature of the body of the lens or scope further to the right, hopefully far enough so that when you do get cold conditions the body will stay to the right and no dew will form.

In the company I work for we make chilled mirror hygrometers which combined with an accurate PRT can measure dew points in gases to an accuracy of 0.1 Deg C. These use the above theory in a practical manner.

Any questions please feel free to ask.