Water evaporation with a cover on?

[mscdman]

So been wondering about this. If you had a normal fitting new cover and a temp set at say 104 but never really opened and never used the tub (just an experiment - no obvious questions like like why have a tub then :)) would you still see any water loss over a month?

im thinking no, but then again these covers aren’t 💯 hermetically sealed…

[mscdman]

Anyone?

[Cusser]

A little loss in a month, less than 1/2 inch for me.

[mscdman]

Yeah but you had a leak 🤣. I’m talking if there was a zero leak Covered tub that was set at 104 and never used would it be reasonable to say see a 1/16 or so inch water loss drop?

[RDspaguy]

On 5/2/2021 at 7:40 PM, mscdman said:

a normal fitting new cover

Define normal. Spa cover manufacturers give themselves an inch and a half of extra "wiggle room" on their cover sizes, guaranteeing only a minimum dimension. So no two covers can be assumed to fit the same.

Some covers have a seal along the hinge and others don't. 

Some spas have dips, cut-outs, etc on the spa shoulder affecting seal.

The greater the difference between water temp and air temp the greater the evaporative loss.

In other words, there is no answer to your purely theoretical question. It's a spa, expect it to lose water.

[mscdman]

Ok so in my case the cover is a 4 inch taper to 2 inch.  There’s about 1/2 inch overlap around the perimeter and there is a “foam and vinyl covered flap) that is at the hinge and wedges between. 
 

im okay with water loss.  Provided it’s not a leak.   I’ve noticed on my tub even with the cover on 💯 of the time over a month I might see 1/16 loss.   
 

I think you’ve answered my question.  It’s unlikely a spa cover will 💯 seal in all evaporation even without being opened.  So some very minimal water loss could be expected even without use.  Especially as the water temp greatly exceeds the ambient temp and with minor imperfections in the cover design