Coolant evaporated?

[SunSoaked]

Is it normal for 50/50 mix coolant to evaporate over time?
I've noticed my level slightly lower in my surge tank. I know it's not an engine issue. I don't have the issue on any of my other vehicles so I'm wondering if the "vented cap" on the surge tank is the culprit. Was this just a common maintenance issue back in the day?

[geezer#99]

Shouldn’t change,
Either it’s leaking somewhere, being burned away from a crack in the heads or your catch can is the culprit.
How big is your catch can?
How low do you keep the low level at?
Do you have an overflow hose on the vented cap or on the catch can?

[SunSoaked]

It's the OEM setup. It's obviously not a sealed system so it's open to the atmosphere. The cap has slots in it, kinda like a castle nut for lack of a better description.
Here in S. Arizona, it's dry and temps reach 115 daily in the summer. Most any liquid will evaporate over time. I just thought that glycol wouldn't?
I only fill the jug to the fill line that is maybe 20% of it's volume.

[geezer#99]

You got capacity.
Is that the cold line marking on the side?

[SunSoaked]

Yes, the cold line is on the front. After a month or two, the level usually drops about a quarter or half inch. Further research on the net revealed that it is normal for a small amount to evaporate over time. Maybe our 115 degree summer days have more of an effect than others.

[geezer#99]

To see if it’s evaporation you could put some coolant in a similar container and see what happens.
A gallon jug with a hole in the cap would work.
Put it on a high secure shelf to protect it from kids, pets and varmints.

[100%Chevy]

Glycol won't evaporate but the alcohol and water will
Replace it once a year to keep the mix correct.
Mike.