Question - Please explain the difference between " hydroscopic " and
" hygroscopic ".
Would it be better to refer to (for example)the water absorbing nature of
nylon as a hydroscopic or hydroscopic material?
---------------------------------
Henry,

The answer to your question is, neither.

My dictionary offers the following definitions:

hy.gro.scop.ic Pronunciation: "hI-gr&-'skä-pik Function: adjective Etymology:
hygroscope, an instrument showing changes in humidity + [^1]-ic; from the use
of such materials in the hygroscope Date: 1790
1 : readily taking up and retaining moisture
2 : taken up and retained under some conditions of humidity and temperature
- hy.gro.scop.ic.i.ty /-(")sk@-'pi-s&-tE/ noun

"hydroscopic": <=== Dictionary has no entry for hydroscopic

Nylon is not at all good at absorbing water. It is polymeric structure is
hydrophobic (water hating) rather than like cotton which is hydrophilic (water
loving). Thats why nylon clothing is so uncomfortable in hot and humid
weather.
Under those conditions, I will take cotton every time.

Regards,
ProfHoff
================================================== =======
As far as I am aware and can find in dictionaries, there is no word
"hydroscopic." "Hygroscopic", on the other hand, is a respectable word,
meaning the tendency of some materials to absorb moisture from humid air.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

Originally posted by flubyux2
i think youre wrong and im right... i thought it was hyGroscopic....


http://www.newton.dep.anl.gov/askasc...9/gen99896.htm

POLY GLYCOL ETHER BASED FLUIDS
Fluids containing Poly glycol ethers are regarded as DOT 3, 4, and DOT 5.1. These type fluids are hydroscopic meaning they have an ability to mix with water and still perform adequately. However, water will drastically reduce the boiling point of fluid. In a passenger car this is not an issue. In a racecar it is a major issue because as the boiling point decreases the performance ability of the fluid also decreases.

Silicone based fluids are non-hydroscopic meaning that they will not absorb or mix with water. When water is present in the brake system it will create a water/fluid/water/fluid situation. Because water boils at approximately 212º F, the ability of the brake system to operate correctly decreases, and the steam created from boiling water adds air to the system. It is important to remember that water may be present in any brake system. Therefore silicone brake fluid lacks the ability to deal with moisture and will dramatically decrease a brake systems performance.

Hydroscopic products are those which readily retain moisture based on temperature and humidity. They can be dried to their equilibrium moisture content in accordance with their desorption isotherms as related to air humidity and product temperature.

The DOT brake fluid classifications (49CFR571.116) include a set of minimum specifications that are guidelines for manufacturers as to how impervious their fluid is to heat. The dry boiling point (when the fluid is fresh and contains no water) is the temperature at which the fluid turns to vapor. The wet boiling point (measured when the brake fluid contains 3.7% water) is related to how easily the brake fluid will absorb water (or how hydroscopic the fluid is). The lower the wet boiling point, the more water the fluid will absorb. Absorbing water is bad, so a higher wet boiling temperature means better brake fluid. And, of course, a higher dry boiling point is good too.

Essentially there are five groups of commercially available brake fluid: DOT 3, 4, 5 and 5.1. Other types are purely synthetic or custom-made. This item will discuss the 4 common types above.

• DOT 3:

Advantages: cheap, readily available.

Disadvantages: highly corrosive of paint; hydroscopic (readily absorbs water in the atmosphere or in lines, such as condensation); short shelf-life once opened - do not retain for more than about 2 weeks once opened; can corrode the sytem very quickly if not replaced regularly.

• DOT 4

Advantages: generally easy to find in parts stores; less hydroscopic than DOT3; higher boiling point than DOT3, therefore more suitable for high performance uses; slightly longer shelf-life than DOT3, but best to apply the same rule: discard after 2 weeks of being opened.

Disadvantages: highly corrosive of paint; can sometimes be a little or a lot more expensive than DOT3, depending on country; corrosion can occur in the system if not replaced regularly.

• DOT 5 (also known as 'silicon' or 'synthetic' brake fluid)

Advantages: is not corrosive of paint or metal; is NOT hydroscopic.

Disadvantages: Is NOT compatible with DOT 3 or 4. If DOT5 is to be used, then all seals must be replaced or the system will fail. Since it is not hydroscopic, then water in the system has a tendency to pool or form puddles leading to localized corrosion of components. Bleeding must be done VERY carefully; small air bubbles will eventually collect to form large ones. DOT5 is slightly compressible and has a lower boiling point than DOT4.
Verdict: Avoid using this fluid!

• DOT 5.1 (should have been called DOT6 !)

Advantages: superior to all the above. It has a very high dry and wet boiling point (dry = about 275 degrees C; wet = 180-200 degrees C). Racing fluid, in comparison, has a dry boiling point of about 300 degrees C, so DOT5.1 is pretty close. Compatible with DOT3 and 4, but it is best to flush the system completely. Seal replacement is NOT required.

Disadvantages: is non-silicon and therefore hyrdoscopic; corrosive; more expensive than DOT3 and 4.

Verdict: if you can get it, use it! You will still need to replace it every 12 to 18 months though.

Originally posted by flubyux2
ok sounds good to me... except this is the article that made me think it was "hyGroscopic" in the first place.

http://stoptech.com/whitepapers/brakefluid1a.htm

stoptech is a reputable company so i figured i could listen to them

Why the heck do we use brake fluids that absorb water in the first place?

Believe it or not, one of a brake fluid’s most vital characteristics is its ability to absorb water. Yes, you read that correctly – brake fluids absorb water by design and that is really a good thing.

What?

Whether we like it or not, water is everywhere and finds its way into everything. That’s just the nature of the beast. Even our brand-new sealed brake system will eventually absorb water given enough time.

The magic of diffusion allows moisture in the air to permeate microscopic pores in the rubber brake hoses, the nylon master cylinder reservoir, and the various rubber seals in the hydraulic system. Sadly, there is nothing we can do about it and if left unchecked the water would sit in our brake system and rot it away from the inside out.

Hence the need for brake fluid to absorb this unwanted house guest. Because brake fluid absorbs water into solution, the local concentration levels are typically low enough that corrosion is slowed dramatically. As an added benefit, when exposed to low temperatures, the solution state prevents the water from pooling and freezing on its own. While water in brake fluid will certainly increase the solution viscosity at low temperatures, this is much more desirable than having little chunks of ice plugging up the system!

Originally posted by Icemark
I personally don't like the hydroscopic properties of the Valvoline Synth brake fluid.

Castrol LMA (low Moisture absorbing) seems to be better.

I suppose if you changed the brake fluid every year or you lived in a very dry (non humid) climate, other brands might stay dry enough.

Originally posted by casio
<snipped the useless ultra-brave drivel>
i'll change my fluid and let ya know. and what on the system should i check out in particular?

Originally posted by theloudroom
Icemark, do you have any more info on this?
I have the valvoline synth in both my cars right now but it's been there for less than a year.

Originally posted by Icemark
No scientific data to back it up, but more the old school method of if you boil the fluid (and have brake fade) you keep trying a fluid that doesn't boil when you use it in your application.

When I raced my RA64 (almost every weekend), I use to boil all the Valvoline brake fluids (despite Valvoline Synpowers's claimed higher temp boil point (343F wet), but I never have [boiled fluid] when using the Castrol LMA (even with Castrol LMAs supposedly lower wet boil point of 311F). My conclusion to that informal testing was that the Castrol resisted absorbing humidity better and longer than the Valvoline Synpower brake fluid.