Moisture visual indicator and relative humidity versus dew point.

[lowviz]

...//... Both ends of the whip are sealed between uses and the air should remain dry in the whip. ...

I think you'll find that within a day the ambient moisture will leak in the line even with end plugs (moisture actually leaks through the thermoplastic hose wall) ...

That is a reasonable concern.

Cheapest and most effective workaround (IMHO) would be a moisture sealed storage pail as shown in attached pic. Lid is upside down to show gasket. Put 5# of indicating silica gel into the pail and cover the desiccant with a screen or rack to keep your coiled whip off the granules.

If that doesn't do it, go nuclear by putting a small dish of phosphorous pentoxide or lithium bromide in there too. Both are corrosive. Keep their dust off your whip.

 

[Guy Kuo]

Thanks for the tips. I didn't know there would be seepage in through the tubing.
I will keep an eye on that. If I see things changing between sessions, I'll take more drastic measures.

One other idea...
What if I add another cut off valve BEFORE the indicator. Then, I could shut it off on both sides. Really not wanting to add more mass to the end of the whip, though. Seems pretty heavy already.

 

[lowviz]

I agree. See what happens first, you might be fine as is.

If your most sensitive section of the detector does change on storage, then I would definitely add the cut off valve. You obviously risk humid air entering when you disconnect and cap so the valve seems to be a good idea. That and better storage should do it.

 

[swamp diver]

Yes all you really need to know is the state desiccant saturation at the end of each run. If you haven't run the compressor for a week the disc will likely be completely pink if the whip is not pressurized but capped. On a bigger compressor purifier with a decent PM valve it will hold pressure for a good couple days. On our system it will hold the pressure at 1000 psig for about 10 days. Unfortunately on the Junior I do not think that is the case with its built in PM valve.

There is a thread on here about 5 years ago regarding leaking PM valves and it was determined the LF deluxe PM valve leaked the least.

With an upstream shut off valve you could hold the air in the fill whip at the end of the fill session at 1800 psig, and come back in an hour or two and examine the disc. It should reflect the moisture content of the last fill. Then drop the pressure down to 500 psig which will keep the moisture from entering the system if you are going to run it again in a few days.

If you find the disc is completely pink you could remove the disc (only handle by its edges) and dry it down initially with a hair dryer. It only takes a minute or so to change it from pink to blue with the hair dryer..

 

[Guy Kuo]

My Bauer JR will hold pressure in its whip above 1000 PSI for only about 5 days. Don't laugh. My usage needs are very tiny compared to you guys. Only about once a month I need to top a 80 CF SCBA tank from 3,000 to 4,500. My needs are high pressure, very small volume, but definitely dry. Sometimes, I even have to power up the compressor so it doesn't sit idle too long.

My whip is going to definitely be depressurized before uses. I will see soon enough whether additional measures to keep the disc from turning pink between sessions. A valve or drying storage bucket may well be in my future.

THANKS again for the experienced advice. Much to learn.

 

[Luis H]

They both should give you the same answer but I'm not sure if the HumiCalc allows the choice of Greenspan enhancement factor. It's been a while since I used that one but I think they have Sonntag and Hyland and Wexler as options. If you call Thunder Scientific they will tell you which option is closest to the Greenspan enhancement factor.

I tried the COSA Xentaur Dewpoint Calculator software. It is easy to use and it is free.

The first thing I did is to try to replicate the table 551-D-2. I had to change the preferences to apply Greenspan water vapor Enhancement Factor in order to get the number to come close. I didn’t actually take the time to try some of the other options since the numbers seemed enough.

The odd thing was the dewpoint temperature when the pressure was 4000 psi or higher. The dewpoint started to go back down a bit as the pressure increased above 4000 psi. That doesn’t seem right, but the numbers are not far off.

Here is my table trying to partially duplicate the table below it.

Here is the software interface for COSA Xentaur Dewpoint Calculator and the interface window for the preferences.

Here is the interface window when using the %RH (Relative Humidity).

So here is the final product that I was trying to produce. I did not fill in all the blanks because this was a very tedious process with all these data. My original intention was to fill all the blanks, but it quickly became apparent that the higher number for %RH are not very useful and I also don't plan to operate at very high pressures.

The first column on the left is the %RH readout.

Next column would be the air temperature inside visual indicator chamber.

The row on the top shows the pressure in the visual indicator. To the right of each pressure it reads atm, representing the 1 atm dew point for the air in the visual indicator.

My conclusion is that the 10%RH indicator is the only useful warning, which I think is what you suggested in one of the early posts.

Even the 10%RH indicator doesn’t seem to be an early warning as the air temperature is higher than 80F.

If anyone see any mistakes, please let me know. Any suggestions are also appreciated.

This was an interesting exercise, but it became a bit tedious.



Here is the document from HSM ENGINEERING TECHNOLOGY LTD that you posted earlier for comparison. The numbers are somewhat off, but not too far.

 

[swamp diver]

Luis,

Well that is quite a bit of work you did, but does raise some interesting points.

The original HSM table above was done over a decade ago if I recall using a previous version 2.? of the Cosa software, but not entirely sure. Either there was a mistake made in the original table which is unlikely as it was checked independently by two people, possibly the Cosa software at the time did not apply the enhancement factor correctly, or the vapor enhancement factor calculations have been revised since 2006. If you Google Greenspan water vapor enhancement factor you will see quite a few papers on updates to these uncertainty factors.

I did a quick cross check at 3000 psig, 80 F, and using the Sontagg vapor pressure equation without any uncertainty factor applied and came up with these pressure dew points (F).

Cosa 20.1
Michell 18.6
HumiCalc 18.6

Now applying the water vapor uncertainty factor (the Michell software won't allow this apparently because it only applies to air and their customers are using the software for other gases,....see FAQs) we have the following pressure dew points which match those in your table.

Cosa 15.6
Humicalc 13.7

Converting those to atmospheric dew point (F):

Cosa -61.8
Humicalc -63.0

Michell Humidity Calculator
Be Certain with Humidity Calculation Software | HumiCalc® | Humidity Conversion Software | By Thunder Scientific (demo version at bottom is free and does allow you apply under options the water vapor enhancement factor although they do not say which one)

Bottom line as mentioned in my previous post and only confirmed further by your updated findings is that once that 10% RH sector turns pink at any pressure it is time to switch out the cartridge for cold water diving.

If looking solely at the ability of the AC bed to remove volatile hydrocarbons from the air stream a cutoff of 20% RH is fine in Florida when the water temperature is 80 F.

We won't mention that the humidity strips in the Lawrence Factor after market cartridges change from blue to pink at 40% RH but they were never meant to be used for a solo purification tower rather the humidity indicator strips were meant to be used in a two or three filter 'train' where when the first cartridge turned pink you disposed of all the cartridges. If the front tower has an RH% of 40 the downstream towers would be in the 10% to 20% RH range. In a solo tower set up though you might approximate 10% to 20% RH when the first third of the strip had changed color.

For you in Maine or us in Canada we need the -65 F or colder atmospheric dew point to prevent first stage free flows. Work out the pressure dew point at 3000 psig and there is not much safety margin when the ocean water is - 2 C and one is breathing heavily or filling the dry suit or BCD to rapidly.

The figure attached is a thermocouple embedded in the outer brass covering of a Sherwood regulator while under the Antarctic ice. You can see the temperature drop with task loading at the start of the dive as drysuit and BCD are inflated and respiratory rates are higher all of which will drop the first stage temperature with air expansion from 3000 psig to intermediate pressure.

Taken from: Mastro, J and Pollock, N "Sherwood Maximus Regulator Temperature and
Performance During Antarctic Diving"
Proceedings of the fifteenth annual scientific diving symposium: Diving for Science.
Nahant, MA: American Academy of UnderWater Sciences, 1995:51-62.

 

[Guy Kuo]

You guys called it. The indicator is changing color after a week on the bare whip despite having the output valve and bleeds closed. 20% definitely pinking. 40% starting to change. I guess I'm going to get another valve and regenerate the disc.

 

[rjack321]

Bottom line as mentioned in my previous post and only confirmed further by your updated findings is that once that 10% RH sector turns pink at any pressure it is time to switch out the cartridge for cold water diving.

If looking solely at the ability of the AC bed to remove volatile hydrocarbons from the air stream a cutoff of 20% RH is fine in Florida when the water temperature is 80 F.

Another strategy which I have employed for a decade now is to put the RH disk in between my tiny on-compressor P1 sized filter and a larger 16" secondary filter. Use the RH disk as a guide for when to change the tiny prefilter - I change mine at 20%. Then you know the secondary filter is barely working on trace water and hydrocarbons and only serving as a polishing filter. And because its being fed with pretty clean gas, when you do have an unexpected breakthrough of the prefilter (trust me it will happen those P1 sized filters can go from 8% RH to 35% RH in less than one tank) the secondary filter will easily have the capacity to remove that water and hydrocarbon load. Change the secondary filter based on rated hours under the hypothetical ideal conditions (that rarely exist) since you are not working it hard at all.

If you were to install a second RH indicator after the secondary filter, even the 10% sector will never turn pink (has to stay pressurized though). That gives you the highest confidence that you aren't getting even partial water or hydrocarbon breakthrough.