Moisture visual indicator and relative humidity versus dew point.

[Luis H]

I am thinking about getting one of the visual indicators like the one in this link:

August Industries - Visual Indicator

August Industries - Visual Indicators

I have two options for the actual indicators disk. They both measure relative humidity. One reads 20, 40, 60% relative humidity and the other reads 10, 20, and 30%.

See the links:

August Industries - Moisture Indicator Disk

August Industries - Moisture Indicator Disk

In the links, it recommends to use the 20, 40, 60% relative humidity (RH), for normal use, but I would like to see how I can correlate the high pressure RH measurements to the one atmosphere dew point (as normally published).

I assume that I am going to have to know (measure and/or estimate) the air temperature and pressure in the visual chamber to evaluate the RH readings.

In NSTM 551 (appendix D), I have found the best tables showing moisture content (in PPM) for high pressure air and how it relates to the one atmosphere dew point (plus the actual high pressure dew point). I think I can calculate the relationship between high pressure RH and dew point using the graph below, but I was wondering if there is an easier way.

 

[lowviz]

Yep, that is a problem. You would think that table exists, I can't find it.

Before I go crazy on this, need to see if I'm pointed in the right direction.

Look at this first: Water Vapor in Air Now, look at PPMw.

Is this the same thing only scaled by 1000 ("M" for mille?) If so, then you have to resolve two tables, and that is a total PIA. So we turn the RH / Pounds per Pound table into an equation and feed it to your Table 551-D-2. Yes?

I assume that the second column in your table labeled "atm" is really meant to say "Dew Point at one ATM"?

So I'm guessing that you want to input your color disc reading of RH, and your temp, and PSIA in order to get a value for "One ATM Dew Point"? Please confirm.

If so, we convert both tables to equations (I have that software) and put the resulting single composite equation into an Excel spreadsheet so that you just have to enter your variables into input cells and you get your one atmospheric dew point in the output cell?

Let me know. Most interesting...

 

[swamp diver]

Wow Luis I'm very surprised to see that 1955 LDS chart on Scubaboard. I believe that table was originally done by/for the US Navy submarine program which used HP compressed air in the torpedo tubes or that is what I remember someone telling me.

Yes the eyeball humidity indicator from August Industries works like a charm but given you'll be diving in Maine where I assume the ocean water is about -1 C you should purchase the 10/20/30 RH % disc. The point to remember is the discs are reusable and once pink you can dry them back to blue quickly with a hair dryer otherwise it will take a few hours of compressor use depending on the FAD of your compressor. We usually changed our discs out annually or sooner if in contact with free water.
"blue is new and pink is extinct"

The RH percentage will be dependent on temperature of the air after the final separator and back pressure valve setting. If you have a setup where the back pressure valve does not hold for at least a few days if sitting idle you'll have to check the color after a few hours of run time given it changes quite slowly.

In Maine if you want to maintain an atmospheric dew point of -65 F ,which the older CGA Grade E scuba air required, you should switch out your filters just as the 10% section turns pink.

If you want to verify the attached table you can use the free Cosa software available here:
Software - COSA Xentaur

Just remember to apply the Greenspan water vapor enhancement factor to the calculation to allow for the non-ideal behavior of the gas.

 

[Luis H]

Yep, that is a problem. You would think that table exists, I can't find it.

Before I go crazy on this, need to see if I'm pointed in the right direction.

Look at this first: Water Vapor in Air Now, look at PPMw.

Is this the same thing only scaled by 1000 ("M" for mille?) If so, then you have to resolve two tables, and that is a total PIA. So we turn the RH / Pounds per Pound table into an equation and feed it to your Table 551-D-2. Yes?

I assume that the second column in your table labeled "atm" is really meant to say "Dew Point at one ATM"? Yes

So I'm guessing that you want to input your color disc reading of RH, and your temp, and PSIA in order to get a value for "One ATM Dew Point"? Please confirm. Yes

If so, we convert both tables to equations (I have that software) and put the resulting single composite equation into an Excel spreadsheet so that you just have to enter your variables into input cells and you get your one atmospheric dew point in the output cell? I think all the information we need is in Figure 551-D-1. I think that I can get the saturation moisture (in PPM) out of that table at the output temperature and pressure and then calculating the RH is the ration of actual moisture to saturation (please correct me if I am wrong). Fitting does curves into a table would make it a lot easier to work with.

Let me know. Most interesting...

See my replies in red inside your post.

I think that we are thinking the same thing, but Swamp Diver may have provided some tools with an easier solution.
I don't have too much time this morning so I am going to have to look at it this evening.

Thanks for the help.

 

[Luis H]

Wow Luis I'm very surprised to see that 1955 LDS chart on Scubaboard. I believe that table was originally done by/for the US Navy submarine program which used HP compressed air in the torpedo tubes or that is what I remember someone telling me.

Yes the eyeball humidity indicator from August Industries works like a charm but given you'll be diving in Maine where I assume the ocean water is about -1 C you should purchase the 10/20/30 RH % disc. The point to remember is the discs are reusable and once pink you can dry them back to blue quickly with a hair dryer otherwise it will take a few hours of compressor use depending on the FAD of your compressor. We usually changed our discs out annually or sooner if in contact with free water.
"blue is new and pink is extinct"

The RH percentage will be dependent on temperature of the air after the final separator and back pressure valve setting. If you have a setup where the back pressure valve does not hold for at least a few days if sitting idle you'll have to check the color after a few hours of run time given it changes quite slowly.

In Maine if you want to maintain an atmospheric dew point of -65 F ,which the older CGA Grade E scuba air required, you should switch out your filters just as the 10% section turns pink.

If you want to verify the attached table you can use the free Cosa software available here:
Software - COSA Xentaur

Just remember to apply the Greenspan water vapor enhancement factor to the calculation to allow for the non-ideal behavior of the gas.

I just did a google search and found that information.

The PDF attachment you provided may be all that I need (at least to start with). The table seems to be for 80F output temperature, which may be close enough for what I am looking for. At least it provides one data point that can be used to validate what I was thinking about.

I am not familiar with the software link you provided. I will have to look at it.

Thanks for the help.

I will look more at this later this evening.

 

[swamp diver]

The table seems to be for 80F output temperature, which may be close enough for what I am looking for. At least it provides one data point that can be used to validate what I was thinking about.

Use a post separator temperature of 80 F which represents an ambient temperature of ~ 68 F.

If I recall it was the Cosa software that was used to calculate that table with water vapor enhancement factor applied.

 

[lowviz]

Closer analysis of Table 551-D-2 brings up more questions...

Another piece to the puzzle: http://www.edgetechinstruments.com/docs/humdef.pdf

 

[lowviz]

This is useful: Free Professional Online HUMIDITY CALCULATOR

Edit: The link above is good for checking your thinking, but won't go to desired pressures.

Also useful:
Dew Point Calculator | Howell Laboratories
Air Dry Co. of America

2nd Edit: Humidity calculations using temperature, relative humidity, pressure

 

[Guy Kuo]

Funny this is being discussed at the same time I was looking at the indicator to help an air gunner out. Although I will never be breathing air from my Bauer JR, I have ordered one one these for my compressor. It seems a cheap, cross check that things are working correctly. I'm planning to install the indicator on my fill whip just before the gauge and valves. That way, It can remain sealed between compressor runs.

Wow. $100 with fittings and shipping. if I want this to protect just equipment, it seems a no brainer for those who rely on their air for breathing. Sure, follow procedure and schedules, but this is objectively warns of malfunction.

 

[lowviz]

@Luis H : Table 551-D-2 predates modern correction factors.

In all honesty, RH inputs in increments of 10% make this moot, but it would be nice to get an analytic solution to the moisture content of HP air using today's tools. I think that we are close.