Martin Relative Humidity vs Temperature chart, please explain!

[Palladio]

I found this chart in an old thread, but can't find it on the Martin Website or definitively explained anywhere else. I understand it has to do with the actual available water volume in the air at certain relative humidity and temperature levels, and now wonder if I have been storing my guitars incorrectly.

I live in San Diego by the coast where the climate is very mild, but relative humidity levels are usually higher than 60%. I prefer leaving my windows open instead of using heat or air conditioning all but a few weeks a year. Since my apartment isn't climate controlled, I have been using bluetooth hygrometer/thermometer units in my cases, along with reusable desiccant packs. This system has allowed me to maintain the relative humidity of my stored guitars well within the widely recommended range of 40% to 60%. The hygrometers connect to an iPhone app, which alerts me if levels get out of range in any of my cases. So all I have to do is remove a desiccant pack from a case and dry it in the microwave for a few minutes, put it back in the case, and I'm all set for several weeks to months depending on how often I play that particular guitar.

The average temperature in my room is around 65 degrees. Does this chart indicate that I should be accounting for that lower temperature and keeping my guitars between 55% and 80% relative humidity as indicated by the in-case hygrometers? In other words if my average in-case temperature is 65 degrees, my target indicated relative humidity should be around 67%, not the usual 47% recommended by Taylor and many other sources?

I know this is only a target range, and that guitars can survive just fine in most circumstances in our SoCal Climate, short of being left in a hot car too long or taken inland to the desert. However, since I already have come up with a very easy system to maintain and monitor relative humidity, I would like to know what range and/or target number I should be using for my guitars while they are stored.

[Earl49]

Don't get too caught up in this chart. Data from a hygrometer in your house is what matters. What you really care about is the RH inside your home on an extended basis. Hour-by-hour changes are not critical -- it takes days for low or high humidity to substantially affect your guitar, not minutes. Also irrelevant is the outdoor humidity from the TV weather report. That is only a factor if you have open windows all the time, ie: no heat or AC. Keep your guitars on the back porch all the time? Then the TV weather-guessers and their RH numbers become relevant.

Your monitoring and control actions seem fine to me. Your guitars will tell you if it isn't working (swollen tops, sinking tops, sharp fret edges, etc). Basically any conditions in which you are reasonably comfortable will be OK for your guitar for half day periods. As with pets or kids, don't leave your guitar in a hot car or a freezing car all day. We obsess too much about humidity around here....

[JackC1]

Originally Posted by Palladio

Does this chart indicate that I should be accounting for that lower temperature and keeping my guitars between 55% and 80% relative humidity

No, RH already accounts for temperature. Just keep it at 50% RH +/- and you'll be fine.

[printer2]

I am at a loss to see how this graph relates to guitars, "adjusted %RH to CFM". CFM is cubic feet per minute. Where has there ever been a recommendation from a guitar manufacturer on the proper CFM to store a guitar? I do not think this is guitar related.

[Earl49]

Originally Posted by printer2

I am at a loss to see how this graph relates to guitars, "adjusted %RH to CFM". CFM is cubic feet per minute. Where has there ever been a recommendation from a guitar manufacturer on the proper CFM to store a guitar? I do not think this is guitar related.

^^ This. Us mechanical engineers use the psychrometric chart to determine the performance and parameters of HVAC systems, and human comfort. This appears to be an odd interpretation of that chart, maybe with a specific application in mind. I'm scratching my head about what they are trying to communicate. "g/cm3" of water content is the absolute humidity, but guitars don't care about that. They only care about the RH, the amount of water vapor compared to the saturation point, or 100% of what the air can contain at a given temperature. Color me puzzled too....

[Palladio]

Originally Posted by Earl49

^^ This. Us mechanical engineers use the psychrometric chart to determine the performance and parameters of HVAC systems, and human comfort. This appears to be an odd interpretation of that chart, maybe with a specific application in mind. I'm scratching my head about what they are trying to communicate. "g/cm3" of water content is the absolute humidity, but guitars don't care about that. They only care about the RH, the amount of water vapor compared to the saturation point, or 100% of what the air can contain at a given temperature. Color me puzzled too....

Earl thanks for you reply. I was doing some searches on ideal humidity for guitars and came across an old thread here from 2012. If you're interested you can find it here:

https://www.acousticguitarforum.com/...d.php?t=243982

If you scroll down a bit on the first page you'll find the chart I posted above plus another data table. There is some discussion later in the thread but no one seems to have really explained this chart.

Even if it doesn't apply to my situation, now I'm curious and would still like to understand what this chart was used for at Martin. Or at least in layman's terms what it means vs the usual measurement of relative humidity.

[flatfinger]

Originally Posted by printer2

I am at a loss to see how this graph relates to guitars, "adjusted %RH to CFM". CFM is cubic feet per minute. Where has there ever been a recommendation from a guitar manufacturer on the proper CFM to store a guitar? I do not think this is guitar related.

CFM = CF Martin in the data set posted above. Here is where that graph was posted with supporting data…..

https://www.acousticguitarforum.com/...1&postcount=12

[marciero]

Originally Posted by Earl49

^^ This. Us mechanical engineers use the psychrometric chart to determine the performance and parameters of HVAC systems, and human comfort. This appears to be an odd interpretation of that chart, maybe with a specific application in mind. I'm scratching my head about what they are trying to communicate. "g/cm3" of water content is the absolute humidity, but guitars don't care about that. They only care about the RH, the amount of water vapor compared to the saturation point, or 100% of what the air can contain at a given temperature. Color me puzzled too....

But there are no values corresponding to cubic feet per minute; nothing indicating that the chart corresponds to some given air flow. The two curves give the RH you need at a given temperature to achieve moisture content-abs humidity- between 8.9 and 12.7 gms/cubic meter. On the other hand CFM does sound like something one would be "adjusting" to.

Originally Posted by flatfinger

CFM = CF Martin in the data set posted above. Here is where that graph was posted with supporting data…..

https://www.acousticguitarforum.com/...1&postcount=12

That would be an odd way to reference CF Martin. Why not in the title? Also, you wouldnt be "adjusting to CF Martin"

[flatfinger]

Originally Posted by marciero

That would be an odd way to reference CF Martin. Why not in the title? Also, you wouldnt be "adjusting to CF Martin"

The AGF member who 11 years ago posted the graph and data I referenced said this…..

“I have applied the C.F. Martin Factory temeprature/%RH range and illustrated how it needs to be adjusted across realistic house temperature ranges between 65-85 degrees F.”

The OP of this graph and data is still an active member here. Perhaps he will join this discussion to explain further.

[EZYPIKINS]

That chart doesn't concur with Kansas City.

[sinistral]

Oddly, the file name for that chart says “bourgeois” in it, not Martin.

https://i1238.photobucket.com/albums...50/RHtable.jpg

(//i1238.photobucket.com/albums/ff495/iim7v7im7/bourgeois%20OM-150/RHtable)

Perhaps Bob (iim7V7IM7) will chime in.

[Sir strumalot]

If you're making wooden products,you want to keep the moisture content of the wood in certain limits. I think this chart relates moisture content of the air (rh) to moisture content of the wood at different temperatures - maybe it tells them how fast to run the ventilation system?

[RodB]

The following has already been well explained in this thread, but worth emphasising.

If it is of concern to ensure certain limits are adhered to in terms of Absolute Humidity, then it is necessary to monitor both RH and Temperature.

However, this is not applicable to those of us who wish to ensure that our guitars remain within acceptable limits. This is because the EMC (Equilibrium Moisture Content) of wood and any other hygroscopic substances varies with changes in RH and to a very small amount Temperature. Because the influence of temperature is so small it is then only RH that needs to be monitored, and this is why manufacturers are able to make meaningful recommendations of RH alone.