Temperature and action

[charles Tauber]

Originally Posted by jojobean39

I notice my guitar action gets a little higher in the summer months.

This is typical.

As humidity increases, the guitar top swells, increasing its arch. As it does so, it takes the bridge with it, increasing string height. In the winter, with typically lower humidity levels, it does the opposite: the top flattens, the bridge and strings lower, reducing string height.

Originally Posted by beninma

The relative humidity can stay the same but if there is a big change in temperature the amount of moisture in the air is not actually the same.... I'm not really sure what all that means for wood but the amount of moisture is not staying the same from what you said.

In practical terms, what matters to the expansion and contraction of wood is the relative humidity, not the absolute humidity.

Moderate temperature changes with no change in relative humidity produces little change in the size or shape of wood.

[N+1]

Originally Posted by beninma

Is the guitar in a room that is both temperature AND humidity controlled?

The guitar is kept in a case, and the relative humidity inside the case has always been in the range 45-55%. There is no doubt about the RH in the case.

The only variable in all of this is temperature. And the temperature here has been extraordinarily high for an extraordinary period of time (beyond anything in my previous experience). That's why I'm not going to do anything until the weather gets back to its normal state.

[N+1]

Originally Posted by charles Tauber

Don't use the amount of neck relief to adjust string height.

Thanks for your warnings. I've no intention of doing that.

I want to see what happens when the weather gets back to its usual damp and cold NW summeriness before I make any changes.

[charles Tauber]

Originally Posted by N+1

The guitar is kept in a case, and the relative humidity inside the case has always been in the range 45-55%. There is no doubt about the RH in the case.

Your guitar is a hygrometer and is telling you something different than your other two hygrometers.

Out of curiosity, what is the usual temperature and what is it now? Are your two hygrometers identical, same company, same model? Are they analogue or electronic? Are they within the temperature and humidity ranges for which they were calibrated? If you take them out of the case and expose them to outdoor conditions, do they read the same as local weather reports?

[LeftIsRight!]

Another vote here for having a couple of extra saddles cut to different heights readily available in the guitar case. To me, that’s really the most practical - and fastest - way to change the setup in my guitars if I encounter sudden significant temperature/humidity levels.

This is assuming that the instrument is properly humidified, and that the nut height and neck relief are where they should be before replacing saddles.

[N+1]

Originally Posted by charles Tauber

Your guitar is a hygrometer and is telling you something different than your other two hygrometers.

Out of curiosity, what is the usual temperature and what is it now? Are your two hygrometers identical, same company, same model? Are they analogue or electronic? Are they within the temperature and humidity ranges for which they were calibrated? If you take them out of the case and expose them to outdoor conditions, do they read the same as local weather reports?

I haven't (yet) done those comparisons, but I don't expect any surprises when I do. What I do know is that these two identical (digital) hygrometers are behaving exactly as they have done for the last two years. My other guitars are not exhibiting this curious behaviour of reduced action, so whatever is causing it seems to be specifically an issue associated with the 914.

I really must emphasise the extreme weather conditions here right now. Typical winter temperatures in the guitar room are 12-15 degrees C. Current temperatures have been typically 27-29, for several weeks in a row, and this is truly exceptional.

[N+1]

Just gradually accumulating data here:

The weather chaps say the RH has been pretty high outdoors in NW England during this heatwave - mid 60s or above. The RH in the guitar room is rather less than that - about 55%. But the point is that despite the hot weather, the relative humidity has never dropped below 50%. At the moment the cases aren't really doing all that much beyond slowing down the temperature changes.

I say again: I don't see how this can be a humidity issue. To produce the bridge-lowering effect, the guitar would have had to dry out, and there's no possibility under present conditions that that can have happened.

[Epiphone100]

I am also in the UK in this hot period. I have noticed all my acoustic guitars have reacted somewhat to these unusual UK conditions. I notice an increase in action and neck relief plus a tendancy for the instruments to detune a bit over a preriod of time. I put this down to a slight swelling of the tops due to the sweaty conditions we have here. RH indoors is not too high but I have noticed a slight swelling in wooden doors in my house too........so seems logical ??!!

[N+1]

Here's an update.

We've now had about 4 days in a row at more reasonable and typical temperatures (19-20 degrees C). The RH in the room has hardly changed - it's stayed at about 55%. The RH inside the 914's case has also hardly changed, at about 51%.

Earlier measurements at the height of the heatwave were:
relief 0.13"
action at 12th fret 2.0mm
neck aligns about 1mm higher than top of bridge.

Today's measurements:
relief 0.10"
action at 12 fret 2.25mm
neck aligns about 0.5mm higher than top of bridge.

I reckon it will take as long to come back home as it took to go on holiday, and we've only had 4 days of normal temperature following several weeks at high temperatures, but it seems to me that the problem is resolving itself. Playing it today I noticed hardly any buzzing (which caused me to investigate in the first place).

But throughout all this, the RH has barely changed inside the case - it still sits at 51% today. Humidity can't be the explanation for these changes. It has to be a temperature effect.

[PorkPieGuy]

Originally Posted by vindibona1

However, the neck relief can vary significantly and consequently a large change in relief will be reflected in string height as we typically measure it.

Yup, when I got my Taylor (brand new), I ended up having to do a half turn on the ol' truss rod every six months. I'd turn it one way in the summer, and then turn it back the previous way in the winter. I can't tell you which direction though because it's been so long.

[tadol]

Originally Posted by N+1

But throughout all this, the RH has barely changed inside the case - it still sits at 51% today. Humidity can't be the explanation for these changes. It has to be a temperature effect.

I would suggest a couple of things -

First - make sure the sensor is working properly - there are alot of inexpensive RH gauges that are not terribly accurate, or have an excessive MOE -

Second - pulled from the Wikipedia -

Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Relative humidity depends on temperature and the pressure of the system of interest. It requires less water vapor to attain high relative humidity at low temperatures; more water vapor is required to attain high relative humidity in warm or hot air.

So - if the temperature is high, you could have a great deal more water vapor in the air, which would lead to swelling of certain wood parts (I'm looking at you there, soundboard - ) which would cause the bridge to raise, and raise the action. Heat, by itself, does not really affect the dimensional stability of wood, especially in length, so it would have negligible change in the neck or fretboard length, while a small but prolonged change in RH could have a highly noticable effect on the width of it (note threads about guitars drying and fret-ends sticking out).

If (when) this happens again, try dropping a straigthedge across the soundboard just below the bridge, and measure the height at each edge of the board. you may find that its the softer and more porous spruce that is moving the most, and causing the action to change. You can probably adjust the relief every time it happens to compensate some, but thats not what the truss rod is for, and you should first measure the relief. If it hasn't changed, then you probably shouldn't adjust it. But having a pair of saddles that you can change (or putting a shim under it when its low) might be a better solution -

[N+1]

Originally Posted by tadol

I would suggest a couple of things -

First - make sure the sensor is working properly - there are alot of inexpensive RH gauges that are not terribly accurate, or have an excessive MOE -

I've done enough experiments to ensure that the hygrometers are working perfectly well.

Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Relative humidity depends on temperature and the pressure of the system of interest. It requires less water vapor to attain high relative humidity at low temperatures; more water vapor is required to attain high relative humidity in warm or hot air.

So - if the temperature is high, you could have a great deal more water vapor in the air, which would lead to swelling of certain wood parts (I'm looking at you there, soundboard - ) which would cause the bridge to raise, and raise the action.

Yes, I've seen this happen on other occasions. But here is the puzzle (it has been the puzzle right from the start): in this instance the action decreased as the temperature went up, to the point where I started to get fret buzz. Now that the temperature has fallen, the action is increasing back to where I want it to be.

Heat, by itself, does not really affect the dimensional stability of wood, especially in length

This is what Charles Tauber was saying. But I have no choice in this case except to assume that the temperature change has caused some movement.

while a small but prolonged change in RH could have a highly noticable effect on the width of it (note threads about guitars drying and fret-ends sticking out).

I am absolutely certain that the change in RH through this whole exercise has been negligible.

If (when) this happens again, try dropping a straigthedge across the soundboard just below the bridge, and measure the height at each edge of the board. you may find that its the softer and more porous spruce that is moving the most, and causing the action to change.

I think it's very likely that the effect has indeed been primarily caused by movement of the spruce top, because there has been a shift in the relative position of top of bridge and the neck alignment (extended), as I reported above. The extraordinary thing in this whole exercise has been that for the bridge to sink as the temperature went up, the spruce top would have had to fall - i.e. the spruce must have tightened, slightly reducing the amount of 'bellying'.

I know the effect of a rise in RH - the top bellies upwards and creates an increase in action. The opposite is the case here. And the RH never changed throughout it all. I keep my guitars in cases and control the relative humidity carefully.

Incidentally, the relief has changed too (from 0.013" to 0.010"). The neck straightened a bit as the temperature dropped back to normal.

[DesertTwang]

Originally Posted by N+1

But we're currently in a highly unusual extended heatwave, here in NW England. The temperature is about 25-28 degrees C for much of the day.

Heatwave?

I'm sorry, but that just made my day. Here in Arizona, it's only 35 degrees today (instead of the usual 43), and I was thinking to myself earlier, "how nice, what a balmy, unseasonably nice day it is today."

[N+1]

Originally Posted by DesertTwang

I'm sorry, but that just made my day. Here in Arizona, it's only 35 degrees today (instead of the usual 43), and I was thinking to myself earlier, "how nice, what a balmy, unseasonably nice day it is today."

Here in NW England we fry eggs at those temperatures.

Seriously though, the significant point, I suppose, is not the actual temperature, but the change from the 'normal' conditions at which the guitar was set up.

[FThomas]

Originally Posted by N+1

I'm entirely confident that the RH inside the case has been within the range 45-55% all the time. I have two hygrometers, they both agree, and the system is well established. The only significant variable is temperature.

I live in the Philippines and have to deal with EXTREME heat and humidity. My guitar has developed about 1/16th of belly, which raised the saddle about 1/32. Just enough to be noticable. I replaced the nut after lowering it 4/64ths and had a major improvement in action while at the same time trying to lower the relief as much as I possibly could. I've ordered three more nuts for some experimentation and my goal is to get as close to a flat neck as possible. To lower the string height at the 12th fret 2/64 you have to double what you take off the saddle - 4/64th. Good old geometry.

A couple of days ago a friend of my who is a Luthier and was a writer for Flatpicking Guitar Magazine on guitar setups and maintenance for ten years and I had a very informative discussion on Action and how thing are impacted by the Saddle, neck relief, which is a very misunderstood factor and can be measured three ways: With the String - capo on first and fret the 12th, a Straight Edge - measure under the 6th fret and Visually without taking any measurements, but just watching the string - by feel. And of course string action at the first fret, which can be measured a number of ways. He said that the nut height should allow the string to barely kiss the first fret when the string is fretted between the 2nd and 3rd fret.

He also stressed during our discussion, which makes perfect sense, that all guitars have to have the neck set at a slight angle down from the plane of the body. It's this fact that may be the reason that you measured the straight edge slightly above the bridge by 1mm and not have anything to do with setup or movement in the wood from as a result of humidity and heat. Given that the neck does not move from approximately the 12th fret to the soundhole where it rests over the body.

My discussion with Bryan was enlightening and the three articles he sent me made me realize that I needed to re-evaluate how I was setting things up.

I'll PM you and send you a copy of the three articles. Hopefully, you will share with us how you resolved the setup discrepancies you found. I'm always learning something new and look forward to this being another learning experience.