Why does water condense in my drones?

(Note: Yes, I am a research scientist.  What follows is a bit long and involved, but if you really want to operate from a basis of knowledge and understanding about this issue, it's worth reading and understanding.  Many thanks to the folks at Hyperphysics for a great equation for saturated vapor density!)

Condensed moisture in the drones results in funny noises, instability and eventually drone "shut-down".  It can swell hemp leading to frozen joints.  If it soaks into the wood, it can cause cracks.  

This moisture comes from the passage of your humid breath into the drones where it hits cooler ambient air.   When the extra moisture content coming from your breath is more than the ambient air can hold, you'll get condensation. A "little" condensation can be tolerated, but at some point, you're in trouble.

The amount of condensation depends upon the air flow through your drones (lots of moist air flow -> lots of condensation), the relative humidity of that air (desiccated air contains less moisture than non-desiccated -> less condensation)  and the ability of outside air to accept the extra humidity going up the drones (lower relative humidity in the outside air and/or warmer temperature -> less condensation).

From this simple beginning we can see that pipes that use larger volumes of air (i.e., some "big" or "band" pipes) would be expected to have more of a problem.  We can also see that pipes that use a desiccant system should have less of a problem.  We can also see that the ambient temperature and humidity are important.

Going into all the aspects gets a little heavy, but here goes....

1. When you blow into a pipe, your warm breath contains a lot of moisture.  According to the limited information I can find, expired gases are 100% saturated at about 30C.  I expect that breathing hard may reduce the degree of saturation due to kinetics, on the other hand it may raise the temperature, so 30C/100%RH may represent a reasonable value for a piper working at their craft.
2. If you happen to have bag with a built in desiccant system, the moisture level will be reduced, somewhat.  The level of moisture remaining depends upon the ability of the desiccant to absorb moisture - which in turn depends upon how dry it is to start with, how cold it is and the rate that air is going through. This is a good link about desiccants including clays. 
3. In any bagpipe, the temperature of the exhaled air will also begin to cool in the bag (unless it's more than 30C outside!).  Basically, heat is radiated through the bag.
4. So now we have humid air in a bag at some temperature between the exhaled temperature and the ambient temperature.  
5. In addition, the air in the bag is under pressure - ok - it's only 1-2 psi, but it does have to expand through the reed to get to ambient pressure There will be "some" cooling upon expansion. The degree of cooling depends upon the air flow through the reeds and how the whole instrument is designed and set up.
6. This warm moist air goes up the drones.  The warm air from the drones does warm the inner surfaces of the bore, but probably only slightly. The bores are warmer near the bag and colder further up.
7. The air flowing from the bag goes up the drone and mixes with the cooler ambient air.  The ambient air had some humidity already, so now there is more humidity.
8. If the ambient air can't hold all the moisture, water will condense on any cool surface - like the inside bore of the drone!

Despite all these vague and "unknown" parameters, we can generalize the situation and really need not worry about much of anything except the temperature/humidity in the bag, the temperature/humidity outside the drone and the ratio of mixing that occurs within the drone. 

The first two are rather obvious, but the latter is not.  On a cold day you can see your exhaled breath as a "micro-cloud" of condensate, but it quickly "goes away". The limited mass of moisture in you breath initially overwhelms the ability of the air nearby to absorb it - and you get the "micro-cloud". After a few seconds of mixing with more air, the excess moisture is evaporated and carried away.  Try exhaling gently (low mixing ratios) and you see the "micro-cloud", but if you "blow" out (high mixing), you'll likely see less condensate.

I'll wager you've never seen your breath come out the chanter or drones - even when you can see your breath condense in the air.   That's because of a fairly high mixing ratio of ambient air and your breath in the drones.  The "micro-cloud" of condensate is inside the drones - not outside. The mixing ratio is different for every pipe and depends upon the bore design, the reeds and their set-up.  

This is a good place to bring up the fine point that, on a given day, plastic pipes may have more condensate than non-plastic pipes. The thermal conductivity of plastic is higher than that of wood, so when you are playing on a cool day, the bore of a plastic pipe will be losing heat faster than a wooden pipe (see point #6 above also) and will be colder than that of a wooden pipe. A colder bore represents a surface for more or earlier condensation. This issue is further exacerbated by the fact that the inner bores and flow characteristics of many plastic pipes are modeled after makers such as Henderson. The larger airflow of these drones requires the piper to blow more air - and more water per time into the drone and may overwhelm the ability of the ambient air to carry away that much moisture. (Note: I cannot conclude, based on any data that I'm aware of, that well conditioned African Blackwood absorbs any significant mass of moisture during a playing session: the issues, as I see them, are thermal conductivity and air flow.) (Note #2: The advantage of plastic pipes is not that moisture won't condense in the bores, but that the moisture will do no damage.) (Note #3: Some ferrules and projecting mounts (i.e. silver and/or other metals) also conduct heat very quickly, and may, in a minor way, result in "cooler" spots in the drone bores.) 

OK, so where does that leave us?  We know that the air leaving the pipe bag has some amount of moisture in it.  That moisture needs to be absorbed into the ambient air.  If it cannot be absorbed at the temperature in the bores, the moisture will condense in our drones.

If we know the ambient temperature, we can know the ability of ambient air to hold moisture (i.e., the moisture content for 100% relative humidity or Saturation Level) and we can also measure the relative humidity.  From all these, we can know how much extra moisture the air can hold.  This varies, of course, as a function of temperature.

The numbers in the charts below show this - with temperatures in Fahrenheit and Centigrade, respectively.  For example, on a day at 35F with 30% relative humidity, ambient air can hold about an extra 3.91 gm/m^3 of moisture.  (The saturation level is 5.58 mg/m^3 and the humidity in the air is 0.3*5.58 or 1.67 gm/m^3.  The difference 5.58 minus 1.67 is 3.91 gm/m^3. )  If it's warmer than 35F and the same humidity, the air will hold more.  If it's more humid at the same temperature, the air will hold less.  Also, at 60F and 70% relative humidity, the air will hold about the same amount of moisture (i.e., 3.96 gm/m^3).  (Note: While it's reasonable - to a first approximation -  to expect about the same degree of condensation under both conditions, the truth is that the thermal issues and kinetics of evaporation will be more problematic at lower temperatures, so the issue will be a little worse at the cooler temperatures.)

(Note: The colors are explained below and are related to MY pipes. Yours will probably be different.)

Now, regarding the colors:  For example, for my pipe - as I have it set up and play it, with its inherent mixing ratio, heat loss characteristics, etc.  -  I think I need an effective level of about 4 gm/m^3 of excess capacity to avoid condensation. (Note: I'll try to refine this estimate over time.  I play a set of Lawries with Kinnaird reeds and a Ross Canister Bag.) Needing 4 gm/m^3 means that I can play without fear of condensation under the conditions highlighted above as green.  If yellow, I'm going to be very careful and, if red, I know I'm going to have problems.   

(Note:  My estimate of 4 gm/m^3 should not be interpreted to mean that I believe that the canister bag is removing 26.52 gm/m^3 from my breath.  My simplified estimate effectively incorporates ambient air mixing ratios, heat losses and the like.  In reality, the clay desiccant is probably only removing a small fraction of the moisture, due to the very short contact time.  Someday, I'll work out a way to sample the air on the backside of the desiccant chamber in the absence of mixing effects, measure the relative humidity going to the drones with and without the desiccant chamber in place and have a "real" value for desiccant efficiency.   I'll also include consideration of the 25 or so liters/minute of air that a piper consumes during exercise and try to determine the "rate" of moisture condensation in the pipes.  Maybe then I'll be able to determine how long I can blow into the pipe without drone problems.)

Now, if you're willing to do a bit of work, you can come up with a similar estimate as to when the ambient temperature and humidity will become an issue with YOUR pipe.   You'll have to keep track of the ambient temperature, ambient humidity and whether you observe ANY condensate in your drones.  You can note, in each cell of the table above, when you do (I've done this by coloring them red) and don't (colored as green) have an issue with moisture.  From this you'll find out how much excess moisture capacity you NEED to have to avoid condensing moisture in your pipe.  This value will be different for every pipe and will change or vary with the drone reed set-up, presence/absence of desiccant, mixing ratios and approximate (at least) heat loss issues for your pipe.  

You can then know, based on ambient temperature and relative humidity if you will have an issue with condensation.  It won't tell you how long you can play before your reeds stop functioning, nor will it tell you what to do instead of playing!  However, this information may warn you to not overplay before the event!

You'll need to determine what's right for your pipe.  Let me know what you find out and I'll put some of the information here.

Please contact me for more information or to discuss this subject further.