Aluminium Electrolytic Capacitors:
Epcos:
2 years, cf. this applications information
Cornell Dubilier:
3 years as per this document
Nichicon:
2 years; section 2-6 in this document
Several documents say that longer storage is well possible, but will require reforming before use. Panasonic, amongst others, has a number: Apply the rated voltage via a series resistor of 1 kOhm for 30 minutes (for example http://www.panasonic.com/industrial/components/pdf/aluminum_app_dne.pdf). There is also a military handbook about reforming stored electrolytic capacitors (formerly known as MIL-STD-1131).
Without reforming and by applying the rated voltage after a long storage duration, the reforming current might be so high that capacitors may get (too) warm and even blow up, which we do not like because we are not Beavis or Butt-Head (he he).
Tantalum Capacitors:
I couldn't find similar data after my initial search, but it seems like the usual MSL (moisture sensitivity levels) ratings for surface-mount parts are given and applicable.
Summary:
Yes "polarised" aluminum "wet electrolytic" capacitors can legitimately be connected "back-to-back" (ie in series with opposing polarities) to form a non-polar capacitor.
C1 + C2 are always equal in capacitance and voltage rating
Ceffective = = C1/2 = C2/2
Veffective = vrating of C1 & C2.
See "Mechanism" at end for how this (probably) works.
It is universally assumed that the two capacitors have identical capacitance when this is done.
The resulting capacitor with half the capacitance of each individual capacitor.
eg if two x 10 uF capacitors are placed in series the resulting capacitance will be 5 uF.
I conclude that the resulting capacitor will have the same voltage rating as the individual capacitors. (I may be wrong).
I have seen this method used on many occasions over many years and, more importanttly have seen the method described in application notes from a number of capacitor manufacturers. See at end for one such reference.
Understanding how the individual capacitors become correctly charged requires either faith in the capacitor manufacturers statements ("act as if they had been bypassed by diodes" or additional complexity BUT understanding how the arrangement works once initiated is easier.
Imagine two back-to-back caps with Cl fully charged and Cr fully discharged.
If a current is now passed though the series arrangement such that Cl then discharges to zero charge then the reversed polarity of Cr will cause it to be charged to full voltage. Attempts to apply additional current and to further discharge Cl so it assumes incorrect polarity would lead to Cr being charge above its rated voltage. ie it could be attempted BUT would be outside spec for both devices.
Given the above, the specific questions can be answered:
What are some reasons to connect capacitors in series?
Can create a bipolar cap from 2 x polar caps.
OR can double rated voltage as long as care is taken to balance voltage distribution. Paralleld resistors are sometimes used to help achieve balance.
"turns out that what might LOOK like two ordinary electrolytics are not, in fact, two ordinary electrolytics."
This can be done with oridinary electrolytics.
"No, do not do this. It will act as a capacitor also, but once you pass a few volts it will blow out the insulator."
Works OK if ratings are not exceeded.
'Kind of like "you can't make a BJT from two diodes"'
Reason for comparison is noted but is not a valid one. Each half capacitor is still subject to same rules and demands as when standing alone.
"it is a process that a tinkerer cannot do"
Tinkerer can - entirely legitimate.
So is a non-polar (NP) electrolytic cap electrically identical to two electrolytic caps in reverse series, or not?
It coild be but the manufacturers usually make a manufacturing change so that there are two Anode foils BUT the result is the same.
Does it not survive the same voltages?
Voltage rating is that of a single cap.
What happens to the reverse-biased cap when a large voltage is placed across the combination?
Under normal operation there is NO reverse biased cap. Each cap handles a full cycle of AC whole effectively seeing half a cycle. See my explanation above.
Are there practical limitations other than physical size?
No obvious limitation that i can think of.
Does it matter which polarity is on the outside?
No. Draw a picture of what each cap sees in isolation without reference to what is "outside it. Now change their order in the circuit. What they see is identical.
I don't see what the difference is, but a lot of people seem to think there is one.
You are correct. Functionally from a "black box" point of view they are the same.
MANUFACTURER'S EXAMPLE:
In this document Application Guide, Aluminum Electrolytic Capacitors bY Cornell Dubilier, a competent and respected capacitor manufacturer it says (on age 2.183 & 2.184)
If two, same-value, aluminum electrolytic capacitors
are connected in series, back-to-back with the positive
terminals or the negative terminals connected, the
resulting single capacitor is a non-polar capacitor with
half the capacitance.
The two capacitors rectify the
applied voltage and act as if they had been bypassed
by diodes.
When voltage is applied, the correct-polarity capacitor gets the full voltage.
In non-polar aluminum electrolytic capacitors and motor-start aluminum electrolytic capacitors a second anode foil substitutes for the cathode foil to achieve a non-polar capacitor in a single case.
Of relevance to understanding the overall action is this comment from page 2.183.
While it may appear that the capacitance is between
the two foils, actually the capacitance is between the
anode foil and the electrolyte.
The positive plate is the
anode foil;
the dielectric is the insulating aluminum
oxide on the anode foil;
the true negative plate is the
conductive, liquid electrolyte, and the cathode foil
merely connects to the electrolyte.
This construction delivers colossal capacitance
because etching the foils can increase surface area
more than 100 times and the aluminum-oxide dielectric is less than a micrometer thick. Thus the resulting
capacitor has very large plate area and the plates are
awfully close together.
ADDED:
I intuitively feel as Olin does that it should be necessary to provide a means of maintaining correct polarity. In practice it seems that the capacitors do a good job of accommodating the startup "boundary condition". Cornell Dubiliers "acts like a diode" needs better understanding.
MECHANISM:
I think the following describes how the system works.
As I described above, once one capacitor is fully charged at one extreme of the AC waveform and the other fully discharged then the system will operate correctly, with charge being passed into the outside "plate" of one cap, across from inside plate of that cap to the other cap and "out the other end". ie a body of charge transfers to and from between the two capacitors and allows net charge flow to and from through the dual cap. No problem so far.
A correctly biased capacitor has very low leakage.
A reverse biased capacitor has higher leakage and possibly much higher.
At startup one cap is reverse biased on each half cycle and leakage current flows.
The charge flow is such as to drive the capacitors towards the properly balanced condition.
This is the "diode action" referred to - not formal rectification per say but leakage under incorrect operating bias.
After a number of cycles balance will be achieved. The "leakier" the cap is in the reverse direction the quicker balance will be achieved.
Any imperfections or inequalities will be compensated for by this self adjusting mechanism.
Very neat.
Best Answer
This is said with significant caveats, but the only electrolytic capacitor options for a pressurized environment are ones with a solid electrolyte, so solid tantalum, tantalum polymer, or aluminum polymer capacitors.
Cornell Dublier, for example, specifically states that all of its aluminum electrolytic capacitors have an operational range of 1.5 atmospheres to 10,000 feet (source - page 9).
Aluminum electrolytic capacitors are not perfectly free of voids and their normal operation and initial anodizing ensure that there is a small amount of hydrogen gas already inside, straight from the factory. At modest pressures, any contaminants will be forced into the capacitor past its seals, potentially causing a short or altering the capacitance, and at higher pressures, they will simply get crushed inwards and guarantee a short-circuit failure mode.
Simply put, normal aluminum electrolytics are off the table entirely.
Now, this is where it gets tricky: when designing pressure tolerant electronics, for the most part, you are kind of on your own. What I mean by that is you are not going to find answers to questions like 'maximum operational pressure' of most components, even if you email the company. This is because such a niche is incredibly small and it is simply not worth the time and effort to test or qualify products under such unusual environmental circumstances.
There are a few (very few) companies that make a limited selection of high pressure-rated components like capacitors, some as high as 10,000 psi. These capacitors will be very expensive - I couldn't even find a price, you have to request a quote. If you have high enough volume, I would still expect them to cost well over $500-$1000 per capacitor. They're also huge, 50,000µF of tantalum capacitors, true 10,000 psi monsters. So actually finding pre-qualified parts that are practical is also, I would think, not a realistic option for you.
What this means is it is up to you to qualify components yourself. You have to use an educated decision and select a COTS capacitor, but no one can tell you for sure if it will work or how its properties or longevity will be effected in such an environment as yours. You have to test all of this yourself.
This is how most pressure-tolerant electronics have to be designed. You qualify the parts individually through your own testing, and then you further qualify the entire assembly together under testing, and then you either spend a lot of time and money required to get even a slight idea of the reliability or longevity of your set up, you you just hope for the best (and learn from what happens to the devices in the field - trial by fire if you will).
So you should also be keenly aware of what is at stake, and what the consequences would be if your board were to fail, and make sure that allowances are made so that, for example, no one's safety would be put at risk.
That said, for bulk electrolytic capacitance, solid tantalum capacitors would be your best bet for tolerating the pressure with minimal changes in performance.
Another option is to make sure you really need electrolytic capacitors at all. Ceramic capacitors rated for 10V and 100µF are readily available and not horribly expensive. This Murata capacitor is an option, for example. Just beware of the DC bias graph - most of the high capacity ceramic capacitors use dielectrics that exhibit the ferroelectric effect. Similar to ferromagnetic materials in the presence of a magnetic field, ferroelectric materials are analogous but for electric fields (and energy stored as an electric field is ultimately what the capacitor is ultimately storing). This means ceramic capacitors' effective capacitance drops under DC bias. So you would need to derate their capacitance and use more than one in parallel.
The gold standard in pressure-tolerant electronics has always been the polypropylene metal-film capacitor, but obviously these are much much too low value and simply not suited for any bulk-capacitance application. I thought I would note them here for completeness though.
In closing, aside from some fairly exotic high pressure, deap sea capacitors that are likely not practical for your application, the short answer to your question is that tantalum capacitors as well as most capacitors simply do not have a maximum operational pressure rating. Rating is emphasized on purpose here - do not mistake this to mean that they can operate at any pressure. They certainly have a maximum pressure they can be expected to operate at, but the rating itself simply will not exist.
Don't let all this discourage you, however. The pressures experienced by things like deep sea pressure tolerant electronics are much higher than 30 bar, and quality tantalum capacitors are the first choice here, and all of the purpose-made deep sea 10,000 PSI capacitors are likewise tantalum capacitors.
Just understand that the manufacturer is not at fault if or when the capacitors fail, and you still have to qualify them yourself. This doesn't just mean checking for failure, but making sure their various properties that are of importance to your circuit stay within acceptable levels.
Get some solid tantalum capacitors and test them yourself. You'll probably get it on the first try, but be prepared to try a few different brands or construction types.
Final notes: Other components can exhibit unexpected behavior in high pressure environments. Make sure you don't have anything that has a 'metal can' construction. One easy to overlook is quartz crystals - through hole or SMD, they have empty space inside the can and mechanical stress on the crystal will through the frequency way off, if it isn't simply destroyed.
Also, be wary of wet tantalum capacitors. You should avoid these. There is a common misconception that fluids are not compressible. This is simply not true - they're much harder to compress than gas but it is still compressible, as are solids. That's what bulk modulus is - the compressibility of a substance. Importantly, the difference in compressibility for liquids vs. solids is between 10-100, or 1 to 2 orders of magnitude. This means liquid will compress much more than solids, which would allow for potentially significant mechanical strain.
For water, it will compress by about 46.4ppm per atmosphere. So given volume of water will lose around 0.14% of its total volume if exposed to 30 bars of pressure. This won't make anything implode like a tin can, but for components with very brittle materials inside (like tantalum pentoxide), this could allow enough flex/strain to be worrisome. Solid electrolyte is what you want.