# Pressure inside a light bulb

It is known that nowadays most light bulbs are filled with gas in order to minimise the evaporation rate of tungsten. Is e.g. argon put into the bulb with reduced pressure, so that the glass doesn't explode while heating the wire?

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This is a valid engineering/practical question that has me curious as well. We are all aware of the temperature such bulbs experience (ouch!). The glass must be required to not implode when cold and not explode when at rated power indefinitely, possibly with reflectors around it. Yes it uses an inert gas, but the glass still must maintain the seal and withstand the range of anticipated gas conditions. I suggest there are 2 answers: one for cold and one for hot conditions. –  AlanSE May 23 '11 at 14:39
You are thinking of halogen bulbs. In these, the fill gas is a low pressure hagen gas which reduces evaporation (increases redeposition, really) of the filament. Normal bulbs just use an inert gas to avoid combustion. Low pressure gas is easier to hold for long periods than vacuum. –  Colin K May 23 '11 at 16:43

A great link that details some experiments that looked for the answer to this is Experiment Tests Pressure in light bulbs. But first we need to cover some basis of the gas law to do comparisons. Firstly, most the time a bulb is off, it is near room temperature and most of the time it is on it is at a high temperature but the quantity of gas in the bulb remains the same. Engineering-wise, it is most desirable to have the pressure close to atmosphere when on, because at that time the glass will be hot and more prone to failure. Denoting these states as on and off, I will write the gas law for the case.

$$PV=nRT$$ $$\frac{ P_{on} }{ T_{on} } = \frac{ P_{off} }{ T_{off} }$$

The link I am referencing does an experiment where the bulb is submerged in water and the pressure boundary is broken. Since the bulb has a lower pressure than atmosphere water rushes in, this water mass can be measured and used to find the pressure before it was broken. This state I will denote as test and it is compared to the off state as a constant temperature process.

$$P_{off} V_{off} = P_{test} V_{test}$$

From this experiment they found two 25 W incandescent lamps to have a pressure of about $\frac{6}{1000} atm$, which is consistent with the low pressure inert atmosphere most people expect. For higher wattage lamps, they found the pressure to be close, but slightly lower, than atmospheric pressure. Temperature of the filament, when on, is in the neighborhood of 2700-2900 K, but this is not the temperature of the gas. Any reasonable assumptions will put the $P_{on}$ value for the low pressure bulbs at much smaller than atmosphere. For the higher power, higher pressure bulbs, I will address it more specifically using the surface temperature numbers from this source with clearly no credibility.

$$T_{off} \approx 298 K$$ 40 W $$P_{off} \approx 0.914 atm$$ $$T_{on} \approx 395 K$$ $$P_{on} = \frac{ T_{on} P_{off} }{ T_{off} } = 1.21 atm$$ 60 W $$P_{off} \approx 0.768 atm$$ $$T_{on} \approx 400 K$$ $$P_{on} = \frac{ T_{on} P_{off} }{ T_{off} } = 1.03 atm$$

To conclude, there are two types of constructions of incandescent light bulbs discussed here, one entailing a low vacuum and one that objectively reaches near atmospheric pressure in the on state. There is likely significant variation in products you can buy, and sample numbers given here can't be generalized to any given bulb you're looking at.

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great answer, thanks! –  malina May 23 '11 at 21:06

Most classical light bulbs that are produced today - in countries where they haven't been banned - are filled with (chemically inert) argon or nitrogen at a reduced pressure so that the bulb doesn't explode when it's heated up.

Light bulbs filled with vacuum have been produced, too. Of course, in both cases, the glass has to sustain pressures that are comparable to a significant fraction of one atmosphere.

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