This is a question which is often asked but difficult to answer because it mainly depends on your application's design.

Key points to take into account when you design an application using gas springs

A) The seals natural permeability
B) The gas spring's mounting position
C) The number of cycles
D) The temporary effect of temperature
E) Seals have a shelf life of 5 years
F) The factory tolerance

A) Gas springs will loose their force overtime because the seals natural permeability allows micro amounts of nitrogen gas to escape. Gas springs with a small gas chamber (size 4 and 6) will loose between 5% and 15% of their force each year and bigger gas springs between 3% and 7%.

B) The rate at which a gas spring looses its force depends on the gas spring's design, its mounting position and whether the basic recommendations have been followed or not. For example if you fit the gas spring with the rod up, then the leak rate will be much higher.

C) IGS gas springs will also typically loose 4% of their force every 10000 to 20000 cycles (depending on the length of the stroke). While in many cases it is not critical, some mass-produced gas springs will not last more than 5000 to 10000 cycles.

D) Another point which is often ignored is the effect of the temperature on the force of the gas spring.

Charles Law: [Pressure final N/m2] / [T final °Kelvin] = [Pressure initial N/m2] / [T initial °Kelvin]

In the case of gas springs this equation becomes:

Temp in °K: [Force final] = [Force initial] * [T final °K] / [T initial °K]

Temp in °C: [Force final] = [Force initial] * ([T final °C]+273) / ([T initial °C]+273)

For example, a gas spring is measured with a P1 force of exactly 2500 newtons at 20ºC
The same gas spring, the same day at 0ºC will have a P1 force equal to:
2500*(0+273)/(20+273) = 2330 newtons

As soon as the gas spring temperature comes back to 20ºC, the nominal P1 force will also come back to 2500 newtons.

E) Gas springs seals have a shelf life of 5 years. This means that 5 years after being manufactured the seals will start to degrade and the rate of the leak will slowly increase. For this reason, when we attempt to recharge gas springs which are more than 5 years old, the seals often break or don't hold the charge for very long.

F) Finally, at IGS, standard gas springs are charged with a tolerance of plus or minus (+/-) 5%. If you order a batch of gas springs charged at 2500 newtons, we will supply this batch with a force tolerance of plus or minus 125 newtons.
Now how do you take this into account ?

Because gas springs will invariably loose their force overtime you need to design your application so even when the gas spring has lost some of its force the application will still function safely.

Once you have chosen a mounting position that suits you, try to define the acceptable and unacceptable behaviour for that lid.

For example:

On a machine you may define that lifting the guard with an effort of 2kg every 2 minutes may be acceptable. You have calculated that this behaviour would require a gas spring charged at 200 newtons.

Now, you decide that if the operator has to exert more than 4kg to lift that guard this will no longer be acceptable. You calculate that in order to obtain a lift of 4kg the same gas spring mounted in the same position will have a force of 170 newtons.

A loss of 30 newtons is equivalent to 15% of the force.

If the gas spring was originally supplied at a force of exactly 200 newtons (+5% of nominal force), in the worst case scenario after 10000 cycles and 1 year of usage the gas spring will have lost 19% (4%+15%) of its force and may need to be replaced. In the best case scenario, after 20000 cycles and 2 years of usage the gas spring will have lost 14% (4%+5%x2) of its force and will still make the application behave as expected.

Before you start your design:

When you design an application with gas springs, avoid at all costs to fit the gas springs too close to the hinge. By being very close to the pivot point, the slightest change to the force will have an enormous consequence on the behaviour of the lid. Even a small temperature change may make the application behave in an unacceptable manner.

Unfortunately gas springs are not suitable if you require a device which gives a very precise force for many years in all kinds of conditions. In this case only electronically regulated pressure systems should be considered.