I am working on my handmade radiosonde system and finding an altimeter that can be used in radiosonde(properly working in at least altitude of 80km) and compatible with arduino.
Is there any altimeter that can meet my requirement?

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Hey. I was thinking. There might be other ways of inferring altitude, at those, like, really high places where the pressure drops to nothing.

I think there might be something predictable that happens with temperature, also speed of sound, and I have seen graphs of this, here,

https://en.wikipedia.org/wiki/U.S._Standard_Atmosp...

and maybe other places too.

Atmospheric pressure decreases exponentially with altitude. The approximate formula for this will look like:

P =P0*exp(-z/H)

Where P0 is the pressure at height z=0, somewhere on the planet's surface, and heights z>0 are above height. H, which has the same units as z, is called a "scale height". H for Earth's atmosphere is roughly 8 km.

Essentially what the equation says, is that for every H increase in height, pressure falls by a factor of (1/e).

A consequence of this is, if your pressure sensor has a linear response, (and I think most of them do) then everything above about 4 or 5 scale height, or 32 or 40 km, is going to look like approximately zero pressure.

Let me show you what I mean, like with some numbers; e.g. P0=100 kPa, H=8.0 km.

P(z=0) = 100.000000

P(z=8) =36.787944

P(z=16) =13.533528

P(z=24) =4.978707

P(z=32) =1.831564

P(z=40) =0.673795

P(z=48) =0.247875

P(z=56) =0.091188

P(z=64) =0.033546

P(z=72) =0.012341

P(z=80) =0.004540

I mean it is not going to be a problem to find a pressure sensor that can survive pressures as low as complete vacuum. The problem is getting a meaningful prediction about height from those really low pressure values.