In gliders, we use mainly PVC tubing. 5mm inner diameter.
Where more flexibility is required, silicon is used.
Example at this UK supplier but there is a lot of other places like Halfords, pet shop (aquariums), internet, …
We locak the pipe on the fitting either with a lamb castration ring or a bit of locking wire.
I don’t think the diameter of the pipe is important. The flow of air is small (only the rate of climb/vario need a flow). I recently visted the wind tunnel at work. The pressure line to instrument the model were tiny, in the order of 0.75mm OD.
Thank you Xtophe, I am sure you are right about the 6mm and my memory is poor.
The pipe diameter must be important within some limits. Consider both extremes; it is obvious neither will work. What I don’t know is what range is ok if you want an acceptable propagation delay for any pressure change, for the instruments to not lag excessively, and for the autopilot baro functions to work. If one is not worried about the propagation delay, then any internal diameter bigger than an air molecule will work 
You picked my curiosity so I had to research it.
We speaking about the speed a pressure signal goes through a pipe i.e. the speed of sound in a pipe.
https://neutrium.net/fluid_flow/speed-of-sound-in-fluids-and-fluid-in-pipes/ provides the formula.
PVC is at least 10,000 times stiffer than the air (1.5 e9 Pa vs 140 e3 Pa). So we are in a rigid pipe. So pipe diameter has no influence.
If the pipe is only 10 time stiffer than the air, then you start seeing change in the speed of sound and the pipe diameter (at constant wall thickness) start having effect.
So considering the material and practical diameters available to us, we are very far from the limits
I fully accept that the propagation time for a “wavefront” will be according to the speed of sound and thus independent of the pipe diameter, but in this case we are looking for how rapidly a steady state is achieved at the far end – for that is what say an altimeter is measuring. I think this is viscosity limited at the bottom end of the diameter range. Also, while my intuition does not help me with small diameters (say 1mm) it is intuitively obvious that if the pipe was say 10cm diameter then the far end would not see any significant pressure change, given than the static vent is a hole about 3mm diameter (and there are two of these holes), because the time taken to stabilise the pressure inside such a huge volume would be excessive.
I also suspect those formulae are for incompressible fluids. These transmit pressure changes more or less instantly (limited only by the speed of sound in the fluid) if the container has infinite stiffness.
Doesn’t the VSI work on the delay in pressure equalising through a very small hole?
Maoraigh wrote:
Doesn’t the VSI work on the delay in pressure equalising through a very small hole?
Yes, but the air volume involved is (by design) much greater.
Xtophe wrote:
we use mainly PVC tubing. 5mm inner diameter.
vic wrote:
I´d just get clear PVC hoses, 4 mm i.d. or
I would be cautious using PVC hoses, due to the deformation in hot temperatures / hot summers.
I know that french manufactured aircraft tend to use clear PVC. I have a friend who had just this issue with a DynAero MCR01, where in a not so hot Swedish summer suddenly he lost the speed on ASI.
Luckily he had quite a few hours on the aircraft, didn’t get panicked and used the wind-corrected GPS GS to perform the landing.
After, when he inspecting the hose circuit, he found one of those clear PVC hoses enlarged by temperature and disconnected and no securing clips.
Should I mention the aircraft was purchased by himself from the manufacturer and never mixed with those hoses.
Here for home builds, we use only thick rubber hoses, for reducing the obstruction and deformation risks.
After this, he changed all dynamic and static hoses to rubber hoses 4mm ID and 6mm OD.
Alternatively, I would use hydraulic Teflon hoses, which are more expensive and probably will last more than the aircraft life
Topic touched earlier before regarding the lag between AI and EFIS. All aircrafts I know around the EFIS it is more sensitive than the AI, so it is in my AC too.
