A Custom Made Resistive Heater on my PCP REXIMEX Rifle

 A Custom Made Resistive Heater on my PCP Rifle

I want to talk about PV=nRT (the Ideal Gas Equation) and that basically, if you have forgotten your physics, to remind you that when some pressure vessel gets hot, the gas inside of it wants to expand and hence the pressure will rise in accordance with the temperature change and conversely, if the temperature gets colder the gas pressure will decrease within the vessel.

However since the volume of our cylinder doesn't change we actually need to derive another formula from the ideal gas equation.

From Joseph Louis Gay-Lussac 

P1/T1=P2/T2

Here we are comparing the same substance under two different conditions.

Where P1  and T1 are the following: initial pressure and initial temperature, and P2 and T2 are the final pressure and temperature.

Pressure is measured in Pa  (pascals)

1 bar=100 Kpa = 100,000 Pa

200 bar is then = to 200X 100,000= 20,000,000 (Pa) ie 20 Mpa

or 2 e7 Pa.

Temperature is measured in degrees Kelvin (K) and to convert C to K you must add 273 to whatever temperature is given in Celsius.  In this case we will start at a comfortable 25C and work our way down to below zero C.

On a 200bar system with a 500cc capacity tank, how is this affected by varying the temperature? Let's see!

If we fill our cylinder to 200Bat at 25C and then examine the pressure drop at the following ambient temperatures:

15C, 10C and 0C. 

In order to perform this calculation we need to first convert degrees celsius to degrees Kelvin by adding 273 to each degree C value.

Now we find that the pressures are as follows:

193 bar at 15C

190 bar at 10C

183 bar at 0C

For this PCP, the minimum recommended operating temperature is -5C, for which your starting pressure at this temperature would be approximately:

180 bar.

So we can see the gun is not  recommended to be operated below 180 bar.

Now these values are only what you see here as long as the volume of gas remains the same. So prior to shooting in cold whether check your actual gas pressure.

Hence for the safe operation and or optimal operation of a PCP air gun, the there will be certain limitations upon the functionality of the weapon due to whatever ambient temperature it is operated at and the duration of operation.

Operation outside of the manufacturer's specifications  range for temperature could prove fatal for the operator! Especially if one exceeds the maximum operating temperature (usually around 40C/104F).

However, it is unlikely that one will be shooting at 40C! it is more likely one could be shooting at temperatures lower than 20C/68F during cooler months of the year (if outside). 

BTW what would the pressure be at 40C?

Approx: 210 bar. Well within the safety limits of the original pressure vessel.

This is where I placed a small flexible and water resistant resistive heating element encapsulated within a Silyl ether polymer (can withstand temperatures up to 80C). The heater is powered from X2 AA alkaline cells and sits atop of the steel regulator cylinder of my Reximex throne. The closed cell foam is added for insulation.

Further testing is still required as the mass of metal associated with the regulator pressure vessel is very large and made from a lower conductive steel alloy.

I marked the "Power wheel' so to to be able to reproduce particular power settings on this PCP rifle.

We made a "custom camouflaged' AUTUMN lightweight bottle cover. (See video)

Using a Vortex Red Dot Crossfire sight for close range moving targets up to 25m.

The only other thing I did to this PCP was to replace a small diameter castellated metal washer (inside the grip-not shown) with several wider (but same internal diameter) stainless steel washers to help take up a very small but annoying "slop" in the grip and to evenly distribute the load from the grip bolt upon the plastic molded grip.

BCT