AJC1
Member
i see people asking the difference between these to units of measure and how to convert one to another. This is the most in depth explication I have ever seen and shows why they are not convertible units without an algebraic formula.
Pressure measurement - psi vs. CUP - Questions often arise about these two methods of measuring pressure and whether or not they can be correlated to one another. In short, both scales measure pressure, they just go about doing it in different ways. Piezoelectric strain gauges have a very fast response time and give you pressure (in psi) as a function of time, and the peak chamber pressure is simply the top of the P vs. T curve. The older crusher method had a standardized metal pellet (typically either copper or lead, depending on the pressure range being monitored) inserted into a hole drilled into the side of the chamber, and was then backed by a monolithic anvil. When the cartridge was fired, the pressure generated distorted the pellet, and the amount of distortion was directly related to the peak pressure exerted on the pellet. The length of the pellet was then measured and the length looked up in a table of reference values to determine the peak pressure. These reference pressures were also in psi.
So why do we call these pressure determinations CUP (copper units of pressure)? Very simple, the table of reference values is in psi, but they are determined under static equilibrium conditions. For example, when a static load of 50,000 psi is applied to a copper crusher pellet, it will compress a specific amount, but when that same pellet is subjected to a .30-06 cartridge at 50,000 psi, it will compress somewhat less, leading to an apparent chamber pressure of somewhere around 40,000-42,000, so instead of calling it psi, it was decided to name these units CUP so this offset would be inherently included in the measured result. Why is there this discrepancy? Very simple, TIME. It takes time to move metal, so when a load is applied to the copper pellet it takes time for it to achieve its new equilibrium conformation. The millisecond or so that it experiences the peak chamber pressures of the fired cartridge may not be long enough to complete this rearrangement. How far is it off? That depends on the pressure being measured. For pressures below about 30,000 the two pressure scales are virtually identical (at these lower pressures, the pellet doesn't change much and it doesn't take very long for it to achieve its new conformation). Between 30,000 and 40,000, the CUP scale starts to lag behind the psi scale, and above 40,000 the two scales start to differ significantly (60,000 psi corresponds to roughly 50,000 CUP and 100,000 psi corresponds to about 70,000 CUP). There is a calibration curve correlating the psi and CUP pressure scales in "Firearms Pressure Factors" published by Wolfe Press (this is an excellent book, and is recommended for anyone who handloads ammunition).
Pressure measurement - psi vs. CUP - Questions often arise about these two methods of measuring pressure and whether or not they can be correlated to one another. In short, both scales measure pressure, they just go about doing it in different ways. Piezoelectric strain gauges have a very fast response time and give you pressure (in psi) as a function of time, and the peak chamber pressure is simply the top of the P vs. T curve. The older crusher method had a standardized metal pellet (typically either copper or lead, depending on the pressure range being monitored) inserted into a hole drilled into the side of the chamber, and was then backed by a monolithic anvil. When the cartridge was fired, the pressure generated distorted the pellet, and the amount of distortion was directly related to the peak pressure exerted on the pellet. The length of the pellet was then measured and the length looked up in a table of reference values to determine the peak pressure. These reference pressures were also in psi.
So why do we call these pressure determinations CUP (copper units of pressure)? Very simple, the table of reference values is in psi, but they are determined under static equilibrium conditions. For example, when a static load of 50,000 psi is applied to a copper crusher pellet, it will compress a specific amount, but when that same pellet is subjected to a .30-06 cartridge at 50,000 psi, it will compress somewhat less, leading to an apparent chamber pressure of somewhere around 40,000-42,000, so instead of calling it psi, it was decided to name these units CUP so this offset would be inherently included in the measured result. Why is there this discrepancy? Very simple, TIME. It takes time to move metal, so when a load is applied to the copper pellet it takes time for it to achieve its new equilibrium conformation. The millisecond or so that it experiences the peak chamber pressures of the fired cartridge may not be long enough to complete this rearrangement. How far is it off? That depends on the pressure being measured. For pressures below about 30,000 the two pressure scales are virtually identical (at these lower pressures, the pellet doesn't change much and it doesn't take very long for it to achieve its new conformation). Between 30,000 and 40,000, the CUP scale starts to lag behind the psi scale, and above 40,000 the two scales start to differ significantly (60,000 psi corresponds to roughly 50,000 CUP and 100,000 psi corresponds to about 70,000 CUP). There is a calibration curve correlating the psi and CUP pressure scales in "Firearms Pressure Factors" published by Wolfe Press (this is an excellent book, and is recommended for anyone who handloads ammunition).
