SAAMI question

The 38spl is neutered and the 380 is not

https://www.google.com/url?sa=t&sou...FjAAegQIARAB&usg=AOvVaw3qqvjixjE2ru8YdWWe5M9J

Maybe this will be helpful.

See note numbered 6 near the end.

CIP has some advantages.

sequins said:

The 38spl is neutered and the 380 is not

Click to expand...

concrete evidence?

This stems from the fact that the two measurement systems are not measuring the same thing only closely related things.

The crusher method (CUP) is really a measurement that integrates the entire pressure event into one number. In theory it is the integral of the pressure versus time curve though due to the exact mechanical method used it misses some of the data and the results can be effected by some of the dynamics of the pressure event.

The more modern transducer method (PSI) actually measures the entire pressure vs time curve of the pressure event using a piezo force sensor and the SAAMI specification is for the highest pressure measured over that event. A typical pressure test for a rifle cartridge might produce 100-300 data points.

The fact that you could have several cartridges that all measured the same value with the older crusher method but get different values using the transducer method was some of the first indications back during the development of the transducer method that the crusher method was not giving them the data they had assumed it was. Before the development of the transducer method (1960's) crusher results were reported as PSI because they thought it was an accurate measure of peak pressure. It's not, it is influenced by the pressure rise rate and dwell times of the pressure event along with the peak pressure. So as the newer transducer method gained official acceptance SAAMI started reporting Crusher results as CUP or LUP (Copper Units of Pressure or Lead Units of Pressure) and the newer transducer method as PSI since they were fairly confident that it was a fairly accurate measure of true pressure.

The crusher method may not have been giving them exactly the information they thought it was but they had used the method safely for several decade and had no reason to abandon it but the transducer method gave them more data that was more accurate and easier and cheaper to implement. The transducer specifications were created by using known standard ammunition as measure by the crusher in a transducer setup (a lot of data collected by a lot of different labs). So when a 17,000 CUP 38 Special was fired in a transducer system they measure a peak pressure of 17,000 psi. When they did the same with standard 17,000 CUP 380 ACP ammunition they got 21,500 psi.

This fact that you can have multiply cartridges with the same CUP value point to different PSI values and vice versa is also why mathematically there is not a reliable way to translate between the two even if we ignore the differences in what is actually being measured. A mathematical function cannot have one input result in multiple different outputs. Thus you can never write a function that will accurately convert one to the other.

One of the larger segments of the SAAMI data were a bunch (11) cartridges share the same CUP spec but have several different PSI values.
Cartridge, kCUP, kPSI
221 Rem Fireball 52 60
6mm Remington 52 65
7mm Rem Mag 52 61
7mm-08 Rem 52 61
17 Rem 52 63
221 Rem Fireball 52 60
223 Rem 52 55
243 Win 52 60
270 Win 52 65
308 Win 52 62
35 Whelen 52 62

The above cartridges all have the same SAAMI max CUP of 52,000 CUP but there are 6 different PSI values spread across those eleven cartridges.

-ramblin'

mcb said:

This stems from the fact that the two measurement systems are not measuring the same thing only closely related things.

Click to expand...

Excellent description and explanation above!

The short answer is that as we abandoned CUP, someone had to make a decision as to what specification to set for PSI. Sometimes the decisions were consistent, and sometimes they weren't. That introduces statistical noise into the conversion.

Almost all the deformation of the copper pellet in the CUP system happens at or near peak pressure. It is at least a decent estimate of peak pressure, though not perfect.

Mr. Zorg posted my article on converting between CUP and PSI, which I think covers the topic reasonably well. One important thing to note out of that article is that it only applies to rifle cartridges with 35-60 KPSI peak pressure. So applying it to the 38 Special and the 380 is edgy.

PSI predicts CUP about as well as PSI predicts PSI. If you fire 10 cartridges on a piezoelectric pressure measurement system, and then use that number to predict the pressure of the next 10 cartridges, your prediction will almost always be in error. In round numbers, your pressure measurements will tend to fall in a +/- 1000 PSI range. So knowing the PSI generated by one sample does not perfectly predict any other samples. There is random error in the system.

If you regress CUP vs. PSI, you get a correlation that is about as good as the PSI vs. PSI example above. The PSI CUP correlation hits the mark within about 1000 PSI in the center of the range, and about 2000 PSI near the ends. Only The Almighty gets perfect information. The rest of us have to manage with some sort of estimate.

If you're converting Celsius to Fahrenheit, the conversion is exact. If you're measuring a mechanical system, it never will be.

So what explains the less variable correlation between CUP & CIP PSI? Personally the CIP specs make a whole lot of sense to me especially for rifle cartridges ending in 57mm and 64mm. I kne the SAAMI PSI doesn't have direct conversion to CIP PSI and I'm well aware there are differences in methodology but like CUP they're directionally harmonious and there's no question in my mind that despite the lack of direct conversion that the CIP PSI maximum pressure value for such cartridges exceeds the SAAMI PSI maximum pressure value due to the differences in muzzle energy with cartridges loaded to the SAAMI PSI specifications vs European loaded cartridges loaded to the CIP PSI specifications. But I've received obfuscation regarding those facts explained as being unable to directly convert between SAAMI PSI values and CIP PSI values.

While a conversion between Celsius and Fahrenheit is direct, measuring the value in one set of units for conversion to the other has error no matter which direction you start from, even if the measurement is performed electronically instead of mechanically.

Mr. Zorg said:

So what explains the less variable correlation between CUP & CIP PSI? Personally the CIP specs make a whole lot of sense to me especially for rifle cartridges ending in 57mm and 64mm. I kne the SAAMI PSI doesn't have direct conversion to CIP PSI and I'm well aware there are differences in methodology but like CUP they're directionally harmonious and there's no question in my mind that despite the lack of direct conversion that the CIP PSI maximum pressure value for such cartridges exceeds the SAAMI PSI maximum pressure value due to the differences in muzzle energy with cartridges loaded to the SAAMI PSI specifications vs European loaded cartridges loaded to the CIP PSI specifications. But I've received obfuscation regarding those facts explained as being unable to directly convert between SAAMI PSI values and CIP PSI values.

While a conversion between Celsius and Fahrenheit is direct, measuring the value in one set of units for conversion to the other has error no matter which direction you start from, even if the measurement is performed electronically instead of mechanically.

Click to expand...

There is a conversion from SAAMI PSI to CIP PSI, but it's not as good as the conversion from SAAMI CUP to PSI:

The regression equation is
CIP PSI = 18419 + 0.695 ANSI PSI

R-Sq = 88.5%

I've never figured out why this conversion is not "cleaner", but it is what it is. So I've never staked my reputation on it.

As far as the conversion between CIP PSI and CUP, I think you're left with two choices: 1) CIP equipment is an order of magnitude better than SAAMI equipment, or 2) They are doing all their measurements on piezo systems and converting to CUP by formula, with some exceptions.

the cup method measures gas pressure directly via a gas piston which crushes a copper slug. the piezo pressure transducer (psi) method measures gas pressure indirectly via the case wall pressing on a strain gauge. different methods give different results for the same cartridge.

there is no conversion between the two and don't let anyone try and talk you into one. the 357 magnum cartridge is a great example of why not to trust any conversion formula no matter how good it sounds.

murf

murf said:

the piezo pressure transducer (psi) method measures gas pressure indirectly via the case wall pressing on a strain gauge.

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Actually not.
A piezoelectric transducer is not a strain gauge.
You can buy a consumer strain gauge and glue it to a Contender barrel and make up your own numbers.

I am quoting myself from an earlier thread. I wrote this awhile ago showing the various ways pressure is measured in a firearm by SAAMI, CIP and NATO. Thought it might be helpful to this thread also.

SAAMI and CIP use very different methods to measure the pressure. Mega rambling warning.

SAAMI pressure testing for the current Transducer method uses a piezo force sensor (from PCB Piezotronics) attached to a conformal post of a precise diameter that extends through a matching hole the pressure test barrel and touches the outside of the cartridge case (thus nearly every cartridges has to have a cartridge specific sensor assembly, though some family of cartridges share sensors (ie 243 Win, 260 Rem, 308 Win ). It is typically located forward of the middle of the case but behind the shoulder if the cartridge has one. When fired the pressure pushes on the case wall that pushes on this force sensor. Pressure is inferred from the force measurement. A calibration method/apparatus exists to correlate actual chamber pressure to the force and compensate for the brass thickness over the sensor post. But because this measurement is made through the case wall you have to use new brass and the brass has to all from the same batch both your empty calibration cartridge for calibration and your actual test lot of ammo. SAAMI pressure testing on reloaded brass introduces a lot more error due to variation in residual brass hardness from previous firings. The advantage of the SAAMI method is the sensor is well protected and after calibration testing proceed fairly quickly without special holed ammunition or consumable crushers. The US military method is based directly on the SAAMI method and uses the same hardware.




The much older SAAMI Crusher method is located in a similar location on the side of the case but uses a cartridge with a hole in it (thin tap applied to retain powder). The hole in the case has to be aligned to a hole in the test barrel. This allows allows pressure to push on a gas check (gas seal) that pushes on an piston that acts on the crusher (a cylindrical piece of very pure annealed copper or lead made to very precise dimensions). Pressure is then inferred from how much crush (change in length) is measured in the crusher. Each batch of crushers comes with a calibration chart that correlates changes in length to a peak pressure. This method is still in use by a surprising number of companies but it is moderately expensive (due to consumable crushers) and very slow and tedious to conduct.



The CIP method is similar to crusher in that it uses a cartridge with a hole drilled in it at a specified location. The hole is aligned with a port in the pressure barrel but unlike the crusher method the gases act directly on a piezo pressure sensor (Kistler) and unlike SAAMI CIP measures pressure directly. In the picture below the CIP sensor would be the left one. CIP has the advantage of probably being the most true chamber pressure vs time data curve. The problem with CIP is it also takes special holed cartridges. It does change initial case volume due to space between the hole in the cartridge and the actual sensor diaphrame potential reducing peak pressure very slightly Since the sensor is directly expose to hot propellant gases, sensor life is shorter than the SAAMI method The NATO EPVAT method uses a similar direct pressure sensor like CIP but it located at the case mouth as shown by the right sensor in the picture below. This has the advantage of not needed holed ammunition but it misses the the earlier part of the pressure curve that happened before the bullet moves past the sensor location exposing the sensor to the gases.



Measuring pressures that are this high and this transient is very difficult and the method still has far more effect on the measurement than most people what to admit. I know some US ammo companies that despite the SAAMI transducer method being faster, cheaper and easier to do still use the Crusher method on ammo if the ammo predates the acceptance of the transducer method. Not because the Crusher method is better, it's not, with many short comings, but it is very repeatable and producing consistent ammo is more important than going to a new measurement method.

Here is an article written by someone in the industry that you might learn from:

https://www.shootingtimes.com/editorial/ammunition_st_cuppsireloaddata_200905/100105

Need a treatise on the British axial pressure receiver.

Jim Watson said:

Actually not.
A piezoelectric transducer is not a strain gauge.
You can buy a consumer strain gauge and glue it to a Contender barrel and make up your own numbers.

Click to expand...

and have yet another set of numbers that won't convert to any other.

a piezo pressure transducer has a strain gauge in it. the strain gauge is what transfers pressure to electrical signal. https://en.wikipedia.org/wiki/Strain_gauge

murf

murf said:

and have yet another set of numbers that won't convert to any other.

a piezo pressure transducer has a strain gauge in it. the strain gauge is what transfers pressure to electrical signal. https://en.wikipedia.org/wiki/Strain_gauge

murf

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My apoligies, sir, but you are seriously mistaken.

A strain gauge is a film of copper circuitry, deposited on a substrate. As the substrate is stretched or compressed the resistance of the copper changes slightly. It does not generate an electrical potential difference.

A piezoelectric transducer is a piece of crystal material, in some cases, quartz. As force is applied to it, opposite electrical charges appear on its surface. This is a potential difference, and the voltage is proportional to the applied force.

I have a data set from Ken Oehler, which shows that the strain gauge and piezoelectric systems are capable of equal precision and accuracy. But they are definitely two different systems.

denton said:

My apoligies, sir, but you are seriously mistaken.

A strain gauge is a film of copper circuitry, deposited on a substrate. As the substrate is stretched or compressed the resistance of the copper changes slightly. It does not generate an electrical potential difference.

A piezoelectric transducer is a piece of crystal material, in some cases, quartz. As force is applied to it, opposite electrical charges appear on its surface. This is detected as a voltage, and the voltage is proportional to the applied force.

Click to expand...

Strain gauge are rarely copper, though some of the lower cost gauges use specialize copper alloys. Platinum alloys are probably the most common strain gauge materials as they have much better sensitivity but do cost more. Other common materials include Nickel based alloys like Nichrome (a high nickel and chrome alloy also sometimes know as shape-memory-alloy) and a few other more specialized alloys. (In a former career I help design high-temperature high-pressure sensors using a Nichrome based sense element and super sensitive (sub 1 psi full range) pressure sensors using platinum based sense elements)

Piezo sensors come in two common flavors piezoelectic and piezoresistive. The PCB Piezotronics force sensor used in SAAMI testing and the Kistler diaphram pressure sensor used in CIP testing are both piezoelectric based sensors and thus as force/pressure is applied and strains the peizoelectric sense-element a charge in the semiconductor is create and can be measured as a voltage. That said there are piezoresistive strain gauges. Technically metal strain gauge are also piezoresistive but when most people say peizonresistive strain gauges they are referring to a semi-conductor based not metallic based strain gauge.

Though not an issue in firearms testing but interesting piezoelectic sensors cannot be used for static loads. If you apply a static load to a piezoelectric sensor (force, pressure, accelerometer etc) you would see a voltage that slowly fads back to zero under the static force. When the static load was release you would see the opposite voltage signal that would then also fad after sitting unloaded for a similar length of time. This time constant change depending on the particulars of the piezoelectric sensor. The accelerometer in your phone is piezoresistive for example.

-rambling.

mcb said:

Strain gauge are rarely copper, though some of the lower cost gauges use specialize copper alloys. Platinum alloys are probably the most common strain gauge materials as they have much better sensitivity but do cost more. Other common materials include Nickel based alloys like Nichrome (a high nickel and chrome alloy also sometimes know as shape-memory-alloy) and a few other more specialized alloys. (In a former career I help design high-temperature high-pressure sensors using a Nichrome based sense element and super sensitive (sub 1 psi full range) pressure sensors using platinum based sense elements)

Click to expand...

Thank you for that information.

Mine look like copper, and I never paused to question their composition. My error.

denton said:

I have a data set from Ken Oehler, which shows that the strain gauge and piezoelectric systems are capable of equal precision and accuracy.

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that does not mean that their output scales are the same. that is the point here.

thanks for the clarification on the strain gauge/piezo question.

murf

murf said:

that does not mean that their output scales are the same. that is the point here.

thanks for the clarification on the strain gauge/piezo question.

murf

Click to expand...

But it does, and they are.

denton said:

But it does, and they are.

Click to expand...

There are lots of things that effect the precision and accuracy of these different sensors and very difficult measurements and I am not sure I am following @murf reference to scales.

Can a strain gauge mounted on a rifle barrel get an accurate measure of chamber pressure? Sure it can. But there are a few things that make traditional strain gauges on a rifle barrel exterior less than optimal for some uses. First is calibration. The only way to calibrate a strain gauge placed on the exterior of a rifle's barrel is to fire ammunition of known pressure (standard ammo) in the barrel. This gives you a single point calibration. Usually sufficient but gives you no way to check the linearity of the sensor. The calibration ammo makes a specified pressure in the barrel it was calibrated in (a piezo test barrel), minor difference in your barrel/chamber compared to the calibration barrel may produce slightly different pressures thus affecting the accuracy of your calibration. A strain gauge barrel does a good job of comparing ammo but its absolute calibration typically cannot be as good as the piezo sensors. This is not a deficiency of the strain gauge (other application allow for proper calibration) but a functional limitation of this particular application of the strain gauge. Lastly, to have good resolution with your strain gauge you have to have sufficient strain (stretch) at the surface of the barrel this means you cannot make the barrel super thick or you get no strain at the surface and nothing to measure. Again for normal work this is OK but you have significantly less safety margin to play with if your doing interesting stuff.

With a SAAMI pressure barrel, in a universal receiver, using a conformal piezo sensor, calibration is done in a separate apparatus. This calibration process lets you calibrate the piezo sensor with multiple data points against a "truth" pressure sensor, this results in a very good absolute calibration and confirmed sensor linearity. Since the sensor is not dependent on stain in the barrel's outer surface the test barrel for a universal receiver are massively over build. The chamber area is 2-inches in diameter and the barrels are typically 1.25 inch in diameter forward of the chamber. A strain gauge applied over that chamber would read almost no strain in all but the worst over pressure cases. Remember these test barrel are used to measure production ammo, standard ammo, and proof ammo and they still have a very healthy safety margin above the highest proof pressures.

And we have not touch on the electronics that interface with and record the output signals from both. This can have an impact also but I am just a mech-E and will get out of my depth fast when talking about charge amplifiers, wheatstone bridges, analog to digital convertors, and DAQs. Sampling data 10's and even 100's of kHz takes some decent electronics.

-ramblin' as usual.

Good thoughts, but I do disagree with one point: The strain gauge pressure system is easily calibrated. Direct comparison with an artifact is one convenient way to calibrate, but not only one.

There are seven fundamental physical quantities. Until recently, only one, the standard mass, was a mass artifact. Recently, even that was retired. So all calibration to NIST prime standards is done without direct comparison to standard artifacts.

Duplicate.

denton said:

Good thoughts, but I do disagree with one point: The strain gauge pressure system is easily calibrated. Direct comparison with an artifact is one convenient way to calibrate, but not only one.

There are seven fundamental physical quantities. Until recently, only one, the standard mass, was a mass artifact. Recently, even that was retired. So all calibration to NIST prime standards is done without direct comparison to standard artifacts.

Click to expand...

I agree a strain gauge based pressure sensor (ie a wetted diaphragm with a strain gauge on it to make a sensor) can be calibrated in much the same manner the SAAMI piezo sensor is. It would be similar to how you would calibrate the Kistler sensor used in CIP testing.

But a strain gauge mounted on the exterior surface of a rifle barrel, that I though we were discussing, is a much harder thing to calibrate. All the firearm pressure systems I have seen that use these exterior strain gauges calibrate by comparing the raw output signal to known pressures in the barrel and this is difficult to do short of using standard ammo as I described above. The only other method I know of is to use the factory calibration data (usually a unique value(s) printed on a calibration card and included with each individual sensor from the factory) that correlates the measured resistance of the strain gauge to actual strain. Once you have the actual measured strain you use a FEA model (Finite Element Analysis, a computer model that can simulate the stress and strain relationship in complex geometries and loading conditions) of the gun barrel to calculate the pressure required to cause the measure strain at the location of the strain gauge. This relies heavily on a very accurate dimensional model and material properties for the FEA model of the gun barrel and strain gauge location.

There is no correlation between CUP and psi. NONE. Thus, you get these kinds of things. Have you read all the testing and statistical analysis of pressure that SAAMI mandates for the industry vs. where it used to just be "measure copper pin length, insert copper pin, fire cartridge, measure copper pin length and estimate pressure."