There are three primary ways pressure is measured in a ballistics lab. All of them use one of two different type of piezoelectric transducer. The following methods are described below:
- The SAAMI Method uses a conformal transducer, which conforms to the outside of the case.
- CIP uses a Channel Sensor which samples the gas directly through a predrilled hole in the case.
- NATO specifies a EPVAT ( Electronic Pressure Velocity and Action Time) which is a Channel Sensor placed at the case mouth.
There is a fourth method that is used by hobbyists, and small scale producers. However it does not conform to a SAAMI or CIP standard. An external stress gauge is placed on the outside of the barrel. This is not a process I am familiar with, so I will hold off on going into the details on how it is performed.
The Basic Equipment for Pressure Testing
There are two main transducers that are used for the measurement of pressure in small arms. Both work on the same principles but measure pressure in different ways.
The SAAMI Method uses a PCB 117B Series transducer. This is a conformal transducer, meaning the transducer end is cut in such as manner as to match the curve and taper of a particular chamber. That way the brass will expand and conform to the chamber in it’s normal way. In doing so it presses against the sensor and the gas pressure is measured through the brass case.
The basic equipment to measure pressure using the conformal method as as follows
- A conformal transducer such as the 117B Series from PCB
- A calibration adaptor use to establish brass offset and check sensor sensitivity.
- High pressure calibration pump capable of pressures up to 100,000 PSI
- Test barrels, and a test receiver
- A Charge Amp
- Computer interface, such as an Oehler 85 System
The Equipment needed to measure using the CIP and EPVAT methods are very similar since they used the same transducer. Since the gas is being measured directly there is less required to measure pressure making this a simpler system to use.
- 6215 Transducer made by Kistler, or its equivalent
- A Charge Amp
- Test Barrels and a Test Receiver
- Fixture for drilling holes in cases (CIP Testing only)
- Computer interface, such as an Oehler 85 System
How do Transducers Work?
Before we can get into the specifics of how a transducer works we need to understand a few things about how electricity is measured.
Volt: A volt is a measurement of electrical “pressure”.
Coulomb: The amount of Charge that is present. (This is different from an Amp which measures how fast the charge is moving) as with most things electronic 1 Coulumb is a very large number 6.24 Quintillion, 624,150,907,440,000,000, and is also commonly expressed as 6.2415090744×1018
Pico-: A prefix meaning very small, mathematically it is .000,000,000,001 or 10^12.
PicoCoulomb (pC): a very, very, small amount of charge.
Transducers comprise of a thin slice of highly pure Silicon Dioxide Quartz crystal typically lab grown. When squeeze these crystals produces a charge measured in picocoulombs. (Think of a spark that is generated on a grill lighter. The same concept.)
The sensitivity of a transducer used in ballistic testing its typically between .100pC/PSI to .200pC/PSI. Meaning that a cartridge producing 50,000PSI will produce between 5,000pC to 10,000pC.
This is a very small amount of charge, and takes some special equipment to measure it. These are referred to as charge amplifiers (Charge Amps). Charge amp measures the charge and converts it into a voltage which can be measured by an oscilloscope. Typically the charge amp will have a conversion factor of 10kpsi/volt meaning that same 50,000psi will be represented by a 5 volt signal on the charge amp output.
The Oehler 85 System is basically a fancy oscilloscope which measures the output of the charge amp at about 100kHz, or 100,000 times per second. A peak voltage meter, or an oscilloscope can be used in place of the an Oehler 85 system to get some of the same functionality.
Transducers are sensitive to moisture, shock and dirt. Thus they must be stored in humidity controlled environments, and special care must be taken when handling a transducer. Conformal and channel type transducers both work on the same principles.
SAAMI Conformal Testing
Conformal testing is by far the easiest type of pressure testing for manufacturers that want to do a large amount of development, or in process testing. Here we will highlight a few of the main differences between conformal testing and the other types.
Conformal testing is the most expensive type of testing in terms of initial investment The chief expense is the purchase of the calibration machine.
These high pressure two stage pumps can be hand or machine driven can reach pressures up to 80,000 to 100,000psi. They can also cost somewhere between $90,000 towell over $100,000 dollars brand new. They use a hydraulic oil to create the pressure and this make calibration a little bit messy. Calibration adapters are required for each caliber. These adapters typically run $3,000 or more per adapter.
During calibration the pressure is pumped up to 4 or 5 increasing pressures. The transducer measurement is read, and recorded. By the end of the test a pC/PSI and a pressure offset is established. This offset it the amount of pressure that is required to expand the case to touch the sensor. Depending on the case the offset may be close to 0 or 10,000psi or more.
The last thing that is checked it the linearity of the test. This is a QA/QC check to make sure the test is valid. Linearity is better described and presented in the SAAMI Manual. SAAMI Manuel found here. It is important to note that every time a new lot of case is used it must be calibrated to the sensor. So it is not uncommon to have to do four or five calibrations during a load development. A calibration for the test case, calibration for SAAMI Reference Ammunition, and 2 or 3 different types of competitor samples.
In addition to the basic equipment that was listed previously, to test to SAAMI Standards requires the use of SAAMI Reference Ammunition.
These lots of reference ammunition are produced by one of about six different manufacturers. This lot of ammunition is produced during one run, and it send out to SAAMI Test locations where a Round Robin pressure and velocity assessment is conducted. Once evaluated this reference lot is deemed to have an a given pressure and a given velocity.
This reference ammunition is fire through the test barrel and if the pressure and velocity vary from the SAAMI published standard for that lot, then a correction factor may be applied. This is an attempt to get every test facility, using slightly different equipment to test similarly. To my knowledge SAAMI is the only organization that does this, CIP and NATO testing do not have reference batches of ammunition and do not apply correction factors to the test equipment.
The conformal transducer itself is fairly inexpensive compared to some of the other equipment needed. PCB offers their 117B conformal transducer for $1515. Most transducers can be used for multiple calibers. For example the 1117B13 is used for 357 Mag, 38 Special, 380 Auto, and 7.62 Tokarev.
SAAMI offers the drawings for their standardized test barrel on in the publically available SAAMI Manual and example of which is shown below.
CIP & EPVAT Testing
As mentioned previously CIP and EPVAT testing are very similar in the equipment used and both as substantially cheaper to get into then SAAMI. The CIP Standard is used throughout the world in pretty much every country outside of the United States. It would be incorrect to assume that the CIP method is inferior to the SAAMI method as it is likely responsible for the majority of the worlds safe ammunition production.
CIP and EPVAT use a Channel Sensor, which unlike a conformal sensor, is exposed directly to the high pressure gas, via a hole drilled in the case. Because of this, there is no case calibration required, thus the equipment to do so is not needed. This drastically reduces the entry cost to get into measuring pressure. Like with all Piezo transducers a charge amp is still required.
Also CIP does not use reference ammo, and does not apply correction factors to their test equipment. In theory this may mean that normal variation with test barrels and test equipment may have greater impact on the test results.
The down side to CIP testing is that it requires a hole to be drilled in every case, and the case must be aligned so the hole is pointed in the direction of the port for the sensor. Also, the sensor must be removed after 20 shots, cleaned, and a protective diaphram must be replace. I have seen improperly seated sensors destroyed by gas cutting, so there must be a level of care when installing these sensors.
A single 6215 transducer can measure pressure for just about any metallic cartridge out there, and has uses beyond measuring chamber pressure. The can also be used to measure port pressure. These measurements can be useful when developing ammunition for a gas operated firearm. They are also used when measuring the pressure at the case mouth as is done with EPVAT testing.
With more versatility come and increase cost, each sensor costs and estimated $3,000 dollars. Nearly double that of a conformal transducer. There is also the diaphragms, spacers, and a heat shield which are consumables and will need to be replace. None of those components run cheap.
As stated before CIP and SAAMI standards are different, but it’s hard to say that one is inferior or better then another. They both produce safe ammunition. It is true that sometimes SAAMI and CIP do not agree on what the max Service pressure for a load should be, they are generally fairly close.
EPVAT Testings Differences
As noted before both CIP and EPVAT Testing use the same 6215 Channel type sensor that CIP testing allows for. The main difference is the sensor location. CIP location places the sensor on the body of the case, what they refer to as the powder chamber. While NATO EPVAT testing places the sensor at the case mouth.
Placing the sensor at the case mouth requires no modification to the brass case. It also doesn’t leave a mark on the brass case as the conformal transducer does. Reference ammunition is used, and a specially designated lot is maintained by NATO for calibration of test equipment. Similar to how it is done by SAAMI.
There is drawbacks to measuring at the casemouth, namely the rise in pressure is more abrupt, and can cause oscillations which must be filtered out. The pressure comparison between an EPVAT Test and a SAAMI or CIP cannot directly be compared as the filter used to cut out this oscillations affects the peak pressure data and can cause minute error in the measurement.
As with CIP and SAAMI, NATO has their own chamber dimensions and pressure standards that make it hard to draw direct comparison in results obtain via a test done to the SAAMI standard, verses a test being done to a EVAP standard.
Which System to Get Into?
This can be the hard one. If you are someone who wants to load to a standard, you kind of have to buy into a whole standard and you shouldn’t cherry pick what parts of a standard you are willing to follow and what parts you are not. With that said, if you are a small producer, or you are a hobbyist who has a bit of money and would like to study things a bit deeper, going by the CIP model of testing is probably not a bad way to go.
Typically cost is the barrier to entry, and with the cost of a the calibration pump, the calibration adapter, the transducer, and a charge amp, you probably looking at close to $110,000 at a minimum and it can easily go up from there. By the time you purchase a test barrel, a universal receiver, and expand your set up to shoot a few different calibers it would not be unexpected to spend closer to $150,000 to $175,000.
The CIP method is a bit more forgiving on the front end costs. Allowing you to shoot any caliber, for the entry cost of the transducer, a charge amp, a test barrel, and a universal receiver. These can be had for under $25,000 in total expenses. Each additional caliber you want to shoot will only cost you the test barrel which can run $800 to $1000 dollars.
Some of us cringe at the thought of drilling a hole through a case. Pretty much limits the case to a single use. Though you may get multiple firings from the case if you relegate it only as a test brass… (It’s a thought, not something I’ve done in practice.) There is a lot of brass to be had for $125k difference in start up costs.
Having worked for a company that produced ammunition for the CIP and the SAAMI market we had to test ammunition using both standards. We did a few studies into what the main differences were between a 6215 Channel Sensor in place of a 117B sensor. Obviously we had to use two different barrels, and the chambers were cut to SAAMI or CIP respectively. However we found that that differences were often slight.
Slight enough that if the SAAMI Correction factors were removed then they could be considered near identical results. With that said, there are a lot of unknowns in the world of ballistics, and there may be cases where the results can be vastly different, but I suspect that more often than not the results will be quite similar.
The reason we measure pressure is about 90% safety and 10% quality. So to have a method that will measure accurate pressure that are analogous to what happens in a real firearm is the most important part. The CIP test model does it, and does it in an fairly inexpensive way.
So I were a cash strapped company, I would personally buy into the CIP testing model. However I would still probably use SAAMI standards in terms of cutting the chamber, and for holding to pressure standards. This will give me a fairly cost effective way of periodically checking a load to make sure that it was within a safe pressure. Then down the road, if my business grew, and I wanted to streamline the process, I would look at investing in the SAAMI Model completely.
I realize this is cherry picking standards, but at the same time, having something to check pressure with, is better than having nothing. Getting CIP chamber reamers, or having barrels made to CIP standards may be more difficult in market that is overwhelmingly controlled by SAAMI.
Some Closing Costs and Thoughts
There are some costs that I’ve alluded to but not really gone into detail on. The first is a universal receiver, or called an UR for short. These are large hunks of metal that are bolted to a bench and are pretty near impossible to destroy. You’ll ruin the barrel long before you ruin a UR. These are fairly expensive pieces of equipment, with refurbished units running close to $10,000.
You may be able to get away with testing using old Mauser or Remington 700 Actions. so long as the test barrel is thick enough to support the transducer and you have a means of clamping the barrel into a test fixture. Just be warned that when high pressure event happen, and they will happen, the receivers will crack and fail. This can cause a safety issue for the personal testing. Have some sort of blast shield or protective barrier between the shooter and the action with help protect the individual in such an event.
There have been other means to test pressure, such as a copper crusher, or even lead crusher, but they are long retired means. There is no direct correlation between this antique pressure measurements to the modern Piezo Transducer. So while it is an interesting footnote it is no longer a useful measurement technique for modern ballistics.
I touched briefly on the Oehler 85, which is a box that is designed to combine pressure and velocity measurements and port them to a computer. It’s a streamline system that works well, and is virtually an industry standard. It’s not the only system out there but it is probably the most used.
However, you can get the pressure measurement from the Charge Amp, the velocity measurement from your chronograph, and still have valid results. However without some sort of oscilloscope you will miss out on the P/V curve, which can be a really interesting part of ballistics. (Oscilloscope examples below.)
Someday, I do hope to have a proper ballistics lab set up to do pressure measurements an offer it as a service to handloaders, and small ammunition companies wanting to outsource their QA/QC. This is an eventual goal of mine. In the meantime hopefully this write up provides an insight on how these measurements are conducted, the basics of cost and an overview of some of the standards out there for pressure measurement.