I am writing this to help others who may try to do what I have and avoid some of the mistakes I did. I will try to cover what worked and what worked less than desired.
I would recommend reading the articles form the Kitchen Table Gunsmith (Roy Seifert) on “Strain Gauges for Chamber Pressure Measurements”. http://www.ktgunsmith.com/straingauge.htm
Recording device.
I use a 200 M Hz 4 channel oscilloscope with the ability to save information on a thumb drive. You don’t need this speed but it don’t hurt. I use all 4 channels and sometimes would like another. Anything that will record a 1 M Hz will work. The thumb drive allows me to take the data and screen shot to Excel for filtering and analyses.
Amplifier and PC board.
I used INA849 instrument amplifiers and would recommend it for this application. I set the gain at 100 and it should have a frequency response up to 8 M Hz at that gain. If I were laying out the PC board again I would have added a RC filter on the input and a way to easily convert the input to AC coupled. What I did that did not work and had to be changed was to use DC to DC converters to supply the gauge reference voltage and the negative voltage for the amplifier. What works well is a plus and a minus 15v supply for the amplifier and 7805 and 7905 regulators for the gauge references/bias.
Ground the gun.
I use a 100 Ohm resistor to ground the gun. I fasten the resistor to the gun by using electrical tape to tape one lead to the barrel. Then run a wire from the other lead to the power supply neutral. This eliminated a lot of scope false triggers and electrical noise.
Gauge mounting and configuration.
I recommend buying gauges with the wires already attached. A single gauge can be used but will require AC coupling on the amplifier input. I use a full bridge circuit. The full bridge should give twice as much signal with the same amount of noise. I mount the gauges to the barrel with an epoxy that does not have iron in it. I found that using Gauges as the balancing resistor was susceptible to noise. If they move during recoil there is a capacitive induce signal like a capacitive microphone. What works best is a series pare of resistors to get the correct value. They need to be mounted on the gun not on the amplifier board. If they are on the amplifier board that put connector and cable in the signal generation path and during recoil the connectors impedance changes. All of the wiring near the gauge needs to be restrained during recoil. E6000 glue works well for this purpose. I mounted a ribbon cable connector on the barrel and recoil causes the connector contacts to move frequently causing large noise signal. What would have been better would be to have a short 4 to 10 inch lead with one end connected to the bridge and then anchored to the barrel with E6000 and the connector on the other end. This would keep the connector from being subjected to the recoil.
I would recommend reading the articles form the Kitchen Table Gunsmith (Roy Seifert) on “Strain Gauges for Chamber Pressure Measurements”. http://www.ktgunsmith.com/straingauge.htm
Recording device.
I use a 200 M Hz 4 channel oscilloscope with the ability to save information on a thumb drive. You don’t need this speed but it don’t hurt. I use all 4 channels and sometimes would like another. Anything that will record a 1 M Hz will work. The thumb drive allows me to take the data and screen shot to Excel for filtering and analyses.
Amplifier and PC board.
I used INA849 instrument amplifiers and would recommend it for this application. I set the gain at 100 and it should have a frequency response up to 8 M Hz at that gain. If I were laying out the PC board again I would have added a RC filter on the input and a way to easily convert the input to AC coupled. What I did that did not work and had to be changed was to use DC to DC converters to supply the gauge reference voltage and the negative voltage for the amplifier. What works well is a plus and a minus 15v supply for the amplifier and 7805 and 7905 regulators for the gauge references/bias.
Ground the gun.
I use a 100 Ohm resistor to ground the gun. I fasten the resistor to the gun by using electrical tape to tape one lead to the barrel. Then run a wire from the other lead to the power supply neutral. This eliminated a lot of scope false triggers and electrical noise.
Gauge mounting and configuration.
I recommend buying gauges with the wires already attached. A single gauge can be used but will require AC coupling on the amplifier input. I use a full bridge circuit. The full bridge should give twice as much signal with the same amount of noise. I mount the gauges to the barrel with an epoxy that does not have iron in it. I found that using Gauges as the balancing resistor was susceptible to noise. If they move during recoil there is a capacitive induce signal like a capacitive microphone. What works best is a series pare of resistors to get the correct value. They need to be mounted on the gun not on the amplifier board. If they are on the amplifier board that put connector and cable in the signal generation path and during recoil the connectors impedance changes. All of the wiring near the gauge needs to be restrained during recoil. E6000 glue works well for this purpose. I mounted a ribbon cable connector on the barrel and recoil causes the connector contacts to move frequently causing large noise signal. What would have been better would be to have a short 4 to 10 inch lead with one end connected to the bridge and then anchored to the barrel with E6000 and the connector on the other end. This would keep the connector from being subjected to the recoil.
