To take chamber pressure and thrust measurements while you static test your rocket motor, you will need three basic components. They are a pressure transducer to measure chamber pressure, a load cell to measure thrust and a data acquisition module to record the voltage outputs from the load cell and transducer.
CP Technologies does not directly sell any of these three components as we cannot offer you as good as price as the companies directly making them. We have provide information and links below to the companies so you can buy them directly and save money.
The load cells and pressure transducers output a voltage in direct proportion to pressure or force. You will need a Data Acquisition Module to take these voltages and translate them into force and pressure plus store the information. DATAQ manufactures a simple unit (DI-194) that plugs into the USB port of your desktop or laptop computer. It does not require any external power supply. You simple plug the unit into the USB port of your computer and hook the output wires from the transducer and load cell to the input terminals of the DATAQ DI-194 unit. Along with the DI-194 unit comes the USB cable and a powerhouse software program that will graphically display the data in real time on your computer in the right engineering units. It will store the data on your disk and you can export it into various formats included EXCEL.
The best part is the price for the DATAQ DI-194 including the software is only $59. You can order this unit by going directly to the DATAQ web site.Click here to go to the DATAQ DI-194 web page
Pressure transducers can now be found on Ebay, but if you want to buy a new, good quality pressure transducer, we recommend you buy one from Omega Engineering. The pressure transducers that will stand up to abuse and the harsh environment of solid rocket motor testing as long as you get stainless steel wetted parts. Don't buy pressure transducers with plastic wetted parts. The plastic parts will melt from the heat of the combustion gases.
The pressure transducer from Omega Engineering we recommend is the PX-303 (shown left). It comes in a variety of pressure ranges, but the range most useful to the amateur will be 0 - 1000 psi. If you are making PVC pipe motors, than get the 0 - 500 psi range transducer.
If you are only going to buy one pressure transducer, this is the one to buy. With a 0 to 1000 psi range, you will have all typical chamber pressures covered on your test stand. We have been using ours for years without failure.
Load cells are used to measure the thrust or force generated by the rocket engine during a static test. There a variety of different load cells types, but we like the shear beam or cantilever beam type the best since it is easy to set up. One end of the beam is fastened down to a secure, fixed point. On the other end of beam is mounted the rocket motor so that it pushes against the beam. The load cell can be mounted so that the motor fires horizontally or vertically.
The LCJA model (shown left) from Omega Engineering comes in a variety of different ranges, but the ranges useful to most amateurs is either 0 - 500 lbs or 0 - 1000 lbs.
Load cells come with a low millivoltage output. You can make a simple amplifier to boost the voltage output for use in the DATAQ DI-194 unit. A schematic diagram is shown below using an INA-125 chip. You can buy these chips from a number of online sources for about $7 per chip in low quantities.
Even if you have never made electronic components before you can make this one as it consists of only a chip, a chip socket and a resistor. You just solder wires to the connectors on the pin socket and hook it up as shown in the diagram below. The chip pin numbers are shown in the schematic. If you right click on the schematic diagram with your mouse, you can save the schematic on your computer in a larger image format for better viewing. After right clicking on the image, select the "save picture as" option.
We recommend mounting the chip socket on a small board and mounting the board inside a plastic box. You can route the wires to terminal strips outside the box with the connections labeled for easy connection at the test stand. Make the box big enough so you can easily get your fingers inside to insert the chip and replace it with a new one when necessary. Our chips last a couple years sitting in the test stand in Wyoming's harsh winter environment.
If you use a 9 volt battery to power the chip, the voltage output for the load cell will be 5 volts. If you power it with a 12 volt battery, you will get a 10 volt output. The 10 volt output is better as it will give you more sensitivity.
The resistor shown in the schematic is shown as a variable resistor. We actually recommend that you make it a fixed resistor
based on the amplification requirements for your load cell and data acquisition system. Variable resistors can shift their
position due to handling and vibration causing your recorded values to be off. You should use a high precision resistor
with a tolerance of 1%. The value for the resistor can be best illustrated with the following example.
Example: You want a 0 to 6 volts output for the range of your load cell. Your load cell specifications are a rated output of 3 millivolts/volt excitation with a full scale load of 200 lbs. The recommended excitation voltage is 10 volts DC. We will supply the recommended excitation voltage of 10 volts DC.
The output voltage of the load cell with 10 volts excitation will be (10 volts) * (3 millivolts/volt) = 30 millivolts =
0.030 volts. When a load of 200 lbs is on the load cell, it will output 0.030 volts DC. We want to amplify that to 6 volts
so we determine the required amplifier gain as follows:
Amplifier Gain = (Desired voltage)/(Load cell output voltage) = (6 volts)/(0.030 volts) = 200 = Gain
The value for the resistor can be found with the following equation:
Resistor (ohms) = (60,000)/(Gain - 4) = (60,000)/(200 - 4) = (60,000)/196 = 306 ohms = Desired resistor value in schematic
It probably is not possible to get a single resistor at 306 ohms. You can just pick the resistor value closest to it and use a 300 ohm resistor. You can also hook multiple resistors in series to obtain the desired resistance. In this case, you could use a 300 ohm resistor connected to a 6 ohm resistor or any combination of resistors to give you 306 ohms.