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.
Data Acquisition Module
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 serial port of your desktop or laptop computer. It does not require any external power supply. You simple plug the unit into the serial 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 serial 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 $24.95. 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 of the following models from Omega Engineering. Both are rugged pressure transducers that will stand up to abuse and the harsh environment of solid rocket motor testing.
The PX-181 is economical, corrosion resistant with a voltage output (shown right). We have used the transducer in a lot of motor tests without failure. It is a good transducer for measure chamber pressures up to 500 psi.
The second 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 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 Cell Amplifier Schematic
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. 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.
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.
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:
The value for the resistor can be found with the following equation:
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.