Bob Higgins has been building a programmable back pressure regulator in order to try to precisely keep a maximum pressure (above gauge) in a reactor core so that, for instance, pressure profiles claimed to have worked can be closely emulated - like those of Parkhomov.
The USB driven prototype controlled back pressure regulator circuit is now working but, as of this blog post, it is still missing a 10 micron orifice to slow the out-flow of the small volume of gas that would be in the reactor tube when the solenoid valve is activated.
Figure 1: Bob Higgins' USB backpressure regulator prototype
The valve is supposed to be 12V 6W. What Bob found though was that the coil is 18.9 ohms and it will activate at 3.5V and then will not close until the voltage falls to 1.0V. At 5V, this coil will draw 265mA, so the possibility exists for it to be a fully bus powered device. Bob will program the USB chip to tell the computer that the USB system could take up to 300mA. Bus powered devices can be supported up to 500mA. Once the valve is opened, it only takes about 60mA to keep it open.
The pressure is set to max. for the sensor's operation (in this case 250 PSI) when it is powered. The device appears as a virtual serial port. You send it bytes to program what pressure it will regulate at. Each bit in the programmed byte you send changes the regulation by the (sensor range)/255. The device echo's the byte you have programmed it to. This should be easy to control from almost any control program, including Labview.
Figure2: Bob Higgins' USB backpressure regulator schematic
It is possible for this design to be modified to be bus powered, but the 5V LDO regulator would have to be replaced with a boost or buck-boost switching regulator. For now, Bob will simply make this a <100mA USB device with an external 7-10V isolated supply to power the solenoid (mostly). The drivers come from the USB chip manufacturer (FTDIchip.com). FTDI has good driver support for: Windows from Win98 - Win10 (16b/32b/64b), Linux, Mac, and even Android. The device will appear as a Virtual Com Port. To change the back pressure regulation value, you send 0x00-0xFF to the port with 0x00 being max sensor pressure and 0xFF being 0 pressure. The device will respond with 0xFF and then the value you programmed. The pressure will fall until the programmed value is reached. Use of a 10 micron flow orifice is recommended for the small volume of H2 in the Parkhomov-like experiments. This is calculated to cause the gas pressure to fall at a sensible rate for the solenoid to control to the programmed pressure. The flow orifice has not been tested, as of this blog post, to compare its flow resistance to what is calculated.
Most of the parts are easy to find - most are from Digikey, including the pressure sensor. The hardest part to find was the M12 cable for the pressure sensor, which Bob found at Allied Electronics. Bob bought the Gems solenoid valve from Amazon (believe it or not!).
Bob's prototype is constructed on a solder-less breadboard. I made small adapters for the SOT-23 parts using small pieces of square pad vector board (the software engineer he used to work with called these "spiders"). The prototype is fully operational in evaluation testing so far. Drivers used were the FTDI VCP default x64 bit driver for my computer (Win7). I used RealTerm to communicate with the board over the virtual com port for testing.
Bob will hand wire a permanent version for his experiments.
If others are interested in building this programmable back pressure regulator, Bob - or perhaps another community member - may consider laying out a small printed circuit board. Please let us know in the comments below.