PRESSI-USB Users's Manual PRELIMINARY

This device is intended to make it very easy to measure pressures through a USB interface.

Making the pressure connections

Simple version:

• Insert or remove the ends of the green tubing into the fatter black tubing.
• Don't remove the black tubing from either the PRESSI-USB or the Bulb: leave those attached.

Longer version:

The interface unit and the squeeze bulb both have barbed fittings designed for tubing with 3/32" inner diameters. You can press tubing on to those fittings directly, and metric tubing should also work well if the inner diameter is close to 2 mm. But there are two problems with the barbed fittings. First, they are threaded into the interface unit, and this is a nylon-to-acetal connection with very low friction, so if you connect a tubing directly to that fitting it may unscrew the fitting over time, leading to pressure loss and inaccurate readings. Second, at least for some common polyurethane pneumatic tubing, it is quite hard to press the tubing on to the barbed fittings and almost impossible to pull the tubing off without damaging the barbed fitting.

A simple solution to both of these problems is to use a short length of thick wall silicone tubing (the fatter black tubing in the photo) with a 3/32" inner diameter as a bridge between the longer pneumatic line (the green tubing above) and the interface unit on one end, and the bulb on the other end. The silicone tube can then be used as a flexible fitting for the long line, by pressing the long line into it about 0.5" or 1 cm. This is how we provide the system assembled.

If you need to disconnect the tubing please pull it out from the silicone tubing, and leave the barbed connection untouched.

You can cut the long line (we provide a 50' (15m) length of PUR tubing with 1/16" i.d., 1/8" o.d.) easily with scissors or a sharp blade, and then re-connect it by pressing it into the silicone line.

Making the USB connection

This is obvious -- just connect the cable to the interface and to an open USB port on the computer. But be careful not to install special drivers. PRESSI-USB will enumerate as an HID joystick and should use built in drivers on windows and linux. Please get in touch if you have questions about this.

You can do some basic subjective "are we connected?" tests using the windows game controller control panel. Click on the 'Properties' button and then squeeze the bulb and you should see the cursor move within the small diagnostic square region.

Taking Data

We provide 2 very short python scripts (pressi_test.py and pressi_stream.py) as examples of how to read the pressure data. They are almost identical, but pressi_stream.py writes the displayed PSI data to a file (called pressi.dat) so that they can be plotted. These programs need pygame to be installed on the system in addition to python. It is hoped that they are short enough to be clear but please get in touch if you want help with them.

In both programs, pressing the 'z' key on a keyboard will reset the zero voltage to whatever is being displayed at that moment.

The programs tend to show 9999 PSI until the first actual reading is taken. If the PRESSI-USB device is not connected the program will quit right away.

-- BenDugan - 16 Nov 2022