This is going to be the last blog devoted to purely an electronic venture. This is an amateur pyrotechnic forum, not an electronics site! But fortunately, everything I've made so far has a solid use in pyrotechnics.

For years, I've been envious of those who have what I would call a "deluxe" hydraulic press. My rockets have been inconsistent because I have nothing to go on, in terms of pressure, other than "that feels about right." Like so many others. I've got ove of the exceptionally cheap H-frame presses that cost maybe $100. Even without a pressure gauge, this thing has seen a lot of use in my shop for things other than pyro. I use it for all varieties of mechanical fits, like press or shrink; I've used it to seat ball bearings. It has even been used to do some basic metal forming using dies. But like all cheap presses, it uses a hydraulic bottle that is normally nothing more than a bottle jack to lift a car so as to change the tire. There is an article on Passfire that describes hacking a bottle for a pressure gauge, but it is not a trivial matter. I could see no way to hack this system for a pressure gauge.

My options became one of these:

1. Replace the bottle jack with some sort of hydraulic pack that contains a pressure gauge.
   
2. Insert into the column some sort of "add-on" pressure gauge, like the Wolter unit, or perhaps a short ram tapped for a gauge.
   
3. Utilize a heavy-duty strain or pressure transducer.
   

The first is a bit expensive. Number two can be expensive or cheap, depending upon whether you wanted to buy the commercial unit, or if you wanted to modify a short-ram. Number three I initially dismissed as too complex AND too expensive, but like so many other things, I turned to eBay, and found a nice compression strain gauge for cheap. The only problem with it was that it is an unamplified transducer.

Let me discuss the theory for just a moment. Most strain gauges uses a variety of wheatstone bridge; 2 to 4 resistors arranged in a way so that when weight is applied to the transducer, the resistance changes, and a minute signal is generated. This particular transducer has a half-bridge (2 resistors internally) and can handle up to 7,500 lb. At max, it creates a measly 330 mV signal... too small to be of any practical use, as-is. Being Mr. Electronics for now, I decided to create a full system with an amplifier for the transducer signal, using a differential instrument amplifier chip, the AD-626.

I elected to create a 12 VDC circuit, starting with a cut-apart wall-mounted DC transformer, and a TO-220 12V regulator. With the power supply ready, I focused on the main circuit. The way a differential amplifier works is fairly straightforward. Two voltages are applied to two of the pins, the + and the - signals. The difference between the two is amplified, and the voltage output can be displayed on any simple digital or analog voltmeter. Another pin on the AD-626, by varying the resistance to ground, sets the gain, or amplification level... anything from 10 to 100. I used a 5K cermet trimmer pot for this operation.

The output from the transducer is attached to the "+" input pin on the AD-626. The voltage on the "-" pin must be exactly the same for zero volts to be output. To create this matching voltage, I used a 10-turn, 10K potentiometer, and a 10K precision resistor to ground, with a line between the two going to the AD-626 "-" pin. The potentiometer does two things. It allows me to set dead zero for the unloaded cell, and it also allows me to adjust the system as the resistors heat, which they inevitably do, due to ambient temps or heating of the circuit.

The entire system (except the load cell) is mounted in a simple plastic box.

The voltmeter and the 10-turn pot are mounted on the lid, and the remainder is installed in the box itself. There was not a lot of room. Note the transformer in the lower left... once I had the wall supply cut apart, it was very easy to salvage the power supply from it. The supply outputs 19V, 300 mA. The 7812 regulator steps this down to 12.00V for the circuit. Thats a fat overhead for a regulator, but the current this system creates is minute, on the order of 0.0006 amps for the zero pot, 0.0024 amps for the transducer, and a few milliamps for the amplifier itself, since the voltmeter impedance is nearly infinite.

I elected to go analog with the voltmeter rather than digital, because I find analog to be easier to use in systems like this. The knob for the zeroing potentiometer is also visible, as is the transducer jack to the left.

To use the system, the transducer is mounted in the hydraulic press. I eventually plan on machining a "nest" for it so that it will be as unobtrusive as possible, and will act as a base for normal operations.

For testing, I simply stacked some aluminum and steel plates on it. With no pressure at all applied, I powered the system up, and used the potentiometer to zero the meter. Because it is set up to amplify 32X, it is extremely sensitive, and when I am turning the pot, I am changing the signal by just a few mV. Minute changes using the pot are amplified, and displayed on the voltmeter. Once I find zero, another 1/4 turn of the pot would take the voltmeter to full-range! Don't even BOTHER to make such a device without a precision, 10-turn potentiometer.

With the needle at zero, I stroked the press handle once or twice, and put a good load on the strain transducer. I was rewarded by seeing the needle smoothly move in concert with the pressure applied.

When the pressure is removed, the needle, of course, returns to zero.

I set up the gain on the system so that 10V displayed on the meter will be at about 90% of the rated load limit of the transducer, and will warn me that I am approaching the maxiumum it can handle without damage.

In actual use, there are two basic ways I can use it. The first and most likely way is to keep good notes so that I can replicate a process at a later date. For example, if I pressed a 4 ounce rocket to 3V and it CATO's consistently, but one pressed at 5V works perfectly, then future rockets will all be pressed to 5V. The other way would be to calculate the actual pressure in PSI, and create a graph or two, plotting voltage vs. PSI for a given diameter ram.

Example: The cell can handle 7,500 lb, and the ram is 1 inch in diameter. I have a surface area of the ram of Pi R-squared, or 0.785 square inches. If 10V on the meter is 7,500 pounds, then the pressure under the ram is 9554 PSI. A smaller diameter ram will have a correspondingly larger PSI.

I think this thing is going to work. It is rugged stainless steel, and sealed for washdown duty. There is no messy hydraulic oil. If I were going to do this again, I would do two things. First, I would find a cell with a bit larger capacity. 7,500lb is a bit on the whimpy side. Secondly, if you don't feel like making an amplifier, find a load cell with a built-in amplifier. It'll be a little more expensive, but possibly worth it if you are not into electronics. Transducers with amplifiers built-in typically have output ranges from 0 to 5 or 0 to 12VDC. This can be fed directly into a voltmeter. You'd need three pieces: the transducer, a power supply, and a voltmeter.

Coming up next is a test of the lead dioxide-plated anode with a full suite of data-collection hardware and software tracking the whole thing!