PC² Chronograph

For Rev. Bx Software
Copyright 1993, P.A.C.T., Inc.

Note: This product is no longer manufactured or supported by Practical Applied Computer Technologies, Inc. This knowledge base article is provided as a courtesy to existing customers.

Introduction

We've been building chronographs since 1983. Our design philosophy is to build for the customer, not the engineering department. The PC² (pronounced PC squared) incorporates every practicable (and affordable) neat idea that's been suggested to us over the last ten years. Features like the printer and serial port are optional on this machine to let you buy only the features you want.

The design of your PC² is left wide open for future software up-grades, so you can count on a long and interesting string of refinements and additions, always made available at a minimum cost.

System Overview

The PC² has a completely redesigned "Hot Key" user interface which allows the shooter to examine any aspect of velocity data on a "real time" basis without having to end the particular firing session.

Whenever you turn the PC² on, it will come up in chronograph Mode, waiting for you to fire, with all the defaults reset. If you get "lost" in the software just turn it off and back on again.

In normal operation the PC² will display the current average velocity of your string as you fire. This updates automatically each time with each shot. Because the PC² is primarily intended for use as a chronograph, the majority of the keys are dedicated to easy access of the advanced features of the PC². By pressing the appropriate Hot Key you can make the PC² display:

• EXTM SPRD - Extreme Spread
• STD DEV - Standard Deviation
• MAD - Mean Absolute Deviation
• HIGH LOW - Highest and Lowest Velocities

Chronograph Operation

The on/off switch is located on the back of the chronograph. Switch it to ON. The chronograph will display the model of your machine, the version number of the software installed, such as "Chronograph Revision B1" Our copyright notice is then displayed followed by FIRE WHEN READY. At this point you are ready to roll. Note that the chronograph always wakes up in chronograph mode.

A Few Points to Remember

Setup Tips

• When you set your skyscreens up, put them two feet apart, center to center. (The PACT bracket is already set to an exact 24" separation).
• Chronographs don't like sunlight reflecting off of shiny bullets. If you get odd velocities on sunny days, rig a sun screen.
• Chronographs also don't like florescent lights. For indoor use, rig an incandescent light over each screen.

Skyscreens

Your chronograph comes equipped with the finest Skyscreens on the market. The PACT MK5 Professional Skyscreen contains two lenses that serve to magnify the bullet and bring it into sharp focus, providing more accurate triggering at a given range than un-lensed skyscreens. The diffuser screens provide serve as both an aiming guide and light diffuser.

If you have our optional skyscreen mounting bracket just screw the skyscreens to it as shown in the illustration. The skyscreens should fit snugly against the cross pieces to insure proper screen separation. The center of the bracket is threaded for a standard camera tripod (1/4-20).

If you are fabricating your own mounting bracket take care to insure that the screens are precisely 24 inches apart, center to center. Whatever % error you make in screen separation will result in the same % error in velocity readings.

Connection and Operation

Plug the first screen into the START plug located on the back of your chronograph, and the second screen into the STOP plug. With the screen displaying FIRE WHEN READY fire a round, aiming in such a way that the bullet passes over the center of each screen at an altitude of between 4 and 6 inches above the screens.

Each time you fire, the shot number and velocity of your last shot will appear on the top line of the display and your current average velocity will appear on the bottom.

If an odd-ball velocity appears, like "6952.3 FPS", this is an obvious error. You can remove it from memory by pushing the "NO" key. The chronograph will "back up" to the previous shot.

Note: The PC² needs about 1.5 seconds between shots. This is to allow the smoke to clear. If you fire too soon, the PC² will not record the shot or get a very odd reading and another 1.5 seconds will be required before it is again ready.

Statistical Features

Your PC² actually calculates and updates all of your statistical data each time you fire. Most folks want the Average Velocity displayed so that is what we default to. However you can tell the PC² to display any of the statistics on the current string by pressing the appropriate key.

Examples of Statistical Data Display

Press the EXTM SPRD button and the PC² will display the difference between the highest and lowest recorded velocities on this string.

Press STD DEV. The bottom line will change to show the Standard Deviation of your string and its Coefficient of Variation (CV).

Press MAD. The PC² will show you the Mean Absolute Deviation (average variation) of your shots as a percentage of the average velocity. This number is the key to measuring velocity variations in your loads.

Press HIGH LOW. This indicates the range of velocity that the current string covers.

Review Mode

When you are done with your string, the PC² will provide you with a statistical summary of your string in REVIEW mode as well as allowing you to take a look at the individual shots again. If you have a printer just push the PRT key and you will receive a hard copy of your string number, statistical summary and each shot fired.

Push REVIEW and the PC² will default to the current string number. If you want to look at a previously fired string press the appropriate number and REVIEW again.

The PC² will show the highest and lowest shot velocities and their corresponding shot numbers. Push REVIEW again to see Standard Deviation and Coefficient of Variation. Push REVIEW again for Mean Absolute Deviation.

At this point you can continue to press the review key and review each shot. You can also jump to a specific shot by using the Find Shot feature. Simply enter the number of the shot you want and press REVIEW.

You can EDIT out individual bad shots with the NO button during review. The chronograph will recalculate your statistical summary less the edited shots.

When you are done reviewing your data, you can push "GO" and the PC² will file this string away in memory and display FIRE WHEN READY again.

Options

The REV B software has two features tied to the option button: Lead Calculation for moving targets and the calculation of Kinetic Energy (KE).

Lead Calculation

Press the OPTN button, the display reads: CALCULATE LEAD? (N). Press YES to proceed.

1. Enter the velocity of the round (e.g., 2500 and press SET)
2. Enter the distance to the target (e.g., 50 and press SET)
3. Enter a target speed (e.g., 25 fps and press SET)

The PC² calculates the lead at 18 inches.

NOTE: This is the lead in a vacuum. No allowance has been made for the deceleration of the bullet. The PC² will slightly over estimate the actual lead required. At close range this error is negligible. With long range and/or slow bullet combinations this error becomes significant.

Kinetic Energy Calculator

The second option is the KE Calculator. The PC² will ask you to give it the bullet weight you're using. When you fire, the PC² will display the KE on the bottom line of the display.

Notes On Screen Separation

The screen separation of your chronograph can be set to the 10th on an inch. The default is 24 inches. Unlike some other chronographs on the market which use very dated designs, there is no electronic limit on how close you place your screens.

First of all understand that, unlike some other chronographs on the market, there is no electronic limit on how close you place your screens. Secondly, because the bullet starts decelerating as soon as you drop it into our nice thick atmosphere, the farther apart your screens are the greater the instrumental error.

We recommend a screen separation of 24" for most applications. This is a pretty good compromise between accuracy and ease of transport and set up.

How It Works

Each skyscreen contains a photo-transistor which is constantly measuring the current light level. Because the bullet is darker than the sky, when a bullet goes over the skyscreen the light level drops a little bit. The amplifier inside your chronograph takes note of this drop in light level and amplifies it to the point that the drop is big enough for the computer to notice. The computer then times how long it took for the bullet to travel from the first screen to the second screen where the light level again drops. Accurately converting this time into a velocity is easy; provided the computer knows how far apart the screens are.

Troubleshooting Light Conditions

Your PACT chronograph is looking for a decrease in light level when the bullet passes over the screen. If the light level increases as the bullet crosses the skyscreen, you will probably get no reading at all.

How could the light level increase? Let's say that you are chronographing under a dark blue sky and that the sun is reflecting brightly off of the bullet. Now the bullet is actually brighter then the sky above it. When it crosses over the skyscreen the light level goes up instead of down, thus we get no reading.

If you find that your chronograph is having trouble getting readings on a sunny day with a dark blue sky you should try shooting lower over the skyscreens. You may also eliminate the problem by changing the light conditions or direction of fire.

Muzzle Blast

Most of our original customers were pistol shooters and we designed into our chronograph some special "anti blast" circuitry which really minimizes muzzle blast problems without sacrificing sensitivity.

When you fire a subsonic (below around 1200 FPS) round, the sound of the gun reaches the skyscreens before the bullet does. If it shakes the screens hard enough they will trigger and give you an incorrect reading. For example, let's say that you are firing a .45 ACP at 850 FPS and find that your chronograph is telling you that your round is going 680 FPS. What's happening is that the muzzle blast is triggering the start screen before the bullet gets there, but it lacks sufficient power to trigger the stop screen by the time its traveled that far. So we have a situation where the blast started the computer and bullet stopped it, hence the low reading.

If the blast is severe enough to trigger both screens, you will be measuring the speed of sound instead of your bullet velocity. In either case the solution is simple: BACK UP!

Troubleshooting Muzzle Blast Issues

To correct muzzle blast problems:

• Make sure that the cables are not stretched out (they should be touching the ground between the chronograph and the skyscreens)
• Don't run the cables directly under the muzzle, place them to the side
• Place the PC² behind something (shooting box, jacket, bench) so that the chronograph is not line of sight to the muzzle

Odds And Ends

How do you know if your chronograph is telling you the truth? If the reading you get is close to what it should be you can count on it being within .5%. That's point five per cent not five percent. Normally you will find that the reading are actually better than that. If the unit prints a bad velocity it should be off by enough to be obviously wrong. If you get a bad reading just hit the "NO" button.

Ballistic Calculator

An abbreviated version of the Ballistic Computer has been installed in your PC² at no extra charge. The Ballistic Calculator can help you find an "optimized" zero for your specific gun/load combination and serve as a ballistics teacher.

Turn your PC² on and press TRAJ. The Ballistic Computer needs to know the velocity to base the calculation on. If you're shooting at the range it will pick up the current average velocity from the chronograph.

Using the Ballistic Calculator

1. Pushing SET means that you want the machine to use the data on the screen.
2. If you want to change the default, first tell the machine what it needs to know. For example, if you want the machine to use 2650 FPS for its calculation just type 2650. If you make a mistake entering a number just use the back arrow key GO to erase it. When the number entered is OK with you hit SET and the Ballistic Calculator will accept it and move to the next screen.
3. Yes/No questions are answered with the YES and NO keys. No SET is required.
4. Any time you are messing around with the Ballistic Calculator and you realize you've painted yourself into a corner hit (gently now) the REVIEW button. This will take you out of the Ballistic Calculator. Then hit TRAJ again and it will let you start from the beginning with the last things you entered as the defaults.

The Ballistic Calculator is so easy to use that you really don't need much explanation. It will guide you through the process asking for velocity, ballistic coefficient, sight height, and zero preferences to generate a trajectory calculation.