Re: Chronograph Help... What to get
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: turbo54</div><div class="ubbcode-body">The only concrete thing you can go on is how far apart the sensors are from one another. I believe the CED M2 has them the farthest apart at 2 feet.
None of the manufacturers publish accuracy data of their units. Thus, no one really has any concrete knowledge of how accurate they are. All use the same basic technology: Use an optical sensor to look for a shadow to start a timer, use an optical sensor to look for a shadow to stop a timer, based on a known distance between sensors, calculate rate by dividing seperation distance by time.
The further apart the sensors are, the less accurate the time term can be, yet still provide an accurate solution.
I have used a shooting chrony, a Pro-chronograph and a CED M2. I own the M2. All "work" about the same. I've run the pro-chronograph in series with the M2, and found that at about 2600fps, the M2 reads about 40fps faster. At about 2700fps, the M2 reads about 55fps faster. Which is more accurate? Who knows?
Shortly after I bought my M2, I was on the phone with my dad telling him about it. As an engineer, he asked me how accurate it was. I looked through the manual and discovered there wasn't an accuracy claim. I emailed CED about this, and asked what the claimed accuracy was for their unit. I received a reply basically saying "we won't tell you - and no other company will either....But trust us, its the most accurate unit out there! If you REALLY want to know, pay to have it sent out to a test lab and checked out" </div></div>
turbo54,
I have to ask, how much effort did you take to find how accurate the common consumer chronos are? I took me less than 2 minutes of looking to find this accuracy spec. on CED's website.
CED claims that at 2' their chrono's screens are accurate to within a range of 99.8% to 99.95%. I have to assume this is for a sample that is in perfect condition and under perfect atmospheric conditions. Oehler says about 1/10 of one percent with the second screen shot required to be within one percent or the reading is questionable. The Kurzzeit PVM-21 specs are shown on the website as less than one percent but the PVM-21 appears to be better than 1/10 of one percent in real world demos with multiple PVM-21 chronos checking the same shots.
The CED numbers would be an pretty good numbers if they were true but the only way to know is to buy two CED chronos and shoot through both at the same time which makes the Oehler 35P seem to be a reasonable compromise. The Oehler compares two timings but is standalone and the CED can be connected to a computer but is not integrated and the user must make the decision as to the validity of the two readings from two chronos.
Obviously the problem for all of the old style skyscreen based chronos are the skyscreens reliance on weak sensor technology in this application and the flimsy construction. The problem has never been the time base but the sensors used to detect the events. In sharp contrast the Kurzzeit PVM-21 works day or night or even in total darkness due to the sensor array being very rigidly located in an extremely strong and stiff open box allowing the system a reliable metric to calculate velocity on. These electrical and mechanical features make the PVM-21 very repeatable which is why IMO the PV-21 chrono even given its expense the best of the commercially available chronos.
Chrono numbers are of little value if they cannot be trusted to be accurate within a tight velocity spec. This is why the Oehler chronos were the absolute best for so long given the relatively primitive technology employed. The much improved electronics and sensor design of the Kurrzeit PVM-21 have made the issues of reflective skyscreen based chronos more obvious. It is just a matter of time before the improved design features used in the Kurrzeit PVM-21 are copied and sold for less. Until then I like the PVM-21.
HTH!
