The Ultimate Dogfooding Story

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91

@Philip:

So what happens when you drop 240 to 110? The amps more than doubles.

Incorrect. The relevant relationship is Ohm’s law, E=IR. E is volts, I is current, R is the electrical resistance that the current is passing through; in this context, human tissue. You can see that current is directly proportional to voltage. Current doubles when voltage is doubled, for the same resistance.

R is an important factor in whether any given voltage is lethal. It depends on contact area, contact pressure, presence of liquids, etc. A 12 volt car battery will kill instantly if applied to open wounds on opposite sides of the body – blood is an excellent conductor with a low R. Such a situation will easily drive enough current through the heart to stop it.

Another way to look at it: if your statement was correct (doubling voltage halves current), then merely holding a 1.5 volt AAA battery would kill.

The water analogy is off, too. Current is analogous to flow rate (volume per unit time) while voltage is analogous to pressure.

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@EE

No - I was right:
http://www.ebtx.com/mech/ampvolt.htm

Watts does indeed equal volts times amps.

Ohm’s law is E=IR, but as you can see - that is inrelation to resistance. I was talking about watts - which is a unit of power. You can cross subtitute, but there isn’t much point in this situation…

In other words, if you had the same amount of total watts, and reduced the volts to 1.5 Volts, and were somehow able to put it into something the size of a AAA battery (i.e. a very special type of slow-release capacitor/circuit), then it WOULD kill.

R is imporant - but people don’t vary in resistance that much. Applying 12 volts across the body in relatively low amps DC is unlikely to kill, even if through open wounds. Burn a bit - sure. Kill? No. That’s why MythBusters are always checking the milliamps across the heat rather than volts.

The misconception about volts killing is best shown with a vandergraph machine or tesla coil. The volts flow through the person but it doesn’t kill them.

Oh - and the water anology isn’t off either - but we will have to agree to disagree. I can’t agree with you because I have to agree with my physics text books which use that analogy.

Cheers

94

To the person who suggested a second sensor in front of the blade–the sensor is a CONTACT sensor.

The system is basically one of those touch switches you see on some lights. When it is triggered a spring-loaded block of metal hits the blade. The block is destroyed in the process and it’s trigger was a single-use system anyway–that assembly must be replaced. Furthermore, the blade absorbs enough energy in the process that it’s likely destroyed.

95

I can relate to this story. My brother-in-law did the finger test on my table saw a few years back, without a Sawstop… Not pretty!
I think that any developer should be willing to test what they produce. We have been dogfooding our system for the past 3-years as a test. Now we are ready to hire the coders. But only those who are ready to really put their hands into it!

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My Dad developed a punch press light curtain safety device. At a demo, a prospect asked him if it would really work. He stuck his hand into the press. Press stopped. Sale closed.

97

A Sawstop ad from Jeff? I’m shocked.

Hasn’t this stupid thing been beaten to death 30 million times over on the woodworking forums of the world already? Seriously.

Now video of someone throwing a can of Alpo on Bill Gates, that would have been interesting.

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According to this video employees at Miguel Caballero bulletproof clothing company eat their own dogfood as well. It looks like it applies to visiting reporters as well.

http://www.vice.com/behind-the-seams/bogota-s-bulletproof-tailor

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The inventor went to the major saw manufacturers to sell his
product. They all refused to buy it.
IIRC he also wanted a rather impressive royalty and had an ‘interesting’ attitude to the IP of any future inventions the saw makers came up with on their own.
I worked in a consultancy doing similar safety systems for machine tools and this came up a lot.

ps. the idea of installing a large spike in the car’s steering wheel instead of fitting seatbelts is favored by us cyclists.

100

Phillip
Duhhh… It’s like that Seinfeld lead-in. We have activities that are dangerous that we are likely to get our heads crushed in. But instead of stopping those activities, we wear helmets. But some people are so dumb they won’t stop the dangerous activities AND won’t wear helmets, so we force them to wear helmets.

What I do with my head ought to be up to me (provided I do not harm or restrict the freedom of others) and government coercion removes my freedom. Of course I wore a helmet when I cycled through the streets of London, I’m not stupid but I believe in the freedom of others to be as stupid as they choose.

As for dogfooding, it’s a nice idea but not always practical and as someone above says, it’s no substitute for a rigourous and multi-layered approach to testing; unit, integration, system and user acceptance.

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@Philip: Current is the killer, and a larger potential will allow a greater current to flow through a given resistance. Assuming that the body itself (and the nature of the connection – pressure, humidity, salinity, and so on) and its resistance is a constant in both scenarios, 240 volts will result in twice as much current flowing through the body as 120 volts will. Twice the current AND twice the voltage means FOUR TIMES the power (those Watts of which you speak). Don’t be confused by the fact that a 240-volt 60-watt light bulb requires half the current as a 120-volt 60-watt light bulb. If the 120-volt light bulb is installed in a 240-volt circuit, it automagically becomes a 240-watt light bulb with a very high colour temperature and a very short life span.

So why don’t those wacky science museum displays kill people? Tesla coils certainly will if the input power is not regulated to a low level (they are used extensively in lightning simulations that regularly burn through steel girders and shatter composite airplane wings). Van de Graaff generators depend on friction generators and do not normally accumulate enough electrons to provide any sort of sustained current. In any case, the unlucky volunteer from the school tour is ALWAYS placed on a platform that is not a good return path to the machine or ground, and when the potential is very high a wand with a relatively high-resistance path to ground is used to return the victim to a safe potential before stepping off the platform. (For additional safety, the platform itself is often grounded through a multi-megohm-range resistor.)

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@Stan Rogers and any others.

NO - watts stay roughly the same from country to country in spite of changing voltage. That’s the point. Double the voltage, halve the amps which reduced loss through resistance in the wires.

With all due respect, please do some google searches before posting. Just a little further resarch will show that these common assumptions are not true. Heck - watching mythbusters will be enough!

A 60 Watt light bulb is 60 watts no matter which country you plug it in! Seriously - have you ever seed a 60 watts in USA, 120 in China printed anywhere??? A light bulb is one of those unique devices that can be plugged into any light socket anywhere in the world.

So - if a light bulb stays the same watts, we can work out the current that MUST be passing over it. Let’s assume you have a 60 watt lightbulb.

Amps = Watts/volts
So for Australia, NZ, China and… well… the majority of countries:
Amps = 60 / 240 = 0.25
OR
For America and a bunch of other countries:
Amps = 60 / 110 = 0.55 (approx)

Wow… fancy that - the amps is over double for the same light bulb in two different countries. Same brightness, same amount of energy, but different voltages and amps.

As for vandergraph/tesla coil - yes they can kill - but they sure as heck can build up large voltages that don’t kill. If you don’t believe me, check out the mythbuster’s episode when they make the giant vandergraph. From memory they went off the meter. While makeing it they get almost unbelieavable readings and some very painfull zaps - but nobody dies. Zaps were huge. But nobody died. At my school we zapped the sink (metal pipes, couldn’t get a better ground if you tried). The zap went for what felt like 10 cm (probably closer to 5) and caused an involentary jump. It was probably dangerous - but we lived.

Still don’t believe me? Look up electric fences. THEY HURT. But they don’t kill. Volts are 2000 to 10000 as standard. Not to labour the Mythbuster’s point, but there was an episode where they peed on an an electric fence (talk about a salt bridge!!!). Again - nobody died. They even touched it and got zapped. Nobody died.

Ever rubbed your shoes on the ground and shocked someone? Keep this in mind - 10,000 Volts for electricity to travel 1 cm (0.4 in).

Static electricity can get to tens of thousands of volts and yet when I zap someone. In school we zapped eachother lots of times and were are still alive!

Still don’t believe me? Not interested - not coming back to read any more comments.

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This type of extreme dogfooding isn’t always a good thing. Soviet brass-hats occasionally made the designers of military equipment test it in person to… actually it’s a Russian thing, it’d take some explaining. Anyway, given the Soviet get-it-finished-yesterday attitude and the general Russian attitude towards safety procedures (real men pipette fuming nitric acid with their mouths), it was often fatal dogfood, one extreme example being a Soviet general (Mitrofan Nedelin) who insisted that he and his staff sit outside the safety bunker when a particularly large rocket was tested.

Unsuccessfully.

Spectacularly unsuccessfully.

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@Philip:

Gosh, how can I thank you for sharing your enlightenment. Wow, you’ve got sitting on your couch watching Mythbusters, and Google searches! I never thought of that! All I’ve got are my years of graduate work. And 29 years of work experience. As an electrical engineer (EE). Which I now recognize as worthless, thanks to you. I’m only sorry you won’t get my thanks, seeing as how you said you won’t be back to check – busy spreading your enlightenment elsewhere, no doubt.

(To everyone else: if you value your life, ignore everything Philip said, above. Whether it’s trollish baiting, or profound confusion beyond repair, I don’t know, but it’s too dangerous to leave unremarked. Always consult a certified electrician or degreed electrical engineer, or a P.E. (Professional Engineer), if you have ANY questions regarding any specific real-world electrical situation. Do not listen to TV show experts, whether they are on the show, or on the couch. If you value your life. If you want to know more on your own, correct understanding of the issues mutilated by Philip can be discovered through diligent study of college-level EE textbooks, along with supporting lab work.)

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Oh, and if our brilliant Philip comes back with another confusion-riddled retort (breaking his word that he won’t return), ignor e that too. If you value your life.

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@Philip:

I think you’ve got the wrong end of the stick.
EE and Stan are right.

A 60W light bulb designed for use at 110v has to pass I=P/V = 60/110 amps and must therefore have a resistance R=V/I = 110 / (60/110) which is about 202 ohms.

Similarly, a 60W bulb designed for 220v needs to have a resistance of 807 ohms.

The bulbs are not identical.

If you plug in your north American light bulb in Europe it will draw I=V/R = 220/202 amps and therefore dissipate P=VI = 220220/202 = 240 watts (but not for long!).

(ignoring the variation of resistance with temperature)

Also, I once worked on a radar system that calculated the time taken to shoot down an approaching aircraft.

That might have been fun to dogfood when the 747s from the airport near my home were disturbing the peace on a sunny Sunday morning, but alas I didn’t have all the necessary hardware…

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It’s volts wot jolts, but mills wot kills.

110

I’m an ex-eletrician and I agree with Philip and DavidR. Philip’s off with some stuff, but he is right with most of it. Electric fences do run at about 5000V. Volts can kill, but it is amps that is the real killer. Static electricity can run at hundreds of thousands of volts but not kill. As DavidR points out he was off with the light bulb example, but the rest was all good. If EE is an eletrical engineer, why doesn’t he know that. E=IR is about potential difference and is used to calculate volts given known amps and known resistnace - silly example. Doesn’t make sence. But this doesn’t have anything to do with the dogfooding. How about we stay on topic?

111

@Bobby – yes, electrical fences DO carry potentials in the thousands of volts, but they are incapable (barring power-supply malfunction) of supplying sufficient power over a long enough time to cause any real damage. You can test the effect with a 9-volt battery (one of the little guys we used to use in pocket transistor radios), a switch and a pair of audio output transformers (1 Kohm/8 ohm). Wire the battery through the switch to the 8-ohm winding of one transformer and the 1000-ohm side of that transformer to the 8-ohm side of the second transformer. Attach the 1000-ohm side of the second transformer to yourself (say one wire on each hand). Then flip the switch. Feel the owie? That’s a very transient 15KV you just felt. Still, the power delivered to you cannot exceed what that little battery can provide.

That’s exactly the same thing that happens with static electricity generators. For something like a van de Graaff generator, the limiting factor is generally the size of the accumulator (the dome at the top). For a given voltage, a larger accumulator can hold and supply more electrons, giving a longer and more potentially dangerous jolt. (Tesla coils, on the other hand, can have a tremendous sustained input power – 550 volts at 1000 amps is not out of the question for a lightning simulator.)

You know those defibrillator thingies? Those neat medical devices that can both stop and start a heart? They run at a single voltage (usually 300 – they’re based on the same circuitry as professional photo flashes), and can therefore supply only a single current through the same body, yet they are adjustable for strength. What changes is the duration of the jolt (expressed in Joules or watt-seconds). SOP is to start with a very benign level (200 Joules), one that will have the desired effect on the nerves and cardiac muscles but will not generate enough total heat to cause lasting damage, then (if unsuccessful) increase the length of the jolt bit by bit until a level is reached where any further increase would do more harm than good. (It is interesting to note that the paddles used when the chest is open run at a substantially reduced voltage, beginning in the 30 Joule range.)

Yes, current kills, but that current has to be sustained long enough to cause damage. No, I’m not an electrician. Just a technologist who’s worked on, designed, prototyped and been zapped by enough power supplies to know the difference.

112

Yeah I get that. That was my point, you don’t die with high volts alone. When you hold an electric fence the burst is short. And the fence holds 5000V but not much current goes through the person. Only a small portion of the fence current. Resistance is much higher through the person and ground than the fence. Most of the current keeps going through the fence. But its still 5000V. Was just agreeing that volts could be high and not kill. Yeah, Volts can kill, but current is more likely to kill.

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Another good example of dogfooding, albeit involuntary, involves military parachute riggers. From time to time they are required to make a jump using one of the parachutes they had packed. Typically, this 'chute would be randomly picked from the stack by one of the jumpmasters. This kept the quality control to a high level.