# Arduino sampling speed

I'm trying to calculate the speed of a shock wave through steel due to an impact. I was wondering if I used an Arduino uno with piezo sensors (SEN-10293) spaced about a foot apart would a simple time stamp of when the shock first arrived at each sensor be accurate enough to calculate speed? I've used time stamps in the past but never needed this much accuracy. I'm worried that even at a foot apart the two sensors time stamps may have inaccurate or even the same value.

Thanks for the help

Hi,

What do the sensors give (ON-OFF; analog value, . . .?)

Speed of sound in common solids/liquids

Sound in steel = 20000 feet/second
so a soundwave takes 50 microseconds to travel one foot.
=> use a sensor that gives an on/off signal and use interrupt.

As the (UNO) arduino has a 4 micros accuracy it will measure 48 or 52 micros assuming

• exact distances and
• exact timing and
• right sort of steel

Improve timing by using a hardware timer , start timer in IRQ 1 and stop/read it in IRQ 2.

andreto:
I'm trying to calculate the speed of a shock wave through steel due to an impact. I was wondering if I used an Arduino uno with piezo sensors (SEN-10293) spaced about a foot apart would a simple time stamp of when the shock first arrived at each sensor be accurate enough to calculate speed? I've used time stamps in the past but never needed this much accuracy. I'm worried that even at a foot apart the two sensors time stamps may have inaccurate or even the same value.

The speed of sound in steel is about 6km/sec. Two sensors placed 30cm apart would see a difference of about 1/20000 seconds.

In the Arduino world that's about 800 clock cycles. That's plenty to get a measurement.

robtillaart:
Sound in steel = 20000 feet/second
so a soundwave takes 50 microseconds to travel one foot.

How much is that in furlongs?

feel ashame. What is a furlong?

vffgaston:
feel ashame. What is a furlong?

A unit of length. There are ten chains in a furlong.

22 yards to the chain.

And eight furlongs to the mile.
There is a signpost arround here in Kerby Stephans that still has the distance in miles and furlongs to other places.

vffgaston:
feel ashame. What is a furlong?

A medieval unit of measure, like feet, inches, pints, etc.

fungus:
A medieval unit of measure, like feet, inches, pints, etc.

We learned it in school, I am not that old!

All those units are still used in the UK. I had a pint last night.

Some people already gave the assumed text book values so that makes it easier to design. You know what to expect.

Basically just make sure the arduino time base (micro’s) is a large enough resolution to measure the time difference between a start signal and a end signal via interrupt. 50 micro’s is an “ok” resolution ~4% error at most. On both triggers you might miss as much as 999nS of measurement ~= 1uS, for a total of ~2uS, 2/50x100 = 4% error

If your sensors are any distance away from the impact, be sure to account for that time lag.

confused.

first, if you already know that the speed of sound through the steel is 30 furlongs/second (or any contrived unit of measure)

and you know the thickness.

then, you can calculate the time without need of the measurement ?

in reality, you could use the steel as a way to calibrate the readings, no ?

Grumpy_Mike:

fungus:
A medieval unit of measure, like feet, inches, pints, etc.

We learned it in school, I am not that old!

All those units are still used in the UK. I had a pint last night.

Neither of those statements means they aren't medieval...

according to wikipedia the Metre is from ~1668 , and adapted several times.

The furlong isn''t medieval, it is way older ...
The furlong was historically viewed as being equivalent to the Roman stade (stadium), which in turn derived from the Greek system.
(also from wikipedia)

Steel is a alloy, no 2 batches will be exactly the same, plus steel is highly customizable for different applications.

The steel on your barbecue will be vastly different then the steel holding up a skyscraper. That textbook value is a general average, I admit I can't see sound passing through it varying that much from type to type but it will to some degree.

feel surprised for such a mess around furlongs (it deserves it's own thread!!).

First of all, I have to state that, for me, an inch or a feet is much more "understandable" than cm or m (In fact, I have arguments with my spanish colleagues when I talk of piping diameters in inches), but, does it make any sense measuring in furlongs when talking about speed propagation of sismic waves in steel?.

(I'm not english native. Question is fully naïf). :~

harddrive123:
I admit I can't see sound passing through it varying that much from type to type but it will to some degree.

The table I posted showed 5% difference between "steel" and "stainless steel", 19% difference between "steel" and "iron".

Hard numbers beat "seeing", eh?

The steel on your barbecue will be vastly different then the steel holding up a skyscraper.

The speed of sound in it isn't, though.

I beg to differ. Sound will travel faster through the scalding hot barbecue steel faster than then through the well air conditioned skyscraper steel. ]