justme1968: some ideas about accuracy...
i have just gotten my sr04 and played a lithe with it. i intend to use it to measure oil level in a tank. so accuracy is quite important. after reading on the web there are two main points that seem to influence the accuracy of the sensor and both are not accounted for in the newping library:
one is temperature. it would be nice to be able to set the temperature that newping uses for the conversion to cm/inces. depending on the temperaure the error can easily be in the range of 10% if temperature is not accounted for.
the other one is the which echo is received and evaluated. if i do repeated measurements i notice that the results will be relatively near around two or tree that are themselves further apart. this looks like it is consistent with some articles that say this error comes from reading the second or third echo and not the first and should be especialy noticeable in the short range.
so to be able to make more accurate measurements i started reading 10 or 50 pings, iterating over all of them and throwing away all that are equal to the largest and all that are equal to the smallest and then averaging the rest. if there are non available after throwing the smallest and largest away i take the smallest. the conversion to cm is then done in float.
this seems to give very accurate and reproduceable results. doing this repeatedly in a range of 1cm to 1m gives identical results up to at least 1mm.
maybe a 'slow scan' option could add this to the newping library for applications where fast response times are not needed.
1) Because you need an outside thermometer to measure temperature, it was decided not to overly complicate the NewPing library. However, NewPing IS designed to adjust with temperature. Simply use the ping() method which gives you a time. Then, do your own calculation of distance based on your thermometer attached to the Arduino. In almost all situations, the default distance works well for average indoor temperatures. But, if you need accuracy, you're going to need additional hardware and then (of course) you're going to need to do your own calculation. But, NewPing will work with this situation just fine.
2) This isn't a problem with the library, nor can it be fixed in a library. This is fixed by following the instructions in my sample sketches. Basically, if you ping a second time too quickly after a previous ping, you can get an echo. This can't be fixed by the library as it's only reading what the sensor outputs, which is a digital signal (the library has no amplitude information). The solution is to simply heed my warnings in my sketches. Which is to not ping too quickly. Your pings shouldn't be more frequently than around once every 29ms. If you're in a highly sonicly reflective environment, or your sensor is overly sensitive, you may need to increase this amount. If it's not something fast moving (probably like in your case) ping once every second or two even. That way, you can be assured that you're not reading an echo. NewPing doesn't control the speed of the pings YOU do. If you want to make it slow, just make it slow. Maybe I'm missing something?