rpm detection problem

Projects Programming
1

I have some problem with rpm detection project.
Program is referenced from this site:

http://playground.arduino.cc/Main/ReadingRPM

When i change the number from 30 to 60 in rpm calculation line.

rpm = 30*1000/(millis() - timeold)*half_revolutions;

to

rpm = 60*1000/(millis() - timeold)*half_revolutions;

Serial monitor gives me a strange answer.
For example,the real rpm is 12 but it displays thousand rpm.
What wrong make this happen?

2

What wrong make this happen?

What is 60 * 1000? Does that fit in an int?

What is 60 * 1000UL? What type register will the compiler use to hold the result?

3

I change the rpm calculation line from

rpm = 60*1000/(millis() - timeold)*half_revolutions;

to

rpm = 30*1000/(millis() - timeold)half_revolutions;
rpm = rpm2;

the strange answer disappears
I can't distinguish the different between them.

4

I am not understand the different way between them.

30 is an int.
1000 is an int.

30 * 1000 is an int, and will fit in an int.

60 is an int.
1000 is an int.
60 * 1000 is an int, but the value will NOT fit in an int.

60 * 1000UL will force the use of an unsigned long register to hold the result. So will 60 UL * 1000 or 60UL * 1000UL.

5 
void setup() {
  Serial.begin(115200);
  Serial.print("60*1000 = ");
  Serial.println(60 * 1000);
  Serial.print("30*1000 = ");
  Serial.println(30 * 1000);
}
void loop() {}

60*1000 = -5536
30*1000 = 30000