Low oil pressure alarm after start up engine...Which statement to use??

Hi to all! I am trying to make a car engine oil pressure gauge. I read analog pressure and I have installed a buzzer to make an alarm. My problem is that I want the alarm to ring if pressure goes lower than 0.5 bar of pressure and it should not ring before and starts. So I have to make a loop that if it is lower than 0.5bar ring but only if pressure have been for on time over 0.5 bar. Any idea ?
my code for now

void loop() { 
// read the value at analog input
 value = analogRead(1);
   vout = (value * 5.0) / 1024.0;
   if (vout<0.5) {
   digitalWrite(4, HIGH);  // turn the buzzer on
   }  
else {   
  digitalWrite(4, LOW); 
}

Many thanks to all!!!

you need to memorize the fact that you have been above 0.5 at least for one reading.

void loop() { 
  static bool IWasAboveZeroPointFiveAtLeastOnce = false;

  // read the value at analog input
  value = analogRead(A1);
  vout = (value * 5.0) / 1024.0;
  if (vout < 0.5) {
    if (IWasAboveZeroPointFiveAtLeastOnce) digitalWrite(4, HIGH);  // turn the buzzer on
  } else {
     IWasAboveZeroPointFiveAtLeastOnce = true;
    digitalWrite(4, LOW); 
  }
}

Worked! Many thanks my friend!

Great