Reading pulse and then getting rpm from encoder motor

Other Hardware Sensors
1

i have a hall effect encoder motor

ppm=827.1
i am using the following code for getting no of pulse per sec then converting it to rpm

volatile  float  count=0.00;
 float rpm=0.00;


void setup() {
  // put your setup code here, to run once:
pinMode(2,INPUT);
Serial.begin(9600);
}

void loop() {
  
  //interrupts();
  attachInterrupt(0,countf,RISING);
  delay(1000);
  Serial.println(count);
  detachInterrupt(0);
  
   rpm=count*(60.000/823.10000); //converting pulse per sec to rpm
  Serial.print("rpm=");
  Serial.println(rpm);
    Serial.println(count);
  count=0;
}
void countf()
{
  count=count+1;
}

but i have getting a weird output
its rpm value is not stable it is getting decreasin
please see attached file for output
please help me here

Screenshot (62).png
Screenshot (62).png1366×768 12.2 KB

2

try change the interrupt routine to 'take notes' of timing.
define the two timingvariables "volatile long"
Use the time_difference to calculate rpm.
Remove delay.. do useful stuff instead
Move attachinterrupt to setup()

void countf()
{
   last_time=now_time;
   now_time=micros();
}