Arduino interrupts interfering with TimeAlarms.h - arduino

Question

I have and arduino sketch that needs to do several operations on a timed schedule using the TimeAlarms.h library. However, one of the operations, reading a hall sensor via interrupts, seems to interact poorly with the TimeAlarms library.
I'm using the TimeAlarms library from here: http://www.pjrc.com/teensy/td_libs_TimeAlarms.html
And have adapted the hall sensor script from here:
http://www.seeedstudio.com/wiki/G3/4_Water_Flow_sensor
I can run the hall sensor code on it own fine. However, when I try to run the hall sensor code along with Alarm.timerRepeat it hangs after entering the check_flow function.
Running the code below outputs only enter CF and then hangs. The same occurs if you try the check_flow_alarm_delay function instead, which uses the TimeAlarm version of Delay.
However, if you comment out Alarm.timerRepeat(10, showseconds); in setup
and Alarm.delay(0); in loop the hall sensor works fine.
Strangely, if you comment out sei(); and cli(); in the check_flow function the script works fine, and seems to count properly with the hall sensor. Why would this work? And should I be concerned that I'm not actively setting the time between sei() and cli(), leading to reliability issues in the sensor?
Note: you should be able to run the code without actually having a hall sensor, the output will just be 0 L/hr.
// reading liquid flow rate using Seeeduino and Water Flow Sensor from Seeedstudio.com
// Code adapted by Charles Gantt from PC Fan RPM code written by Crenn #thebestcasescenario.com
// http:/themakersworkbench.com http://thebestcasescenario.com http://seeedstudio.com
#include <Time.h>
#include <TimeAlarms.h>
#include <Wire.h>
volatile int NbTopsFan; //measuring the rising edges of the signal
int Calc;
int hallsensor = 2; //The pin location of the sensor
void rpm () //This is the function that the interupt calls
{
NbTopsFan++; //This function measures the rising and falling edge of the hall effect sensors signal
}
void setup() //
{
Serial.begin(9600); //This is the setup function where the serial port is initialised,
pinMode(hallsensor, INPUT); //initializes digital pin 2 as an input
attachInterrupt(0, rpm, RISING); //and the interrupt is attached
Alarm.timerRepeat(10, showseconds);
}
void loop ()
{
// Serial.println( second() );
// stalls at enter CF
// check_flow();
// stalls at enter CF
check_flow_alarm_delay();
Alarm.delay(0);
}
void showseconds ()
{
Serial.println( second() );
}
void check_flow ()
{
Serial.println("enter CF");
int Calc;
NbTopsFan = 0; //Set NbTops to 0 ready for calculations
// sei(); //Enables interrupts
delay(1000); //Wait 1 second
// cli(); //Disable interrupts
Calc = (NbTopsFan * 60 / 5.5); //(Pulse frequency x 60) / 5.5Q, = flow rate in L/hour
Serial.print (Calc, DEC); //Prints the number calculated above
Serial.print (" L/hour\r\n"); //Prints "L/hour" and returns a new line
}
void check_flow_alarm_delay ()
{
Serial.println("enter CFAD");
int Calc;
NbTopsFan = 0; //Set NbTops to 0 ready for calculations
// sei(); //Enables interrupts
Alarm.delay(1000); //Wait 1 second
// cli(); //Disable interrupts
Calc = (NbTopsFan * 60 / 5.5); //(Pulse frequency x 60) / 5.5Q, = flow rate in L/hour
Serial.print (Calc, DEC); //Prints the number calculated above
Serial.print (" L/hour\r\n"); //Prints "L/hour" and returns a new line
}

Answer 1

delay() uses interrupts. Disabling them interferes with the function.