Methods for measuring flow by means of an ultra sonic flow meter
In a method for measuring flow by means of an ultra sonic flow meter, an ultra sonic signal is transmitted in an upstream and a downstream direction. This is measured upon receipt of an ultra sonic signal and the measurement stopped at a stopping point. A first series of transmissions is made, and the starting time of each transmission is incremented or decremented until a time difference between the upstream and downstream signal is inside a reference band. Flow is then calculated in accordance with the time measurements.
1. A method for measuring flow by means of an ultra sonic flow meter, the method comprising the steps of transmitting an ultra sonic signal in an upstream and a downstream direction, initiating a time measurement at a starting time upon receipt of the ultra sonic signal, and stopping the time measurement at a stopping time
making a first series of transmissions where the starting time in each transmission is incremented or decremented until a time difference (Δt) between the upstream and downstream signal is inside a reference band (Δt span ) and calculating the flow based on the time measurements.
2. A method according to claim 1 , including the further step of generating a second series of transmissions following the first transmissions, where the starting time in each transmission is incremented or decremented until the time period (DS_RUNUP) between the starting time (DS_START) and the stopping time (DS_STOP) is approximately equal to a multiple of half the time period of the ultra sonic signal, but preferably equal to the time period.
3. A method according to claim 2 , including the further step of generating a third series of transmissions following the first and second series of transmissions, incrementing or decrementing the starting time in the third series of transmissions until a difference between a calculated average transmission time and a transmission time reference value determined on the basis of the media temperature is inside a reference band.
4. A method according to claim 3 , wherein the incrementation or decrementation during the third series of transmissions is done in steps of a timely resolution (t sig ) that is approximately equal to the period of the received signal.
5. A method according to claim 1 , wherein the incrementation or decrementation is done in steps of a timely resolution (t res ) defined by the resolution of the microcontroller.
6. A method according to claim 1 , wherein following the starting time, the time measurement is stopped at a first positive zero crossing of the received ultra sonic signal or at a first negative zero crossing.
7. A method according to claim 1 , wherein following the starting time, the time measurement is stopped at a first positive zero crossing following a negative zero crossing of the received ultra sonic signal, or vice versa.
8. A method according to claim 1 , wherein the starting time in the first transmission in the first series of transmissions is a fixed value (DS_START_INI).
9. A method according to claim 2 , wherein the reference band (Δt span ) is delimited by a maximum difference time (Δt max ) and a minimum difference time (Δt min ), the width of the band being smaller than the time period (t sig ) of the ultra sonic receive signal.
10. A method according to claim 9 , wherein a safety margin is added to the reference band, the safety margin corresponding to at least the timely resolution (t res ) of the incrementing or decrementing steps, and preferably the sum of a resolution and the duration of a liming noise (t n ).