Re: [foxboro] flowrate

Dirk,

It seems you want to calculate a flow rate from the rate of change of the
bin weight. I did something similar in order to calculate a rate of change
for a pressure measurement, by using a dead-time (DTIME) block and a calc
block. The dead-time block is on a 1 second period and delays the reading
exactly six seconds (0.1 minutes). The calc block subtracts the current
value from the delayed value and divides by the dead time to get the rate
(or actually multiplies by ten). This arrangement gives readings updated
every second, but they are a bit noisy, due to the additive effect of the
error of the actual measurement. If you use a longer dead time you can
reduce the effect of measurement error on the rate calculated, but it will
be at the expense of frequent updates. You just have to find the best
compromise for your application. You can do better than calculating the
rate once per minute.

The dead-time block uses up to ten discrete "buckets" to hold the old
value. I used six buckets. By default, the algorithm averages the input
values and interpolates the delayed values to give a more continuous
measurement with the limited number of buckets. There's an option (DTOPT)
to disable averaging and interpolation so that only actual values are
stored and indicated. I chose to use actual values.

Once the rate of change is calculated, it can be connected to an AIN block
for filtering and alarming.

Jerry Hidahl
Huntsman Performance Products

 
 
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