entered correctly. Note: At an installed site, the XPC command can be used to automatically
recalculate the off parameters.
For most installation sites, if the update interval is relatively short, there is not a requirement to do a
full purge every reading. See the following paragraph on configuring purge intervals to reduce
power consumption.
Configuring Purge Intervals to Reduce Power Consumption
The operating mode of all software versions prior to V2.2 was to perform a purge for every reading.
Beginning with V2.2, it is possible to configure the unit to only run the pump for a short period of
time for a specified number of readings between readings with a purge. The
aXPP+non_time! extended command provides this flexibility. With this command you
can specify that number of no_purge measurements that occur between measurements with purges.
The factory default is zero to match previous software versions. With this command the pump
on_time for these no_purge measurements can also be specified with 0.1 second resolution. This
on_time is dependent upon the maximum expected water level change between measurements as
well as the length and I.D. of the orifice line. A starting point for determining the on_time is the
following formula:
on_time = 0.1 + ( 0.2 + 0.001 * feet of orifice line) * feet of water level change
or
on_time = 0.1 + ( 0.7 + 0.01 * meters of orifice line) * meters of water level change
The above formula is based on 1/8” I.D (or 3 mm I.D) orifice line and a well charged battery. If the
I.D. is not 1/8” (or 3 mm) then the 0.001 (or 0.01) needs to be scaled by the ratio of the new I.D.
cross sectional area to the standard one.
To handle one foot of water level change between readings, this formula would reduce to:
on_time = 0.3 + 0.1 per 100 ft of orifice line.
For sites not capable of supplying at least 12V to the pump, the times may need to be increased or
accept that when the battery is low, it may take more than one reading to track a one foot increase in
water level. Having an on_time longer than necessary will not have any negative affects on reading
accuracy. Having an on_time shorter than necessary to track water level increases will result in the
readings lagging the water level change until the no_purge readings catch up or until a purge
readings occur.
Note: This is the same phenomena demonstrated by constant flow rate bubbler
systems. If the water level increases faster than a constant flow rate bubbler is set for, its readings
will lag the actual water level.
If there is a leak in site installation tubing, then more air could leak
out between readings than what is pumped during the on_time. If this were to occur, the readings
would be seen to gradually decline between purge cycles and then have a step change back up when
the purge occurred. Although this can be compensated for to some degree by increasing the
on_time, the proper solution especially for substantial leaks is to eliminate the leak.
Example:
Site has 150 feet of orifice line. SDI-12 data recorder is requesting measurements every 5 minutes.
We want to purge once an hour. Never expect to see anywhere near a one foot change in the 5
minutes between readings. Since there are 12 readings an hour and we want one of them to be a
purge, our no_purge number will be 11. For our on_time we will round the orifice line length to
200 feet and calculate an on_time of 0.5 seconds. The command for an unit at SDI-12 address zero
will be:
0XPP+11+0.5!
Bringing the Benefits of Real-Time Data Collection to the World
Sutron Corporation, 21300 Ridgetop Circle, Sterling, Virginia 20166-6520
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