how it works and why it works
Where it started...
THE SENTRY PAC PRO HAS FOCUSED ONLY ON BETTER METHODS TO PRESERVE GRAIN.
This began in 1974 on an Iowa farm. Since the introduction of the first Sentry PAC (Programmed Aeration Controller) in 1983 , Sentry Pac Pro™ continues to set the standard for advances in grain care.
The Sentry Pac Pro™ controls moisture content (MC) and prevents spoilage in grain with an adaptive proactive method of aeration.
Proactive not reactive
It does not wait for the grain to start to go out of condition and then run fans. Instead the PAC monitors the weather - not the grain - to select precise conditions of temperature and humidity to produce your desired result in the grain. Over time the grain equalizes to these conditions and becomes uniform in both temperature and moisture content. The PAC keeps a 21 day running average to use as a targeted temperature.
GRAdualism
NO LARGE FRONTS
The PAC keeps a 21 day running average to use as a targeted temperature. This 21 day average assures the air will be very close to the last time the fans ran so no big fronts are introduced. Big fronts can cause problems, including condensation. This also keeps the total hours of run time lower because never do fans have to be run to undo hours of operation out of range.
Averaging minor differences (ranges) of temperatures and RH over short periods of time works well and is a friend to good grain storage. This promotes uniformity.
The PAC avoids running with large differences over long periods of time which is disastrous due to the relationships between temperature, RH, and the grain mass and the very large energy exchanges involved. Fronts can be set up that are counter productive to desired results.
A brief history...
For centuries various methods of aeration have been used to prevent spoilage: corn cribs use natural winds; ‘turning’ uses an extra bin or silo to remix and cool the grain as it ‘falls through the air’.
Modern day aeration systems force outside air through the grain mass with fans. The challenge has been determining the best time and methods for aeration.
Attempts to improve haphazard aeration
include monitoring the grain temperature and measuring the CO2 levels. Monitoring grain temperature alone does not take into consideration outside conditions. Using CO2 levels is like closing the barn door after the horses have escaped.
when to run fans?
So, how does one decide how to manage aeration? A series of questions need to be considered: First, what is it that needs to be controlled? Is it the temperature of the grain or the moisture content (MC) of the grain? Or both? Then there is the question of how long will it be stored?
Optimum fan control strategies will vary based upon the answers to these questions.
moisture migration
Moisture migration can cause crusting and or bridging that may lead to mold growth. Keeping the grain temperature near the average outside temperature can prevent moisture migration, thus avoiding spoilage.
This requires monitoring the weather and aerating near the average outside temperature for that season.
But what average is best to use? A proven time span for average temperature for grain storage is 21 days. Temperatures on average do not change much from month to month, so it works well to use the gradual changes that occur over the 21 days as the targeted temperature for the grain. Also controlling the temperature uniformly through the grain mass simplifies controlling the moisture content.
It is not desirable to warm grain to the average summer high, nor the average winter low. Having limits for targeting temperatures work best.
The chart shows that an increase of only 2° in temperature each week from the middle of March to the middle of June is required to maintain desired grain temperature.
adaptive
When the outside temperature is within range of the 21 day average temperature the PAC calculates the needed RH% and activates the fans if in range. This decision is repeated every 15 minutes.
For instance, if 60 hours per month were necessary for a certain bin situation the budgeted time would be set to 2 hours per day (60/30 days) and 2 hours would be added to the backlog each midnight. The ranges around the targeted temperature and the TMC expand and contract according to the size of the backlog to make runtime more or less likely to accommodate the variability of the weather.