It appears you may be blending 1910.146(c)(5) and (c)(7) in your attempt to reclassify a PRCS to a non-permit status. You need to follow (c)(7) in your reclassification efforts. It does sound as if your PRCS may be able to be reclassified; however, I am not familar with your type of space or specific situation. OSHA states in (c)(7) that a PRCS can be reclassified if it is has NO known OR potential hazardous atm. It also says, as well in the CPL and preamble, that ALL other hazards MUST BE "ELIMINATED". Using the term "eliminated" is key; as in (c)(5) OSHA allows the hazardous atm to be "controlled" using the force air ventilation. But when reclassifying a PRCS ALL hazards MUST BE "ELIMINATED". In your sand PRCS you would need to utilize one of the three acceptable energy isolation methods to ensure that the space is properly isolated from ALL potential physical hazards (e.g. more sand entering the space burying entrants, agitators/sweep arms/augers, etc.). We can look at OSHA's revised LOTO CPL on page 3-10 where they have finally made clear that ALL "flowable hazardous forms of energy" must be isolated using either Double Block and Bleed, Blanking/Blinding, or Disconnect & Misalignment. This means that all the lines/pipes/ducts/hoses going to and from the space MUST BE isolated using one of these methods. Once you have isolated all the energy sources to and from the space, as well as those inside the space (e.g. sweep arms, augers, agitators, unguarded euipment, etc.) you are ready to reclassify the space from a PRCS to a non-permit status. (c)(7) has one more requirement that many overlook and that is ALL of the actions you took to ELIMINATE the hazards must be documented and made available to the entrants. This documentation must contain the name/location of the space, the date of the recertification, and the signature of the person certifying the hazards have been ELIMINATED. I consider this to be a minature "entry permit".
Your sampling of the atm 5+ times is really no use for reclassifying a space. Remember OSHA states that only those spaces that have no known or potential hazardous atm can be reclassified. If you are sampling for a hazardous atm because there is that potential, then the space can not be reclassified. I will say this, I ALWAYS check the atm and require continous monitoring of the atm even after the space has been reclassified; just my requirment and not OSHA's. The requirement for having a historical record of sampling data arises out of (c)(5), which is for entering PRCS using "alternative procedures". (c)(5) is for those PRCS's that are designated as such because they have ONLY a hazardous atm and NO OTHER hazards AND this hazardous atm can be "controlled" using forced air ventilation. This is not something you will need to do.
One last thing about reclassifying a PRCS...Although it is only mentioned in the preamble, OSHA states that no work can take place inside the reclassified space that could create a hazardous atm. They specifically mention the use of flammable/combustible gases and liquids, as well as welding. These two activities obviously have the "potential" to generate a hazardous atmosphere and I am assuming this is where OSHA is coming from, as (c)(7) clearly states that no employee can be inside the space if a hazardous atm appears.
Some of the previous post to your question refer to rescue and entry/egress methods. When a PRCS has been reclassified to a non-permit status meeting (c)(7), these issues are no longer in play. A space that has been reclassified using (c)(7) is no longer required to meet sections (d) through (k), except for (g), which is the training section of the standard. So no rescue set up is required and this includes non-entry rescue and entry rescue. Means of egress are not a consideration because we are not disputing the space is a still "confined space", we are just ELIMINATING the hazards that made the CS a PRCS, so the size or number of openings does not have a role in the reclassification.