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One of the most commonly asked questions posed by companies who have chambers, rooms or buildings that have critical temperature and/or humidity requirements is, “How many sensors will be needed to map my chamber?”  This paper will explore considerations and mapping strategies associated with product storage chambers.

Temperature and humidity mapping provides evidence that a temperature and/or humidity controlled environment meets specified criteria.  These criteria can be drawn from regulatory requirements such as United States Pharmacopoeia (USP), manufacturers’ specifications, and/or product specifications.  In most cases, these standards and guidelines specify or imply mapping is required, but unfortunately do not provide much guidance as to ‘how’ mapping should be conducted.  Mapping may be used to confirm certain areas in a chamber or the entire volume of a chamber meet the acceptance criteria, depending on how and where material is stored.  Mapping also provides worst case locations to be considered for placement of permanent monitoring/alarm probes.  A small chamber with very critical requirements and a tight acceptable range (such as a pharmaceutical incubator or refrigerator) may require more data points than a large room with a broad acceptable range (such as a warehouse).  Proper evaluations of the chamber design and use will be used to determine the testing locations and number of sensors which will give a true picture of the stability and ultimately, acceptance, of the temperature and/or relative humidity (RH) of the storage area.

The limitation of any sensor in the mapping system is that it is only monitoring conditions at the probe itself, a singular point in space.  The ability to ascertain the conditions throughout the chamber is dependent on having an adequate number of sensors strategically placed in all product storage areas.  The conditions in the “gaps” between the sensors can be surmised by analyzing the total variance in the adjacent sensors and the totality of the variance among all sensors at any point in time.  Some of the causes of large variance in conditions can be inadequate airflow and circulation, locations of heating and cooling equipment, wall openings, windows, doors, and substandard insulation.

If a chamber is well designed and constructed, conditions will always remain within the required ranges in a static state.  But many chambers, especially warehouses and large rooms, rarely experience static conditions.  Changing ambient or outside conditions external to the chamber, personnel/equipment entry and egress, and load changes within storage areas can have dynamic effects on chambers. A mapping strategy should consider dynamic conditions that may occur within the chamber.  Dynamic testing may include power loss testing, door openings and variance in loading.  Results may indicate areas that are not suitable for storage, maximum door opening times and/or quantity of personnel, load size and configuration, and contingency planning for power loss events.

Sensor Types

Common sensors used in temperature mapping include units that use external thermocouples and dataloggers.  Thermocouple systems can provide instantaneous feedback during the testing to observe trends and problems that may need to be addressed before mapping of a chamber is completed.  Thermocouples react rapidly to changing temperatures due to their low thermal mass.  Thermocouples are relatively inexpensive and can be repaired easily if damaged.  Additionally, thermocouples with some mapping systems can have the thermocouples calibrated to a National Institute of Standards and Technology (NIST) calibrated standard prior to, and verify the calibrations immediately after the mapping.  Thermocouples can be used in harsh environments (such as high heat, cryogen, immersion, caustic, or vacuum) and are very accurate.  However, thermocouple placement can be very time consuming and are difficult to use for large areas such as warehouses.  In addition, separate sensors for RH have to be placed if RH requirements are part of the storage criteria.

Dataloggers are small and can be rapidly placed.  Dataloggers can range from inexpensive to very pricey.  Dataloggers calibrated to NIST standards and that include calibration certificates tend to be much more expensive.  Many dataloggers can record both temperature and RH simultaneously.  Some types of dataloggers have multiple channels for calibrated external thermocouples.  Immediate feedback is available from some type of dataloggers that have the capacity to be networked or transmit data by RF.  But, many datalogger calibrations are usually only conducted annually, so it is possible for a unit to have been used for months before finding it has drifted out of the calibration requirements.  Most dataloggers can not be used in harsh environments and have limited temperature/RH ranges that they can be used in without potentially causing damage to the units.  The type of sensor used will be based on the chamber environment and size along with the type of data needed.

Mapping Strategies

To design a mapping strategy for a chamber, the total volume must be considered.  The interior volume of a chamber is mapped by area and strata.  Generally, this means that the sensors are placed in a logical manner to provide three-dimensional coverage (top-to-bottom, left-to-right, and front-to-back) in all product storage areas.  The most critical areas in any chamber tend to be the extremities (such as corners of a unit) and locations close to the heating and cooling sources.  Chambers can be temperature controlled by various means, so airflow and heat transfer needs to be understood to determine monitoring locations.  For example, many refrigerators are cooled via a ceiling mounted fan that directs airflow down the center of the chamber across all shelves.  Alternatively, sub-freezers are cooled by conductive cooling by means of coils imbedded in the side and back walls.  Any air circulation is entirely dependent on convection currents.  Additionally, some sub-freezers have the cooling coils imbedded in the shelves to provide almost direct contact with stored products to provide rapid freezing.

Per USP 36, Chapter 1079: Good Storage and Shipping Practices, the duration of mapping runs should be at least 24 hours for small chambers and 72 hours (three consecutive 24 hour periods) for warehouses.  The 24 hour minimum for small chambers is derived from the need to observe the effects of changing ambient conditions (day and night, cycling of multi-chiller systems, defrost cycles, etc.) on chamber stability when empty and fully loaded.  The 72 hour minimum for warehouses is derived from the need to observe the effects of the external environment over several days.  In addition, temperature (and/or RH) controlled warehouses and large chambers with exterior building walls should be mapped in both the winter and summer to capture the effect of seasonal environmental conditions and extreme outside temperatures.

The following diagrams of mapping strategies for various chambers is neither comprehensive nor covers all scenarios.  Each individual project requires proper evaluation of the chamber and use requirements in determining the best placement options.

The above design for the mapping provides a very comprehensive picture of the entire chamber area that is used for storage.  Validation mapping may be conducted both with the chamber empty and loaded with a maximum load, such as pallets of product stacked to 6’ high.  Sensors are commonly placed by data historian probes in the chamber to document the appropriateness of the monitoring probes placement.  Additionally, sensors may be placed adjacent to chamber controlling probes to document the response of the heating/cooling system to changing loads.

For a chamber with less stringent requirements, the below pattern provides almost as complete of coverage:

Again, the entire useable area of the chamber is mapped. The extremities and all strata have coverage, with about 25% less testing locations than the first example.  For chambers with installed shelving, where product storage is limited to shelving only, the mapping diagram may look more like the below mapping diagram:

This type of mapping strategy only covers the product storage areas of the chamber.  This is a common strategy that is used for warehouses and storage rooms.  Non-product storage areas in the chamber may not require mapping if they are never used for storage.  But if the areas may have to be used at a later date, it might be more prudent to add additional sensors to insure that potential storage areas have been fully mapped.

Very large facilities may have to be divided into separate runs due to the amount of coverage required and the number of sensors available.  Some very large warehouses can require a 100 or more sensor locations to adequately map the chamber.  Warehouses also present some unique challenges, including the areas adjacent to dock doors, external walls, windows, and personnel doors.  Many times, testing will include simulated truck deliveries accomplished by conducting dock door openings to test the building mechanical systems’ ability to respond to a timed (typically five to ten minute) influx of non-tempered air.  These are most important during periods of extreme outside temperatures.  Chamber recovery from the exposure of outside air caused by these door openings should be short enough to prevent temperature changes in the items stored.

Small Chambers

Small chambers, such as reach in incubators and refrigerators, many times have extremely tight criterion for temperature and humidity control.  This leads to having a large number of sensors required to adequately map a relatively small volume.  Generally, the same rules apply as for large chambers.  To adequately map each storage shelf, a suggested pattern is five sensors per shelf, one in each of the four corners and one in the center as shown below.

This can be adjusted depending on if the shelves are solid or wire rack and if the airflow is directed downward and vertical across all shelves, like a refrigerator, or is horizontal from the sides of the unit, like many types of incubators.

Another common pattern used for chambers with a large number of shelves or due to limited sensor availability, is three sensors in alternating diagonal patterns on each shelf (first shelf from left front to right rear, next shelf from left rear to right front, etc.) as shown below.

There are many variations to these mapping strategies.  These are by no means the only sensor placement designs that are used; patterns have to be modified for each different type, use and layout of chambers.

Additional Considerations

Additional considerations for chamber mapping are product placement, product surrogates and monitoring of temperatures within products with critical storage requirements.

The placement of products can have profound effects on chamber stability.  Ill-considered stacking or racking can inhibit airflow and circulation in portions of a chamber.  Adequate mapping will display the adversely affected areas and can lead to solutions for better uniformity.  Heavy loading of the chamber or inadequate airflow may have to be addressed prior to completion of chamber testing.  Many times, simple solutions such as the addition of ceiling fans in critical areas of large chambers can adequately address the problems.  Severe problems may require re-engineering of the control or mechanical systems.  Sometimes the solution is simply restricting certain areas of a chamber from being used for the storage of critical products either by placarding or with physical barriers.  Large chambers with exterior walls may have localized problems.  Many times, these areas are used only for storage of packaging or other items that would be unaffected by temperature/RH variance.

Chambers should be mapped both empty and loaded, with actual product (including in-use mapping) or with simulated product.

Empty loading is many times the worst case conditions for the chamber to control.  The fully loaded chamber is somewhat stabilized by the thermal mass of the stored product at the controlled temperature.  Empty spaces in the chamber can be very reactive and relatively unstable.  The empty chamber mapping can establish the base-line for the chamber performance.

When mapping a chamber retrospectively that is in use, it may be impractical to conduct empty chamber mapping, so the chamber has to be mapped as-is.  Sometimes with walk in chambers that are not in use, empty boxes are used to fill the racks to insure that the effect on airflow can be documented.  This is not equivalent to a true full load test as the boxes have little thermal mass, but this type of testing is many times seen as sufficient to address temperature uniformity concerns with the chamber filled at a very low cost.

When actual product equivalents must be used, water is commonly substituted for liquid products and baking soda or salt make good, inexpensive substitutes for dry bulk substances.  Empty chambers many times react totally differently than loaded chambers and stability under both conditions needs to be determined.  Rarely a chamber in-use on a daily basis is found completely filled or empty, so mapping of the extremes documents the chambers abilities under all conditions of loading.

If products are extremely reactive to small changes in conditions, monitoring of temperatures within the product may be required to insure that door openings and product removals and additions do not adversely affect currently stored products.  Dataloggers can be placed in loading between containers or packaging, and thermocouples can be inserted in products or simulated products, either liquid or solid.

About the Author:

Mark Moody is a Validation Specialist with Performance Validation.  His area of expertise is temperature mapping.  Mark has executed hundreds of mapping studies across the spectrum of cryogenic storage at ultra low temperatures (-196C) to studies that verified depyrogenation (+250C), and everything in between.  When Mark isn’t performing validation studies for our customers he is involved in maintaining our temperature mapping equipment.

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