How many sensors should I use in a temperature mapping study?

Well, although there is no predicate rule establishing the amount of measuring sensors to be used when mapping a volume, there is guidance to help you determine sensor amount and placement. Guidance is available from The International Society for Pharmaceutical Engineering (ISPE), the World Health Organization (WHO) and the United States Pharmacopeia (USP), among others, as listed at the end of this article. This article summarizes them for you.

Commercial-off-the-shelf freezers, refrigerators, incubators and environmental chambers

Recommended sensor placement for internal volumes up to 2 cubic meters

Figure 1 - Recommended sensor placement for internal volumes up to 2 cubic meters

Use 10 temperature sensors in chambers with internal volume of up to 2 cubic meters. Place 9 sensors within the usable volume. Figure 1 suggests sensor placement. Place an additional sensor near the probe of the device that monitors and/or controls the system.

Why is this? You are mapping worst-case locations under the assumption that any other location contained within the usable volume will not experience temperatures exceeding these extremes. The corners represent the intersection of the most planes (3) that are exposed to outside air temperature. Also, corners tend to have the least air circulation. Therefore, each corner represents a potential worst-case location. Conversely, the center of the usable volume is farthest from the outside air. It is also most likely to have good air circulation. Likewise, it represents a potential worst-case location. Together, these locations are most likely to experience temperature extremes. Include a rationale in the temperature mapping protocol to explain all assumptions.

Recommended sensor placement for internal volumes up to 20 cubic meters

Figure 2 - Recommended sensor placement for internal volumes up to 20 cubic meters

Use 16 temperature sensors in chambers with internal volume of up to 20 cubic meters. Place 15 sensors within the usable volume as recommended in Figure 2. The extended length of the sides in bigger chambers makes the center of each pane a point of interest since its temperature might differ significantly from that of the corners. This is due to its proximity to the center of the usable volume. Place an additional sensor near the probe of the device that monitors and/or controls the system. This will be needed to determine alarm and control offsets by correlating temperature differences between these locations and the worst-case locations identified in the study.

Warehouses, cold rooms, walk-in freezers and walk-in refrigerators

Distribute sensors in a grid format every 5 to 10 meters along the width and length of the area3. For very large facilities, sensors can be placed up to 20 or 30 meters apart. At each point on the grid, arrange additional sensors vertically forming layers. Use three layers for rooms with ceiling up to 3.6 meters. Consider adding more layers for rooms with ceiling above 3.6 meters. Place an additional sensor near the probe of the device that monitors and/or controls the HVAC. Map product storage areas only. Provide rationale if adding sensors in other areas of interest. Temperature mapping should consider other factors that may contribute to temperature variability such as the size of the space, location of HVAC equipment, sun-facing walls, low ceilings or roofs, geographic location of the area being mapped, airflow inside the storage location, temperature variability outside the storage location, workflow variation and movement of equipment (weekday vs. weekend), loading or storage patterns of product, equipment capabilities (e.g., defrost mode, cycle mode), and operating procedures.

Bibliography:

  • 1 - ISPE (2016). Good Practice Guide: Controlled Temperature Chamber Mapping and Monitoring.
  • 2 - USP (2018). USP41-NF36 <1079>. Good Storage and Distribution Practices for Drug Products.
  • 3 - World Health Organization (2015). Technical Supplement 8 to WHO Technical Report Series, No. 961, 2011. Temperature Mapping of Storage Areas.
Read 1429 times Last modified on Friday, 18 December 2020 15:27

comments (3)

  • Gary
    Gary Comment Link

    What is the reference source for the following statement: Distribute sensors in vertical arrays with a minimum amount equal to the square root of the room area in square meters plus 1 for each layer. For very large facilities, sensors can be placed up to 30 meters apart. Use three layers for rooms with ceiling up to 3.6 meters. Consider adding more layers for rooms with ceiling above 3.6 meters

    Tuesday, 30 June 2020 12:31
  • Cesar Mirabal
    Cesar Mirabal Comment Link

    Amer, Thank you for your question. There is no cookie-cutter solution for these intermediate volumes. Here is my line of thought and recommendations: The use of the square root of the room area in square meters plus 1 per layer as an estimate for the minimum number of mapping sensors to use is a widely adopted industry interpretation of WHO’s guidance for large temperature-controlled storage areas that hold pharmaceutical products. For these large areas, WHO recommends installing sensors every 5 to 10 meters, hence the formula, in at least three layers along the width and length of the area. For very large facilities, this can be up to 20 or 30 meters. This is intended to ensure that the volume is mapped symmetrically in all three dimensions while reasonably covering the volume under study. This method of estimation is intended for large volumes. For high ceilings, the relatively small volume of the 34 and 70 cubic-meter stability chambers might result in a calculation yielding less sensors than those recommended by ISPE for a 20 cubic meter volume. If this is the case, you might consider using 16 sensors following the same ISPE rationale as for a 20 cubic meter volume. For low ceilings, the calculation might yield an adequate number of sensors provided that the area layout and intended product location still support symmetrical mapping in all dimensions. Applying the calculation to the 90 cubic-meter stability chamber will most likely yield an adequate number of sensors to symmetrically map the volume. Keep in mind that you can use more or less sensors depending on the chamber height, area layout, intended product location and other factors discussed in the article. You should consider mapping only the intended product locations based on the product rack configuration and layout. There is no need to map corridors if product will never be stored in them. If in doubt, err on the safe side. Finally, it is important to always include a rationale in the mapping protocol explaining the selected number of sensors. This will help auditors understand your decision. I hope this helps. Cesar

    Thursday, 19 March 2020 20:04
  • Amer
    Amer Comment Link

    Dear Sir : Kindly according to this article ( Use 16 temperature sensors in chambers with internal volume of up to 20 cubic meters. ) , what about stability chambers of volume above 20 m3 (34, 70 and 90 m3) ?how many sensors shall we use for these volumes? can I use the square root of the room area in square meters plus 1 for each layer as mentioned above ? Kindly advise .

    Wednesday, 26 February 2020 17:13

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