Video Q&A: Guidance on warehouse mapping sensor placement

See below for an edited transcript of this video. Here is another brief video question and answer session from our warehouse mapping webinar. In this webinar we suggest a cost-effective way of mapping your warehouses continuously. Senior Regulatory Expert Paul Daniel answers questions we did not have time for during the webinar. Enjoy and feel free to leave further comments below.

[00:00:06] Good day and welcome to another Q&A video with Vaisala Senior Regulatory Expert Paul Daniel. Today we're going to cover some of the questions that we received from our webinar: “Continuous Mapping: Better Data, Better Compliance.”

[00:00:30] Our focus in that webinar was warehouse mapping. We were very grateful to have over two hundred people in that webinar, but we didn't get a chance to answer all the questions. So we're doing it today. *See an earlier Q&A video from that webinar.

[00:00:54] Question one: “Why stack the sensors vertically, on top of each other? And can I use stacks of two if the room is not very tall?”

[00:01:04] Paul: In the webinar I recommended starting with stacks of three. That's a stack of sensors with one low, one middle, and one high, and then distributed through the warehouse. This attendee is asking why we should stack sensors vertically. Also, can we use shorter stack stacks [in mapping studies]? This is a perfect question for a thought experiment; we only have three choices when it comes to sensor placement. Option one: distribute sensors randomly throughout the warehouse. This sounds like fun to me, but I guarantee your quality department will hate it.

Option two: we can place sensors only at the places that we think are going to provide interesting data. Again, this sounds like fun, but that's not going to be good enough. Finally, option three: we distribute the sensors evenly and geometrically throughout the warehouse covering the entire space.

As fun as it sounds, nobody does random [sensor placement in] mapping. We map with evenly distributed sensors. That is option three and occasionally, we put extra sensors in places of interest. If you distribute sensors evenly in space, geometrically distributed, you will inevitably end up with stacks because rooms tend to be square. So with sensor in an upper corner and one in a lower corner you automatically have to stack sensors.

[00:02:23] Let's look at the guidance. USP Chapter 1079 Good Distribution Practices states that sensors should be placed in three planes in three dimensions.

A temperature mapping study should be designed to assess temperature uniformity and stability over time and cross a three-dimensional space. Completing a three-dimensional temperature profile should be achieved by measuring points at not less than three dimensional planes in each direction/axis—top-to-bottom, left-to-right, front-to-back, where product will be present.

That's a confusing way to say stacks of three. The only way to meet these criteria is to use three stacks of sensors, one on top of the other. If you don't have a stack of at least three sensors, you can't meet that requirement.

Another guidance source for this is the WTO, the World Health Organization. "Temperature mapping of storage areas Technical supplement to WHO Technical Report Series, No. 961, 2011" The WTO also recommends three sensors in a stack if your ceiling is less than 3.6 meters. They go up to five sensors in the stack when your space is 6m or more. I think five sensors in a stack is sort of overkill. I've never seen anyone use that many sensors in a controlled space. For a warehouse I recommend stacks of three in general. To our question asker, if the ceiling is low, say less than 8 feet, or 2.5 m, then place sensors in stacks of two.

[00:03:28] Next question: “What lateral distance between sets of stacks is appropriate?” You said 20 m maximum lateral distance. Where does this number come from?”

[00:03:52] Paul: In the webinar, I recommended a 20 meter distance laterally (sideways) between those stacks of three. I have not found a guidance or regulation yet that gives us a specific number for placing sensors like this. It is just not there. That 20-meter limit is just a rule of thumb that I came up with myself and it is based entirely on my own experiences and warehouse mapping. In practice it works. It is a distance that gives reasonable confidence to most people, including auditors. But, it doesn't hit that point of overkill where we begin to feel like there are too many sensors in space.

Remember that 20 meters [lateral distance] is for ambient warehouses. We sometimes map more extreme environments, say large cold rooms and large freezer rooms. For those, I would decrease the distance to 8 meters. It is a bit of a sliding scale determined by the size of the space, the temperature dynamics, the product stored, and the data that we get from mapping studies. But for a warehouse storing ambient products, I found that 20 meters is a good maximum lateral distance between stacks of mapping sensors. Given the huge variety of floor plans you might run into, even that parameter should be adapted to the particular situation at hand.

[00:05:08] Question three: “How many mapping sensors for 3 x 3 meter room?” Is there any guideline we can use as a reference for?”

[00:05:33] Paul: Yes, we actually have a good guidance for that. The threshold is not so much the floor plan, but the volume and the volume threshold we need to be worried about is 20 m3 of volume so that 3 x 3 meter room would probably be right there at a limit of 20 m3. Any rectangular place that is less than 20 m3 volume is going to need 16 sensors, with one sensor in the middle and a sensor in each corner. Since there are eight corners, we have a total of nine. Then we would put one sensor in the middle of each side. A room has six sides and that takes six sensors. That makes 15 so far. The last sensor, number 16, we would place adjacent to the control center. The outline for this kind of sensor placement is detailed in the ISPE’s “Good Practice Guide for controlled temperature chambers”.

For a webinar on this topic, See: “Mapping Made Easy”

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