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Monitoring probe locations: how to justify them from your mapping study

When the inspector asks why a probe is where it is, the answer should already be written down. What the rules say, what a good rationale contains, a worked example and a free template.

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Written by Ahmad Salih, Account Manager, Pharmaceutical, JTF WirelessUpdated 19 October 2026

Reviewed by Jake Farrow, Managing DirectorLast reviewed 19 October 2026

If an inspector asks why a monitoring probe is where it is, the answer should be one line in a controlled document: this probe covers the hot spot the mapping study found here, at this height, and the Responsible Person approved it. When monitoring went in before the mapping, or the mapping was never checked against the monitoring, that line is missing. This guide shows how to write it, with a worked example and a free template.

Why the question comes up

The rule is one sentence, and inspectors test it

EU GDP 3.2.1 says temperature monitoring equipment should be located according to the results of the mapping exercise, so that monitoring devices sit in the areas that experience the extremes of fluctuation. Section 9.4 asks for mapping of temperature-controlled vehicles too, taking account of seasonal variations. The European Commission's GDP Q&A (question 7) adds that the number of probes and their placement depend on your risk analysis and should agree with the mapping results.

The MHRA reads it the same way. Its mapping blog says your risk assessment should determine the most appropriate positions for the permanent probes, and that the RP should be party to the whole mapping process. The 2016 GDP deficiency data cite a site where mapping had not been done well enough to place the thermometers in the areas with the extremes of fluctuation.

What the WHO mapping supplement adds

WHO Technical Supplement 8 to TRS 961 Annex 9 is the most detailed public source. It says the purpose of finding hot and cold spots is to identify where the monitoring sensors should preferentially go, that hot and cold spots need to be found seasonally because summer and winter can differ, and that the report should recommend the optimum location of the routine monitoring sensors. If a monitoring system is already installed, the study should identify where to relocate sensors, if needed.

Our recommendation

A JTF position, not a regulatory requirement

How many permanent probes a room needs

Start from the risk: The extremes matter more than the floor area. A small room with a heater and a sunny wall can need more probes than a large, stable one.
What we recommend: Start from the mapping. Put one probe at each mapped hot spot and each mapped cold spot, at the height where it was found, and classify those as critical. Every separately controlled area (each room, store, mezzanine or cold room) gets at least two: one at its warm extreme and one at its cold extreme. Where storage is over about 3.6 m high, cover both high and low level in each zone.
Scale it to the risk: Small, single-zone rooms at room temperature (up to about 10 square metres, no unusual heat source): one probe at the risk-assessed worst point can be enough.
Cold rooms: at least two, one at the warm extreme (often near the door) and one at the cold extreme (often near the evaporator outlet).
Extra probes at docks, doors or handling areas: classify them as supplementary, not as substitutes for the mapped points.
A warm extreme by a door: put the critical probe where stock sits, buffered so door openings do not trip it, not on the threshold. See worst-case location vs nuisance alarms.
What to record: A probe schedule: probe ID, serial number, area, position, height, critical or supplementary, the mapped location it represents and the reason.

The common situation

Monitoring first, mapping later

In practice the order often goes wrong, in one of three ways:

The monitoring had to go in before the mapping report existed. Positions came from the layout, site guidance and engineering judgement.
The mapping was done by someone else and never reconciled. The report recommends positions; nobody checked that a probe actually sits at each one.
The estate has grown. The original probes followed a mapping study, but rooms, fridges and humidity sensors were added later, each without a recorded reason.

What a good rationale contains

The sections that answer the inspector

A rationale is short, honest about what happened, and ends in actions with owners. Our sections (the template has the prompts):

Purpose and status: what the document is, and that it does not replace the mapping report or your quality risk assessment.
Chronology: when the monitoring went in, when the mapping happened and what was known at each point. Never backdate.
Mapping evidence: dates, logger positions and heights, empty or loaded, HVAC state, results and any seasonal limits.
Mapped critical points: each hot spot, cold spot and worst-case position, with its height and the reason it matters.
Quantity: the mapped minimum against what is installed, and the status of every extra probe.
Alignment, probe by probe: does a probe sit at each critical point? Note doors and docks, HVAC influence, height and temporary fixings.
Communications: wireless signal under real conditions (doors closed, racking full).
Responsibilities: you own the risk assessment, the mapping provider owns the results, the installer owns the as-built record.
Actions and review triggers: relocations, an as-built schedule, re-tests, the next mapping, and what makes you re-justify.
Approval: prepared, technically reviewed and approved by the RP or QA.

Worked example

Twenty-two probes installed, eight mapped points

Based on a real retrospective rationale for a GDP-licensed pharmaceutical warehouse; the site, layout and numbers are changed. The store is 15 to 25 °C: a secure store with storage to about 3.7 m, and an open warehouse with two dock doors.

What happened. The monitoring had to go live while the mapping study was still running. With the RP, the team installed 22 probes for broad coverage: 14 in the secure store at high and low level, 8 in the warehouse at mid-height. The mapping report arrived later and recommended a minimum of 8 critical points: two high and two low in each area.

What the rationale found. 22 exceeds 8, but the installation was not yet aligned. In the secure store the heights were right, but no record tied a probe serial to each mapped position. In the warehouse every probe was at mid-height, so none covered the mapped high and low points. The probes at the docks, goods out and the store door were sensible, but supplementary. The study was run empty, with air conditioning started part-way through, so the critical points stay provisional until a loaded winter study.

What they did. Moved four warehouse probes to the mapped high and low points, cross-referenced the secure store probes to the mapping plan, labelled 8 as critical and 14 as supplementary, fitted a gateway after a closed-door signal test, kept the top racking level out of use, and issued an as-built schedule for the RP to approve. No new probes were bought.

IllustrativeThe worked example as a plan: 8 critical points, 14 supplementary probes (10 in the secure store not drawn)

Critical: at a mapped hot or cold point
Supplementary: operational risk or coverage
Old mid-height position, moved

Probe	Where	Height	Role	Why it is there
S-H1, S-H2	Secure store, wall opposite the AC	3.5 m	Critical	Mapped warm points under the ceiling, furthest from the air conditioning outlet
S-L1, S-L2	Secure store, floor level near the AC	0.2 m	Critical	Mapped cool points at floor level, below the air conditioning airflow
W-H1, W-H2	Warehouse, top of racking	High	Critical (moved)	Mapped warm points under the roof; the probes were at mid-height
W-L1, W-L2	Warehouse, near the docks	Low	Critical (moved)	Mapped cool points by the dock doors; the probes were at mid-height
D1, D2	Dock doors	Mid	Supplementary	Door-opening events; not a substitute for W-L1 and W-L2
GO, SD	Goods out, secure store door	Mid	Supplementary	Handling areas and the store interface
10 others	Secure store, spread	High and low	Supplementary	Representative coverage and redundancy, kept by the RP’s decision

When they don't match

Reconcile, don't rewrite history

If your monitoring and your mapping disagree, the fix is usually moving and relabelling, not buying more. In order:

Get the final mapping report and its numbered plan, and the heights of every logger.
Walk the site with the plan and record each probe's serial, position and height.
Match a probe to every critical point. Where none matches, move one and recommission it.
Classify every other probe as supplementary, with its reason, or retire it through change control.
For probes added since the original mapping, link each to its own basis: change control, equipment qualification, a change of room use or a later study. If none exists, say so and raise one.
Re-test alarms and communications at the new positions, issue the as-built schedule and get RP or QA approval.

Our recommendation

A JTF position, not a regulatory requirement

When to re-justify

Start from the risk: A rationale goes stale the day the room changes. Racking, doors and air handling move the extremes; a new mapping study can move them too.
What we recommend: Review the probe schedule at every self-inspection, at least once a year. Re-issue the rationale before stock goes back into any area where racking, doors, dock arrangements, HVAC or the set point have changed, and within one month of any new mapping report.
Scale it to the risk: Probe replaced like for like in the same position: update the schedule (new serial), no new rationale.
Probe moved, added or retired: update the schedule and record why, with RP approval.
Unexplained trend in monitoring data: re-map that area, then re-justify.
What to record: The trigger, the date, what changed in the schedule and who approved it.

How we help

Part of our mapping service

Temperature mapping is a JTF managed service, and the report ends with recommended probe positions. Where monitoring is already installed, we cross-reference it to the mapping and write the rationale with you, including retrospective rationales like the example above. Your RP or QA still approves it. See temperature mapping or use the template yourself.

In summary

The short version

Monitoring goes where mapping found the extremes; the number follows your risk analysis.
Put one critical probe at each mapped hot and cold spot, at the height it was found.
Classify every other probe as supplementary, with a reason.
If monitoring came first, write the chronology honestly and reconcile probe by probe.
Give later additions their own basis: change control, qualification or a later study.
Keep a probe schedule with serials, positions and heights, approved by the RP or QA.
Re-justify after layout, door, HVAC or set-point changes and after every new mapping report.

Free download

Probe quantity and location rationale: a template

The document an inspector wants to see when they ask why a probe is where it is. Ten sections with prompts, a probe schedule, an alignment table, review triggers and approval lines, ready to fill in for any store, warehouse or cold room.

A template to help you document your own decisions. Your RP or QA approves the final rationale.

Questions

Common questions

The mapping report already lists probe positions. Do we still need a rationale?

Usually, yes. The report recommends positions; the rationale shows that a specific probe now sits at each one, at the right height, and that your RP or QA approved the arrangement. WHO Supplement 8 treats repositioning sensors as a normal outcome of a mapping study, and the rationale is where you record that it happened.

We have more probes than the mapping recommends. Is that a problem?

Not in itself. Make sure the mapped critical points are covered first, then label every other probe as supplementary with its reason (a dock door, a handling area, redundancy). Problems start when nobody can say which probe covers which extreme.

Can a rationale be written after the event?

Yes, as long as it is honest. Record the real chronology, what was known when the probes went in and what the mapping later showed, then close the gaps with dated actions. Never backdate a document to look as if the mapping came first.

Who signs it?

Your Responsible Person or QA owns the quality risk assessment and approves the final quantity and positions. The mapping provider owns the mapping results. The installer documents the as-built system. Our template has a line for each.

Does a probe by the dock door count as our cold spot?

Only if the mapping found the cold spot there. Door and dock probes are often good supplementary positions because they catch door events, but they do not replace a mapped critical point elsewhere in the room.

Our mapping was done with the room empty. Is the rationale still valid?

It is valid for what the study showed, and it should say so. EU GDP asks for mapping under representative conditions, and WHO Supplement 8 describes mapping empty and in a normal loaded condition. Mark the critical points as provisional, plan a loaded or seasonal study, and review the probe schedule when it reports.

Keep reading

More on mapping and monitoring

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