Correctly Positioning Cold-Chain Data Loggers: Distinguishing Product Core, Packaging and Air Temperature

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A temperature data logger can only document the temperature at the location of its sensor. If the logger is positioned directly in front of the evaporator, it may record very low air temperatures. If it is located near the door of a cold room, it responds to incoming warm air each time the door is opened. Both positions may show real extreme values, but they do not necessarily represent the temperature of the stored products.

Particularly for food, pharmaceutical products, laboratory samples and temperature-sensitive technical products, selecting the right data logger is therefore not the only important factor. It is equally important to determine whether the air temperature, packaging temperature, a product simulation or the actual core temperature is to be measured.

Short air-temperature peaks may have almost no effect on a thermally inert product. Conversely, a product that has already warmed up may remain outside its permissible range for a considerable time, even though the air in the cold room quickly returns to the setpoint. Without a clearly defined measurement task, logger data can therefore easily be misinterpreted.

Suitable devices for storage, cold rooms and transport can be found in the ICS category Temperature Data Loggers. Additional recording devices for temperature, humidity, pressure and other measured variables are summarised under Data Loggers and Universal Measuring Instruments.

Define the measurement task first

The correct position is not determined solely by the geometry of the refrigerator or shipping carton. It primarily depends on the conclusion that is to be drawn from the data later.

Measurement task	Suitable measured variable	Typical position
Monitor the function of the refrigeration unit	Air temperature	Representative storage area or critical point identified by mapping
Detect door openings and loss of refrigerated air	Fast-responding air temperature	Within the relevant air space, but not directly at the door or evaporator
Document the thermal exposure of a shipment	Temperature inside the package	Between the products or at the thermally most critical position
Approximate the product temperature	Buffered probe or product simulation	In representative packaging close to the monitored goods
Check the actual core temperature	Penetration or core probe	In the thermally relevant product core
Qualify the temperature distribution	Multiple air-temperature or product-simulation values	Within a defined three-dimensional measuring grid

A single logger cannot answer all of these questions simultaneously. If a buffered sensor is used, for example, it intentionally responds more slowly to air fluctuations. This makes it suitable for approximating the behaviour of a thermally inert product, but it may indicate a short-term refrigeration fault later than an exposed air sensor.

Distinguish between air, surface and core temperature

Air temperature

Air temperature describes the immediate environment of the sensor. It responds quickly to door openings, defrost cycles, fan operation, newly loaded warm goods and changing cooling performance.

Air measurement is particularly suitable for monitoring the function and temperature distribution of a cold room. However, it must not automatically be equated with the temperature of a product.

Packaging and surface temperature

A probe attached to packaging measures a temperature that is influenced by airflow, packaging material, contact area and heat exchange with the product. It is often between the air temperature and the core temperature.

A logger on the outside of a carton responds more quickly to the environment than a logger positioned between several densely packed products. When monitoring an individual shipment, it must therefore be documented precisely whether the logger was attached externally, beneath the insulation, between the packages or directly to the product.

Core temperature

Core temperature describes the temperature inside a product. It is particularly relevant when product quality depends on the actual internal temperature and the external temperature provides insufficient information.

A true core measurement usually requires a penetration probe or a specially prepared sample. This is not always possible for hygienically sensitive or saleable goods. The test product must not be damaged or contaminated unnoticed by the measurement and subsequently returned to stock as an undamaged product.

Why products respond more slowly than air

The air inside a refrigerator has a low thermal mass. Even a short door opening can therefore cause a rapid temperature increase. A filled bottle, a solid block of food or a complete pallet, by contrast, stores considerably more heat or cold.

The rate at which a product heats up or cools down depends, among other factors, on:

• the mass and dimensions of the product,
• heat capacity and thermal conductivity,
• initial temperature,
• temperature difference relative to the environment,
• packaging and insulation material,
• air velocity and contact surfaces,
• the arrangement within a carton, pallet or container.

A small individual item at the edge of a pallet can respond much more quickly than a product in the densely packed pallet core. For transport qualification, it is therefore often insufficient simply to attach one logger to the pallet.

Differences also occur during cooling. The air in the cold room may already have returned to the required range, while newly stored warm products have not yet cooled sufficiently inside.

Correctly positioning data loggers in a cold room

A logger should not be attached only where it is easy to access. Its position must either be representative of the stored goods area or deliberately selected as a critical measuring point.

Door area

Short-term heat ingress frequently occurs near a door that is opened regularly. A logger positioned directly in the door gap records these events particularly clearly, but may exaggerate the temperature exposure of products stored further inside the room.

The door area can be useful as an additional risk-monitoring point. However, it is often not representative as the sole routine monitoring point for an entire cold room.

Evaporator and cold-air outlet

Significantly lower temperatures and strong fluctuations may occur directly at the evaporator or in front of a cold-air outlet. A logger installed there can detect frost or freezing risks, but does not automatically represent the average storage temperature.

Particularly sensitive products should not be stored directly in the cold airflow unless this is specifically permitted by the system design and product specification.

Return air and areas with poor airflow

Warm zones can form in areas with weak air circulation, behind densely stacked goods or in corners of the room. High shelving, areas near the ceiling or floor and locations close to external walls may also be relevant.

The final routine monitoring position should therefore preferably be derived from temperature mapping or a documented risk assessment.

Positioning loggers in transport packaging

For refrigerated transport, it must first be decided whether the vehicle environment, pallet, shipping carton or individual product is to be monitored.

A logger attached to the vehicle wall documents the air temperature at that position. It may be influenced by solar radiation, thermal bridges or the airflow from the refrigeration unit. This does not automatically indicate the temperature inside an insulated shipping package.

Possible positions inside a transport carton include:

• between several product packages,
• near the outer side of the package,
• in the thermally inert centre of the shipment,
• near a cooling pack but without direct contact,
• at a critical location identified during packaging qualification.

Direct contact with a frozen cooling pack can produce a local extreme value to which the rest of the product is not exposed. Conversely, a position deep within the pallet core may respond too slowly to detect heat ingress at the outside in time.

For qualified transport packaging, the logger position should be adopted from the packaging test results and clearly documented for every shipment.

When core temperature measurement is useful

Core temperature measurement is particularly useful when the actual condition of the product, rather than only its environment, must be assessed. This may be required during cooling processes, for frozen foods, large containers or when investigating a specific temperature deviation.

The probe must reach the thermally relevant core. For irregularly shaped products, this is not always the geometric centre. The penetration depth, probe diameter and heat conduction through the probe cable also influence the result.

Invasive core measurement is often impractical for continuous transport monitoring. Representative test products or thermal simulations are therefore frequently used. However, their suitability must be demonstrated for the respective product and packaging.

Correctly using product simulations and buffer bottles

A buffered temperature probe is placed in a medium or material that responds more slowly to temperature changes than the surrounding air. Depending on the method, suitable liquids, solid materials, glass beads or specially developed thermal buffers may be used.

The buffered sensor reduces its response to very short air fluctuations and can approximately reproduce the behaviour of a thermally inert product. However, it does not automatically measure the actual temperature of every type of product.

For a reliable product simulation, the following factors must be compared:

• mass and heat capacity,
• shape and surface area,
• packaging and insulation,
• heating and cooling behaviour,
• position within the storage or transport container.

A small buffer bottle may be suitable for compact pharmaceutical packages, but may inadequately reproduce the behaviour of a complete pallet or a large liquid container.

The use of a buffer should therefore be defined in a work instruction. The medium, filling quantity, container, probe position and attachment method must be reproducible.

Hot spots, cold spots and temperature mapping

Temperature mapping investigates the spatial temperature distribution within a cold room, refrigerator, vehicle or transport package. Multiple loggers are used simultaneously at defined positions.

The mapping should be performed under representative conditions. Depending on the application, these may include:

• typical and maximum loading,
• empty and loaded conditions,
• door openings and working procedures,
• defrost and refrigeration cycles,
• different external temperatures or seasons,
• power failure or restart, where safe and intended,
• different packaging or loading configurations.

The hot spot is the area that shows the highest relevant temperatures under the investigated conditions. The cold spot is the area with the lowest temperatures. For products sensitive to freezing, the cold spot can be just as critical as the hot spot.

The routine monitoring logger is not necessarily positioned in the geometric centre afterwards. Its position is selected so that relevant deviations are detected reliably. In pharmaceutical storage areas, monitoring points are typically defined based on the mapping results and a risk assessment.

When multiple data loggers are required

A single logger is only representative of a limited spatial area. Multiple loggers are particularly useful for:

• large cold rooms and freezer rooms,
• high shelving and multiple storage levels,
• strongly varying loads,
• long vehicle bodies or containers,
• multiple air outlets and return-air areas,
• large pallets or grouped packaging,
• qualification of new packaging,
• investigation of recurring complaints.

The loggers should be time-synchronised and clearly identified before the measurement begins. After the measurement, the position, serial number, measurement interval, start time and installation situation must be clearly assigned to the respective data.

If multiple instruments with different response times or calibration statuses are used, apparent temperature differences may partly be caused by the measuring instruments themselves. For mapping applications, loggers should therefore be as comparable as possible and checked beforehand.

Coordinating response time and measurement interval

The response time describes how quickly the measured value approaches a change in temperature. A small exposed air sensor generally responds more quickly than a solid probe inside a buffer bottle or product core.

The measurement interval defines how frequently a value is stored. A logger can only document events that are recorded with sufficient temporal resolution.

A very short interval increases temporal resolution but requires more memory and may affect battery life. A long interval conserves memory but can completely miss short temperature peaks.

At least the following time-related parameters should be considered during selection:

• expected duration of critical events,
• response time of the sensor and buffer,
• door-opening and defrost cycles,
• transport and storage duration,
• required documentation accuracy,
• memory capacity and battery life.

A measurement interval of several minutes may be sufficient for long-term product simulation but too coarse for investigating short airflow or door-opening events. There is therefore no universally correct interval.

Correctly assessing short temperature excursions

An exceeded limit in the logger report initially means that the sensor at its position exceeded the configured limit. This does not automatically mean that the product has become unusable.

At least the following information is required for the assessment:

• measured temperature,
• duration of the deviation,
• sensor position and measuring principle,
• thermal inertia of the product,
• product specification and stability data,
• temperature development before and after the event,
• possible repeated exposure.

A short air-temperature peak at the door may be uncritical for a densely packed product. A moderate deviation lasting several hours in the pallet core may, by contrast, be considerably more relevant.

The logger limits must not be extended arbitrarily or combined with a general delay merely to avoid frequent alarms. The alarm limit and permissible duration must be derived from the product requirements and operational risk assessment.

For pharmaceutical products, the release decision should be made by the responsible quality function based on the available stability and transport data. The logger provides evidence of the exposure, but does not automatically determine the final product decision.

Ensuring calibration and comparability

A carefully selected position is only useful if the logger provides reliable measured values. The calibration status, measurement uncertainty and operating range must be suitable for the application.

Before mapping or comparative measurements, a plausibility check of all loggers under conditions that are as similar as possible is recommended. Larger deviations between the instruments should be clarified before use.

For quality-critical applications, the following information should be documented in particular:

• instrument and serial number,
• calibration or test status,
• measuring range and accuracy specifications,
• measurement interval and start mode,
• time zone and system time,
• position and attachment method,
• external probe or buffer used,
• evaluation software and report version.

If an external probe is used, it forms part of the measuring chain. Calibrating the logger without the probe actually used does not necessarily represent the complete measurement system.

Systematic planning and verification procedure

1. Define the product requirements: Document the permissible temperature range, possible sensitivity to freezing and the relevant exposure duration.
2. Define the measurement objective: Select air temperature, packaging temperature, product simulation or actual product core.
3. Identify the risks: Assess the door, evaporator, cold airflow, external wall, pallet core and poorly ventilated zones.
4. Select the logger: Check the measuring range, accuracy, probe design, memory, battery and reporting function.
5. Define the position: Clearly describe the representative or deliberately critical measuring point.
6. Set the measurement interval: Consider the expected event duration and sensor response time.
7. Synchronise the instruments: Check the time, start point and measuring-point designations.
8. Document the installation: Photograph the position or mark it on a plan.
9. Perform the measurement: Record loading, door openings, refrigeration cycles and special events.
10. Check data plausibility: Assess the trend, minimum and maximum values, duration and relationship to operating events.
11. Establish the product relationship: Do not interpret an air-temperature value as the product temperature without verification.
12. Define the routine position: Adopt the results from mapping, packaging testing or the risk assessment.

Typical errors when positioning loggers

Error	Possible consequence	Suitable measure
Logger directly in front of the evaporator	Very low and strongly fluctuating values	Derive the position from mapping and monitor the cold spot separately
Logger directly at the door	Short air-temperature peaks are overestimated	Use the door area only as an additional risk-monitoring point
Logger on the outside of the transport carton	The inside of the package is not represented	Position the logger inside the qualified packaging
Direct contact with a cooling pack	Local cold value instead of product exposure	Maintain the specified distance and packaging arrangement
Air temperature described as core temperature	Incorrect product assessment	Clearly document the measured variable and probe position
Arbitrary buffer bottle used	The product simulation has incorrect thermal inertia	Define and verify the buffer arrangement for the application
Only one logger used in a large cold room	Hot or cold spots remain undetected	Perform temperature mapping with multiple measuring points
Measurement interval too long	Short events are not recorded	Adapt the interval to the event duration and measurement objective
Logger wedged between goods	Unintended contact measurement or damage to the device	Install the logger mechanically securely and reproducibly
Excursion assessed without considering its duration	Unnecessary quarantine or impermissible release	Assess temperature, duration and product stability together

Practical example: Refrigerated shipment with a noticeable temperature peak

A shipment of temperature-sensitive products is transported in an insulated reusable container. According to the product specification, the permissible range is 2 to 8 °C. A transport logger is positioned directly beneath the lid of the container.

During transshipment, the report shows a maximum temperature of 11.5 °C for twelve minutes. The entire shipment is initially quarantined because the limit was exceeded.

The investigation finds that the logger was located directly beneath the lid and was immediately exposed to warm ambient air when the container was opened. A second logger was positioned between the product cartons in the centre of the container. During the same period, its temperature increased only from 4.8 to 5.6 °C.

For a subsequent test, an external probe is additionally installed in a documented product simulation. During the same handling procedure, it reaches a maximum of 5.3 °C. The results show that the first logger recorded a real air-temperature event that had only a minor effect on the product temperature.

The solution is not to remove the upper logger. Instead, the tasks of the measuring points are separated:

• The fast-responding logger beneath the lid monitors opening of the packaging and warm-air ingress.
• The logger between the cartons documents the temperature inside the package.
• The product simulation is used during qualification tests to estimate the thermal response of the product.

For routine shipping documentation, the position between the products derived from the packaging qualification is subsequently used. The logger beneath the lid remains in use only during selected test shipments as an additional diagnostic measuring point.

Which products and solutions are suitable?

testo 175T2 Temperature Data Logger

The testo 175T2 has an internal temperature sensor and a connection for an external NTC probe. This enables, for example, the air temperature in a cold room and a second temperature at packaging, within a product simulation or between the goods to be recorded simultaneously.

A suitable external probe must be selected for the application and installed so that the required air, contact or penetration measurement is actually achieved.

testo 176T2 Temperature Logger

The testo 176T2 has two inputs for external Pt100 probes. It is particularly suitable for precise comparative measurements in which, for example, two packaging positions, two product simulations or a hot spot and a cold spot are to be investigated simultaneously.

The external probes enable clear spatial separation between the logger housing and the actual measuring point. The probe design, cable routing and calibration must be suitable for the application.

testo 184T1, testo 184T2 and testo 184T3

The transport loggers testo 184T1, testo 184T2 and testo 184T3 are designed for documented temperature monitoring of sensitive shipments.

Depending on the version, they provide an LED or display indication, configurable limits and a directly generated report. However, the position of the logger within the shipment remains decisive for the significance of the recorded data.

ICS Schneider Messtechnik supports the selection of the data logger, probe design, measurement interval and memory capacity, as well as the planning of measuring positions for cold rooms, refrigerators, pallets, transport vehicles and qualified shipping packaging.

Conclusion

The position of a cold-chain data logger has a major influence on the temperature that is subsequently assessed. A sensor in the airflow measures something different from a probe attached to packaging, placed in a buffer bottle or inserted into the actual product core.

Air temperatures respond quickly and are suitable for monitoring the refrigeration system and detecting door openings and operating faults. Product and simulation temperatures respond more slowly and are often better suited to estimating the actual thermal exposure of the goods.

Loggers positioned directly at the door, evaporator or cooling pack may provide important extreme values, but they are not automatically representative of all the goods. In larger storage areas and qualified transport packaging, positions should be derived from temperature mapping, packaging tests or a documented risk assessment.

Short excursions must neither be ignored as a matter of principle nor assessed as product damage without considering the product. Temperature level, duration, sensor position, thermal inertia and product specification must be evaluated together.

Reliable cold-chain documentation is only achieved through the combination of a suitable logger, a clearly defined measured variable, a reproducible position, an appropriate measurement interval, valid calibration and professional data evaluation.

Frequently asked questions about positioning cold-chain data loggers

Should a data logger be placed in the centre of a refrigerator?

The centre may be a useful starting point for small, evenly loaded units. However, the final position should take account of the actual storage areas, airflow and possible hot or cold spots. Temperature mapping is preferable for critical applications.

Can the logger be installed directly next to the evaporator?

This position can be useful for monitoring a cold spot, but it frequently does not provide a representative storage temperature. For routine monitoring, it should be determined which measuring point best represents the critical products.

Where should a logger be positioned during refrigerated transport?

It should be placed at the critical position defined in the packaging or transport concept. This is often between the products or at a location identified during qualification. Direct contact with cooling packs should normally be avoided.

Is the air temperature the same as the product temperature?

No. Air usually responds much more quickly to door openings and refrigeration cycles. Product temperature also depends on the mass, material, packaging, position and duration of the temperature exposure.

What does a buffer bottle do?

It slows the probe’s response to short air-temperature fluctuations and can approximate the behaviour of a thermally inert product. However, it must be suitable for the specific application and does not constitute a universally valid product measurement.

Is one logger sufficient for each cold room?

Often not in large rooms or rooms with uneven airflow. The required number and position of the loggers should be derived from the room size, loading, airflow, risk areas and temperature mapping.

How should a short temperature peak be assessed?

In addition to the maximum temperature, the duration, measuring position, sensor response time and thermal inertia of the product must be considered. The final decision depends on the product specification and the available stability data.

Does a logger positioned between packages measure air or product temperature?

In most cases, it is an air-temperature measurement in a highly shielded area. However, because of the close contact with the packaging, the value may respond more slowly than freely circulating air. The position should therefore be described precisely.

Can one external probe measure both air and core temperature simultaneously?

A single probe only measures at its own measuring point. Two separate measuring channels or two synchronised loggers are required for a direct comparison between air and core temperature.