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How Do Data Loggers Work: A Complete Guide

A data logger is a small, self-contained unit that measures and records a stream of information. This information can be stored for later retrieval and analysis, or transmitted to a central location in real time. A common use for data loggers is to monitor environmental conditions in areas where sensitive products are made, stored, and transported. In these situations, data loggers are a key part of the systems that ensure the quality of medicines, foods, and other safety-critical products.

Digital data loggers have enabled the distribution of sensitive products across geographically expansive networks, through temperature-controlled supply chains, including cold chains. For example, data loggers have been a key part of the safe distribution of vaccines and other medicines to remote locations with unpredictable infrastructure. The importance of environmental monitoring using data loggers is reflected in federal regulations, FDA guidance documents, and quality processes like the AS9100 standard for aerospace manufacturing.

Environmental measurements with data loggers are also important in less regulated applications, like climate research, agricultural monitoring, and HVAC monitoring.

In this article, we’ll cover four of the most common types of data loggers, and some important aspects of their use.

What functions do data loggers perform?

The primary role of environmental data loggers is to ensure that critical locations and process units are within specified limits, which is important for protecting valuable products, ensuring consumer health and safety, and regulatory compliance. Monitoring data can also be used to detect issues with and improve the efficiency of control equipment through reviewing historical data.  

The basic elements of a digital data logger are the sensor (or multiple sensors), electronics for converting the sensor signal into digital data, memory, communication hardware, and a power source. More sophisticated data loggers can also have display screens and alarm features. We’ll talk about the available options for some of these components throughout this article.

First, let’s consider the sensor. There are four main types of sensors used in data loggers, which can either be embedded within the unit, or external to it and connected by a cable (if, for example, the sensor is in an extreme environment or needs to be enclosed in a buffer).

Temperature monitoring is the most common use for data loggers. These are found in industries that deal with pharmaceuticals and vaccines, medical devices, perishable food storage, and aerospace manufacturing. Some of the raw materials and products in these areas can degrade if they aren’t maintained within a specified temperature window. In addition to measuring ambient environmental conditions, temperature loggers can also be used to monitor  temperatures in freezers, refrigerators, shipping containers, or in manufacturing equipment like reactors and ovens.

A number of different sensors are available for this type of data logger, including thermistors, resistance temperature detectors (RTDs) or thermocouples. The best sensor for a given application is a balance between cost and performance specifications like temperature range, accuracy, and drift.

One area where temperature monitoring is needed is the distribution and storage of vaccines. Most vaccines will lose potency if they aren’t kept in a narrow temperature window, and when they degrade, there may not be obvious visual indications. This can have wide-ranging consequences, since patients who receive ineffective vaccines will incorrectly believe they have been immunized. Because of these risks, the CDC, WHO, and the VFC (Vaccines for Children) program provide extensive guidance for vaccine cold storage, including recommendations for the use of digital data logging.

Relative Humidity (RH) is similar to temperature monitoring, since it can impact the shelf life of some products, and cause problems during manufacturing. For example, humidity can affect pressing powders into tablets, or curing advanced coatings and composite materials. Because temperature and humidity are commonly measured in the same locations, some data loggers incorporate sensors for both in a single unit.

Another reason that humidity is a safety-critical parameter is that a minimum level of RH is needed to reduce static electrical discharges, making it important to monitor in areas with sensitive electronics, like surgical robotics. Likewise, RH must be kept below a critical level to prevent microbial growth.

Data loggers that measure differential air pressure can be used to monitor areas where air pressure is used to control the flow of small particles (viable and non-viable), like bacteria, viruses or dust. This is used in hospitals, labs, and semiconductor manufacturing facilities to prevent cross-contamination. (Loggers also exist for measuring higher fluid pressure, which can be used in process monitoring and water treatment.) Depending on your application, negative pressure rooms used to contain particles (Emergency rooms, BioHazard areas) or positive pressure rooms (e.g clean rooms, surgical suites) need these sensors to ensure safe conditions.

Voltage and current data loggers can be combined with external sensors to measure less common properties. For example, sensors are available to measure conditions like fluid level, flowrate, pH, or dissolved oxygen, typically generating industry standard signals (1-5 V or 4-20 mA), that can be used as the data input for this type of logger.

Loggers set up to measure higher voltage and current ranges can be used in manufacturing and HVAC applications, for example to monitor the equipment health and power consumption.

How long do data loggers last? Do they ever need to be replaced?

Most data loggers themselves are not consumable, but contain parts that should be replaced or calibrated on a regular basis.

Calibration is needed when sensors are first installed, and periodically to correct sensors that drift or become contaminated over time. Sensor recalibration is an important aspect of ongoing data logger maintenance and should be included in the overall quality management system for facility and process monitoring. To simplify calibration and minimize downtime, data loggers can be designed with easily replaceable sensors so that calibrated sensors can be quickly swapped in.

Most modern data loggers are powered through building AC, auxiliary DC, or power over ethernet (PoE), with rechargeable battery backup. In locations where this is not possible, loggers can also be battery powered, with replacement intervals in the range of months to 1-2 years.

A notable exception to this are compact, simple temperature monitors that are meant to travel along with shipments of sensitive materials. These monitors are made to be inexpensive and simple, often with non-replaceable batteries. These are useful in cases where temperature monitoring is needed during shipping, but returning the monitor to the supplier is not feasible.

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How do data loggers store my data? How do I retrieve it?

There are different approaches for this, and the right choice will depend on your application. For cases where real-time access to data isn’t needed, data can be stored in the logger, for later retrieval via USB. Local storage is inherently limited, so an important specification for this type of logger is the number of data points that can be stored.

These loggers typically have user-defined data sampling intervals, and the total amount of time the logger can run without retrieving data is determined by both factors. For example, a data logger with a capacity for 100,000 readings set to collect 1 reading per minute can run for 100,000 minutes, or about 70 days, before exceeding its storage capacity. The data sampling interval can also affect battery life.

A second option is to transmit data wirelessly from the logger via bluetooth or NFC (near-field communication). This can be useful in applications like shipping containers, where real time access to data is needed, but the logger cannot be easily accessed.

However, the state of the art in this area are data loggers that upload monitoring data continuously to a cloud-based software tool. This makes it possible to monitor and react to environmental monitoring data in real time. This approach is particularly powerful when you have a distributed network of many sensors, and is well suited for businesses that plan to scale up in the future. These loggers transmit data to a central location via WiFi or ethernet, and usually have a data backup within the unit in case connectivity is lost.

How do data loggers alert me if there is a problem?

Here again, there are different levels of sophistication depending on the application. The simplest data loggers can trigger audible and visual alerts, so that on-site staff can respond to issues.

Using a data logger that can remotely transmit data also allows the use of customized alarms. For example, advanced data loggers can be set up to send an SMS text, phone call, or email when a problem is detected. These messages can include a description of the problem, so that response staff are prepared when they arrive.

Historical data can be used to set the optimal alarm triggers, to ensure fast response by the right personnel, escalation when needed, and avoid alarm fatigue caused by excessive false or nuisance alarms.

Conclusions

Data loggers are an important part of a comprehensive environmental monitoring and quality management system. They are used across a wide variety of industries, including some where environmental conditions are important for ensuring consumer health and safety.

There are a number of different features of modern data loggers, including the variable they measure, the way they are powered, store, and transmit data, and how they alert users of a problem.

For assistance on selecting and using data loggers, contact the experts at Dickson.