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How to Choose a Data Logger

Digital data loggers represent the state of the art in environmental monitoring technology. They're compact, versatile, and can easily be scaled up to keep pace with a growing business.  

Modern data loggers are available with many different features and options, from the environmental variables that they measure, to how they store and transmit data. In this article, we’ll provide a list of questions that are important to answer when choosing data loggers for your business.

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What variable are you monitoring with your data logger?

This is a good starting point when filtering data loggers. The most common variables they measure are temperature, humidity, and pressure. These three will cover most common situations, but we’ll explain how to deal with less common measurements at the end of this section.

Temperature loggers are the most common, since temperature is a critical variable in many industries. For example, temperature is important for preserving the potency of vaccines and other medicines, preventing the spoilage of produce, curing advanced composite materials, and many other sensitive processes.  

Data loggers can be used to monitor temperature in ambient settings, like in warehouses and processing areas, or in temperature-controlled chambers used in manufacturing, like ovens, incubators, or freezers.

Within temperature data loggers, you’ll find that there are different sensors to choose from. In most data loggers currently on the market, the sensor is either a thermocouple, thermistor, or resistance temperature detector (RTD). All three of these generate an electrical signal (voltage or resistance) that is proportional to temperature, which the data logger automatically converts into a real temperature. Each type of sensor has its own strengths and weaknesses, so the sensor is usually selected on a case-by-case basis depending on the application.

Humidity is also an important environmental variable, and in fact some loggers incorporate sensors for both humidity and temperature. Like temperature, humidity affects the degradation of finished products, as well as the processing of moisture-sensitive materials like fine pharmaceutical powders. Poor humidity control can also lead to microbial growth.

Humidity is an important factor in environments where static electrical discharge poses a fire hazard, or a risk to sensitive electronics. This is because moisture in the air can help to dissipate static buildup before a discharge happens. (This is why we feel static electric shocks around our homes more in dry weather.) For this reason, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends specific humidity ranges for sensitive areas in healthcare facilities in their ASHRAE Standard 170.

Data loggers for pressure are configured to measure in one of two ranges: differential pressure (low range), and static (or process) pressure (high range).

The first type of monitor is used in situations where specific parts of a facility are held at small positive and negative pressures relative to other parts, or to the outdoor environment. This is used to prevent cross-contamination between these sensitive areas and the outside. For example, some semiconductor manufacturing facilities are held at a positive differential pressure. This causes a small current of air to flow out from places where the walls of the facility are not airtight, which prevents particles from flowing in. Hospitals use positive and negative pressure in sensitive areas in a similar way.

Loggers configured for higher pressures are used to monitor process fluids like cooling water and pressurized gases. Different sensors are used for the two ranges. Static pressure monitors typically have a full range of hundreds of psi, while ambient differential pressure monitors have a typical range of 1-2 inches of water, where 1 inch of water is equal to 0.036 psi.

Some applications might call for measuring less common parameters, such as pH, flowrate, power consumption, rotational speed, or light intensity, to name a few examples. These can be measured using a data logger too, by combining custom sensors that output industry standard signals (0-10 V or 4-20 mA) with a logger configured to take these electrical signals as an input.


How many monitoring points do you have?

Among other things, the scale of your monitoring network will drive the decision of how your data loggers should transmit data. For example, if you have a simple application, with a limited number of monitoring points in a single facility, loggers that store data locally for later download by USB could be a good choice (we’ll talk about how often data has to be downloaded from this type of logger in the memory/storage section).

For larger operations, where there are many monitoring points, or where the points are spread out over a large area, you can consider loggers that continuously upload data to a cloud-based storage and analysis tool. This approach saves staff the time and effort needed to manually download data, while integrated monitoring software provides a way to quickly visualize and analyze data from the network of sensors.

Another way to think about this is the relative scalability of different data logger setups. A system of remote loggers tied into a software tool is much easier to scale up by adding new monitoring points compared to a set of independent monitors that require manual data collection.

What conditions will the data logger be placed in?

Since data loggers are a relatively mature technology, a few features have been developed over time to make them usable in difficult environments. Here are a few things to consider:

  • Extreme temperatures. For measurements in very high- or low-temperature environments, external sensors can be used so that the entire logger is not exposed to extreme conditions. Keep in mind that the target temperature also affects the choice of sensor.  
  • Temperature fluctuations. In some environments, the air temperature can fluctuate quickly, even under normal conditions. For example, opening the door of a vaccine storage refrigerator briefly increases the air temperature inside the refrigerator. However, due to the higher thermal mass of the vaccine vials, these fluctuations will not significantly affect their temperature. In these cases, the temperature sensor can be placed in a buffer vial that better reflects the temperature of the vaccines (this is the recommended practice by the CDC).
  • Dust and moisture. Particularly in outdoor environments, the ability of the housing to protect the sensitive internal hardware from dust and moisture is key. If selecting a data logger for this type of environment, make sure to check its IP (ingress protection) rating.
  • Availability of wireless signal. There are a number of ways that a remote logger can transmit data. WiFi is one, but if this is not available, other types of remote communication are available, including RF, ethernet, and bluetooth.
  • Availability of power. In some cases, it will be possible to power data loggers using the building AC or power over ethernet (PoE), but battery-powered loggers are also available. Hard-wired loggers should also include a battery backup for uninterrupted monitoring during power outages.


Does the data logger need to fit in a tight space?

Very small data loggers, reduced to the bare minimum of features, are designed for applications where space is at a premium. For example, there are compact, remote temperature monitors that are designed to travel inside packages of materials that are being shipped via a refrigerated supply chain (a “cold chain”).

These loggers can have non-replaceable batteries and can even be treated as consumables, and might not even be returned by the receiver of the shipment when it’s impractical.


How Much Memory/Storage Is Required?

While digital data loggers have a relatively large storage capacity compared to older technologies like paper chart recorders, their internal storage space is still limited. For a logger where data is periodically downloaded by USB, internal storage is used to hold the data between downloads. Loggers that transmit data remotely can also have internal storage, used as a backup when access to the network is unavailable or the network is down.

The storage space on a digital logger is usually specified as a maximum number of data points. However, the more meaningful figure for operation is the amount of time the logger can run without exceeding its memory. To calculate this, you’ll also need to know your data collection frequency. This is typically a user-defined parameter, and will depend on the time sensitivity of your application.

Once you have that information, you can use this equation to calculate the maximum time the logger can run before exceeding its storage capacity:

            Maximum storage time = (data point capacity) ✕ (measurement interval)

For example, if the logger has a capacity of 16,000 points, and you’re using a measurement interval of 2 minutes, the amount of time the logger can run without exceeding its memory is 32,000 minutes, or a little over three weeks. Shortening the measurement interval to 30 seconds would reduce this time to about 5.5 days.


Conclusion

Environmental data loggers are adaptable pieces of equipment that can be deployed in diverse situations.

Because of their versatility and the maturity of data logger technology, there are a number of customizable features that you’ll need to consider when purchasing a data logger. The questions presented in this article will give you a good start for determining the exact logger to match your application. For further assistance and more detail, contact the experts at Dickson.