Temperature, Humidity, Pressure & Voltage: A Guide to Data Logger Functions

At the most basic level, data loggers are stand-alone units that record a stream of information from a sensor, or a set of sensors. The data logger stores this information for later review, analysis, and archiving, and may even trigger alarms if a problem is detected. They are a critical component of environmental monitoring systems, and are commonly used to measure temperature, humidity, pressure, and voltage.

If you’ve reviewed the data loggers on the market today, you’ll know that there are many variations available. For example, they can differ in the way they are powered, how they transmit data, how many data channels they have, and what types of environmental conditions they are set up to collect.

In this article, we’ll take a closer look at some of the important considerations when choosing a data logger, and discuss real world examples of how the four common measurement functions are used.

Why Data Logger Choice Matters

Data loggers are a key component of modern environmental monitoring systems. These systems are used to protect sensitive assets, like medicines, biological samples, or perishable foods, with the goal of preserving their value and protecting the health of consumers.

There are many suppliers of data loggers, and many customizations available from each supplier. So it’s important to take some time to assess your needs to make sure you’re selecting the right one.

Here are a few things you’ll want to consider:

1、Regulatory compliance. In heavily regulated industries, like drug manufacturing, the environmental monitoring system falls within GxP (good practices) regulations. We’ll talk about this in more detail in the sections below. In highly regulated industries, it can be very helpful to work with a vendor who has designed their data loggers and other equipment with the regulations and best known practices for environmental monitoring in your industry in mind.

2、Cost savings. This becomes a more critical issue when setting up a network with many monitoring points. For example, there can be significant cost savings if you can tie a number of sensors into a single data logger, or integrate various types of sensors into a single logger. It almost goes without saying that, in the long run, the cost of lost assets, regulatory compliance problems, and potential hazards to human health far outweigh the time and capital cost of setting up and maintaining a robust monitoring system

3、Ease of use. Particularly in fast-paced environments, where onsite staff have diverse and demanding job roles, like clinics where vaccines are being administered, having the right data logger setup can remove an unnecessary burden. In these environments, data loggers that automatically transmit and store environmental conditions for later review, and flag potential issues only when intervention is needed, can allow busy staff to focus on critical tasks.

4、Scalability. In some cases, it can be appropriate to use very simple data logger technology, like a pen and paper chart recorder. If, however, you plan to grow your business by adding new production lines, new warehouse space, or additional distribution routes, you should plan for that when selecting what type of data logger to use. For example, data loggers that wirelessly transmit data to a cloud-based software tool are inherently scalable.

5、Trust, reputation, and vendor expertise. Since safeguarding your assets is one of the roles of the environmental monitoring system, it is important to work with a vendor that has an established track record, and whose products and people can adapt to the changing regulations and needs of your industry.

Temperature Data Loggers

Temperature is the most common data logger function. As we’ll discuss below, temperature monitoring and control is critical across a wide range of industries and applications. These data loggers have sensors that are generally based on thermistors, resistance temperature detectors (RTDs) or thermocouples, which can be either integrated into the data logger box, or external. The choice of sensor type is based on cost, response time, temperature range, accuracy, and other factors.

One area where temperature loggers are used is pharmaceutical manufacturing and distribution. In manufacturing, environmental temperature can be critical for overall process control to ensure consistent, high quality final products. Examples of sensitive areas include rooms where raw materials are stored, or where products are held and packaged. Finished pharmaceuticals can also be temperature sensitive, and holding them at a specified temperature range can be critical for maintaining their value and protecting patient safety. For this reason, temperature control as it relates to pharmaceutical manufacturing and distribution is federally regulated and enforced by the FDA.

Vaccines are a special case, since they must be held within a narrowly defined temperature range after manufacture to maintain their potency (typically +2 to +8 °C, -50 to -15 °C, or even lower for sensitive mRNA based COVID-19 vaccines). This means they are shipped via a temperature controlled cold chain, and held under refrigeration until use. The CDC has published clear, detailed guidelines for vaccine storage in their toolkit, which recommends that digital data loggers be used for temperature monitoring in areas where vaccines are stored. (Also note that digital data logging is a requirement for vaccines distributed through the Vaccines For Children program). Choosing the right data logging approach for a vaccine cold chain can greatly simplify this and ensure regulatory compliance.

Temperature monitoring is also used in healthcare for human plasma and tissue sample storage. This ensures, for example, that blood products or pathology specimens don’t degrade during storage.

Quality control in the production and testing of medical devices through GMP is regulated in a similar way to pharmaceuticals. Due to safety concerns, manufacturing and maintenance in the aerospace industry is also highly regulated, as discussed in the AS9100 quality management standard. In both of these cases, temperature monitoring and control is part of assuring quality and repeatability when manufacturing precisely controlled parts from advanced materials.

Storage and distribution of perishable foods and beverages is another area where temperature data loggers are commonly used. The needs in this area are diverse, with frozen foods, produce, meats, prepared foods, dairy, and dietary supplements all requiring different, controlled environments. Increased concern over foodborne illness has caused evolution in regulations in the industry in the last 10 years, mainly through passage of the Food Safety Modernization Act (FSMA) in 2011.

Humidity Data Loggers

Humidity sensors in environmental monitoring data loggers typically measure relative humidity or RH. This is the concentration of water in the air relative to how much the air can hold at the current temperature. At 100% RH, fog or dew would begin to form. 30-50% RH is considered a comfortable, safe range for indoor environments.

Humidity monitoring goes hand in hand with temperature monitoring. In fact, it is common to have sensors for both integrated in a single unit. Similar to temperature, relative humidity is important to monitor in pharmaceutical, aerospace, and medical device manufacturing, food storage, and any other industry where the presence of moisture can affect the quality of the finished material.

Let’s look at pharmaceutical manufacturing first. Here, humidity can have direct and indirect effects. One direct effect is the absorption of water from humid air into in-process materials, which can affect their properties and the quality of the final product. For example, the level of humidity in the air can affect the pressing of fine, high surface area powders into tablets. When water-based coatings are used, humidity can affect the drying rate and the characteristics of the coating.

Indirectly, humidity helps to dissipate static charge buildup. As a result, static discharges are more likely in low-humidity environments, which is why we feel static shocks in dry weather. This can be hazardous when flammable solvent vapors are present. This effect is one reason why sensitive manufacturing environments have a minimum RH limit.

Similar to temperature fluctuations, changes in humidity can impact the manufacturing of sensitive components for aerospace and medical device applications. Moisture content during curing of advanced composites and coatings can affect their properties, while electrostatic discharge in low humidity environments can damage electronic components.

Humidity is a special concern in certain critical areas of healthcare facilities, like operating rooms, newborn nurseries, and exam rooms. In these areas, humidity must be kept within certain ranges, which are published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) in the ASHRAE Standard 170. Humidity levels that are too high can lead to the growth of microorganisms, while static discharges caused by low humidity are a hazard in procedure rooms where robotics or other sensitive electronics are used.

Pressure Data Loggers

When we talk about pressure data loggers, there are two quite distinct applications. First, pressure loggers that are designed to measure and record the pressure of process gases and liquids. For example, measuring the pressure of compressed air running to a pneumatic tool, or of a liquid being fed into a reactor, which could be critical process parameters for municipal water systems or sensitive manufacturing operations. The relevant pressures for these applications can be up to tens or hundreds of psi (atmospheric pressure is about 14.7 psi).

A second type of pressure monitoring is differential pressure. In this context, this refers to holding certain areas in a building at slight negative or positive pressure to control the movement of airborne materials, typically particles or microorganisms. For example, cleanrooms used for sensitive semiconductor or pharmaceutical manufacturing processes are held at positive pressures relative to the outside environment, to prevent particle-laden outside air from rushing in.

Similarly, positive differential pressure can be used to prevent bacterial contamination in operating rooms and other sensitive areas in a hospital. Negative differential pressure in rooms where infectious diseases are present reduces the risk of transmission outside the room.

Voltage Data Loggers

Voltage loggers are also available in different ranges, and can commonly measure both voltage and electrical current. High range loggers can be used to directly measure the electrical power supplied to industrial process equipment, which can be useful in manufacturing and HVAC.

Lower range voltage and current loggers are designed to monitor and record the outputs of remote sensors. Remote sensors with their own power supplies typically send output voltage or current signals, normally 1-5 V or 4-20 mA, that can be recorded by this type of logger. This makes a voltage logger highly adaptable, since they can be combined with any sensor that uses one of these industry standard output signals. This is helpful when measuring a property that isn’t available on a commercial data logger, or needs to use a custom remote sensor for some other reason. Examples include logging tank levels, flowrate, pH, or rotational speed.

Conclusion

Data loggers are versatile tools that are a critical part of any modern, efficient environmental monitoring scheme. Here we’ve covered four common measurement functions of data loggers: temperature, humidity, pressure, and voltage. Monitoring and controlling these parameters is part of a comprehensive plan to protect assets and ensure regulatory compliance.