Chart recorders were the industry standard for monitoring temperature and humidity for more than 70 years. But the technology is simply not accurate enough for applications such as cleanroom monitoring, pharmaceutical cold-chain validation, and the manufacturing of medical devices and equipment. Data loggers provide a more reliable and precise method for such critical processes. Wireless and pager-sized devices record temperature and relative humidity (RH) in a unit that combines a sensor, memory, power supply, clock, and microprocessor. Many data loggers use proven thermistor and capacitive RH-sensor technology to provide accurate and reliable readings and some systems have tamper-proof options for complying with 21 CFR Part 11.

Hazards of humidity

Environmental monitoring is critical, for example, because static electricity caused by low levels of RH can compromise the integrity of electronic devices such as defibrillators or pacemakers. And high RH levels can change component dimensions, reducing the quality of close-tolerance assemblies. Excess humidity can also undermine the effectiveness of product coatings and even prevent devices from functioning properly. Make these considerations when comparing chart monitors with data loggers:

Ruggedness

Chart recorders comprise complex and interconnected parts that gradually deteriorate. Pens clog, leak, or dry out, which results in data loss. Paper charts are easily damaged and susceptible to shrinkage or expansion due to heat or humidity.

Data loggers, on the other hand, are unsusceptible to mechanical failure. Missing a download won't fill the logger and halt recordings. Data loggers have plenty of excess storage capacity. A logger set to sample once per hour, for instance, would take more than a year to fill its memory. And while paper charts can get lost or fade over time, users can back-up data-logger information just as they do other electronic files.

Accuracy

Using a chart recorder to monitor RH in a room may not provide an accurate reading because RH can vary a lot through a given space. However, a strategic placement of several data loggers provides a thorough and accurate reading.

Effect on ambient conditions

Chart recorders generate enough heat to affect the ambient temperature in small spaces. Even a small temperature variation can significantly affect humidity levels, so the RH readings may not be accurate. Data loggers, though, consume significantly less energy and generate virtually no heat.

Reliability

Chart recorders rely on power connections, making them vulnerable to power outages, the exact situation in which they would be critical to identifying how power loss may have affected RH. Data loggers, on the other hand, operate on long-lasting (up to 10 years) internal lithium batteries.

Alarm capabilities

Chart recorders do not have reliable alarm-notification systems to warn users when RH levels rise outside of acceptable parameters. Users must frequently check recorders to see whether RH is out of compliance. A few data-logger systems, however, feature alarms that, for example, trigger an e-mail message, freeing staff to complete other tasks.

Data handling

In most cases, paper charts limit the amount of data that can be recorded to seven days. Data loggers, though, provide electronic information that users can, for example, export into spreadsheet software to generate tailored graphs and reports and calculate minimum, maximum, and average values. With many recorders, users must be physically present to view results. Some data loggers, on the other hand, let users view real-time readings on PDAs, desktop PCs, or network connections.

Cost

Though cost is not the primary reason companies switch to data loggers for RH monitoring, most are pleasantly surprised at the savings. For example, it can take 15 minutes a week to service a recorder and a technician's time typically costs about $40 per hour. This is a labor cost of $520 annually. For a recorder requiring calibration twice a year the annual cost is around $300. Recorder paper and pens can cost about $20 and $30 each per year. So the annual cost of maintaining and calibrating a chart recorder can hit $870.

In contrast, a typical data logger costs $450, the software about $250, and calibration $150. The first year total cost is about $850. But a data logger requires no weekly maintenance. With calibrations necessary just once a year, a data logger easily pays for itself in less than a year.

Fear of change

Fear of change and the conservative nature of the medical manufacturing industry causes many companies to continue using the old-school technology despite readily available modern and effective options. The fear is understandable because most individuals are apprehensive about switching from a known system even if the new technology has proven, significant capabilities. Many other industries have all but abandoned chart recorder technology as the RH-monitoring standard.

Users having experience with first-generation data loggers also tend to be wary. Originally, the HVAC market drove the development of data-logger technology. The stakes were relatively low, and reliability and accuracy reflected this fact. But over time, data-logger technology evolved and today's devices are exceedingly accurate and precise.

How data loggers work

Most relative humidity (RH) data-loggers use a capacitive RH sensor system. The sensor is made up of two conductive metal plates separated by a thin insulating (dielectric) layer. Upon absorbing and diffusing water vapor from the surrounding environment, the conductive plates retain a small electric charge, known as electrical capacitance, the value of which depends on the amount of water vapor detected. The logger measurement system translates the electric capacitance into a raw number, which the data logger then converts into an RH percentage.

The electric capacitance generated by different sensors within the same environment could potentially be quite different because no two pieces of equipment are exactly identical. To eliminate this variation, data loggers, like any measurement system, should be calibrated before use. Calibration involves comparing raw measurements from a specific sensor with known RH values in a reference chamber and assigning the actual RH percentages for each potential raw measurement.

The sensitivity of capacitive sensors to the surrounding RH changes as a function of temperature. All other things being equal, sensors may give a different reading for the same RH at 25°F than at 75°F. To ensure the most accurate RH measurement, data loggers are calibrated at multiple temperature points.

Once installed, the data logger measures the ambient temperature (and subsequent changes) and assigns RH values based on calibrated response of the RH sensors to the temperature. A basic data logger requires calibration at two or three different temperature points to be suitable for non-demanding applications. More advanced systems for critical applications, however, are calibrated at many different temperature and RH points to ensure consistent and accurate RH readings.