Dataloggers

Introduction

Datalogger refers to a battery powered device equipped with sensors and a microprocessor to monitor and record data such as temperature [T], relative humidity [RH], light, voltage, etc., Datalogging systems usually include proprietary software to initiate and set parameters for monitoring, downloading, viewing and analyzing data.

This discusses selection criteria focusing on stand-alone loggers (each unit operating independently) rather than networked systems that provide real-time data using wireless or hard-wired technologies. Criteria are divided into two sections: the first lists basic hardware specifications, and the second discusses software related issues. At the end of the page is a listing of dataloggers that were evaluated in 2011 using these criteria. It is hoped that additional products will be added by other users over time. Links to pages on the various dataloggers will be given below and on their pages will list how the loggers performed according to the criteria here. This informatin is an update of the National Park Service Conserve O Gram 3/3 Datalogger Applications in Monitoring the Museum Environment, Part I: Comparison of Temperature Relative Humidity Dataloggers 2001. See also the NPS Museum Handbook, Part I, 4, Museum Collections Environment.

Methodology

The loggers discussed below are currently the most prevalent in the cultural heritage community, based on queries and responses posted on several of the largest preservation listservs, or were suggested in discussions with manufacturers and distributors. Information was obtained from specification sheets or the manufacturers and distributors. Demonstration units of each logger were obtained from the manufacturers or distributors and run in a series of three environmental chambers at stable temperature with saturated salt solutions of low, mid and high RH. The data was used to evaluate the software and assess logger performance.

Datalogger Hardware Specifications

This section compares hardware specifications for ten different dataloggers. How to evaluate these specifications is explained below.

Operating Range

The operating range of a datalogger is determined by sensor type and quality. Manufacturers specify range using different terms (e.g. operating, working, reading, or sensor range) but they are not necessarily synonymous. Specification sheets may use one term to indicate the physical limits to which a unit can be exposed, and another to indicate the working range for the sensor. All the loggers listed have functional ranges beyond the temperatures expected in a collection environment and may even function during heat or freeze treatments. A few do not function well at low RH (under 20%). None are intended for outdoor environments or exposure to high RH for prolonged periods.

Accuracy

Accuracy is a function of the sensor type, quality and type of calibration and will greatly impact cost. Accuracy may differ across the functional range of the logger.

Calibration

Some loggers are given a basic, one point calibration, generally performed at room temperature and 50% RH, guaranteeing that the logger will be accurate at a mid-range RH. A logger calibrated at two or more points is checked at an additional low, or high RH level and is more likely to take accurate readings across its range. Loggers arrive factory tested for accuracy within the manufacturer’s specifications, which is generally sufficient for collections monitoring. Calibration to a traceable standard (e.g. National Institute of Standards and Technology NIST) may imply greater accuracy, but is rarely necessary for museums.

Electronic sensors are more durable and have faster response times than hair hygrothermographs or hygrometers but they also require periodic recalibration. Manufacturer recommended recalibration times range from 6 months to 3 years, with most advocating annual calibration. Some loggers allow the user to compensate for minor sensor drift by recalibrating the unit using accompanying software or adjusting the data in the software after download. Most, however, must be sent back to the manufacturer if they are not performing within their accuracy parameters. Factor the cost of factory recalibration into the lifetime cost of a logger.

Before returning a logger to the company, check the calibration of your unit using small chambers and saturated salt solutions. Use this procedure before installing newly purchased units. Discard inexpensive units that are found to be inaccurate, as recalibration is not cost efficient.

Power Source and Battery Life

Battery life is a function of battery type and the sampling rate. A logger set to read every five seconds will not last as long as one sampling every hour. Specifications are usually based on a moderate sampling rate (i.e. sampling every 20 - 30 minutes) as is generally recommended for long-term collections monitoring. Features such as Liquid Crystal Display (LCD) or Light Emitting Diode (LED) alarms also impact battery life. All loggers listed below have non-volatile memory, i.e. data is saved even if the battery is removed or dies. For general trend monitoring, a logger should have enough battery life to provide one full year of monitoring. A shorter battery life may be acceptable for special needs projects.

Memory Capacity and Run Time

Memory capacity is listed by the number of paired readings (T and RH readings) while others list the total number of individual readings. To compare these figures, divide this total by the number of operating channels, which is generally two: one for T and one for RH. Some loggers allow extra channels to be enabled for additional features (e.g. external probe) or other types of readings (e.g. light). If more channels are activated, fewer sets of readings will be collected. To facilitate comparison, all product specifications have been translated into the number of pairs of T and RH readings.

Memory capacity is impacted by the sampling rate (see below). To compare the combined impact of memory capacity and sampling rate, the loggers were launched for a 15 minute sampling period. The duration that the logger can run at this interval before reaching memory capacity is listed. Alternative data is provided for the few loggers where 15 minutes was not a selectable option.

Sampling Rate and Start/Stop Options

Most loggers allow the user to set the sample rate (how frequently a T and RH reading are taken) during the launch process. Intervals range from 1 second to 24 hours, lasting days or years. Users can also select various start and stop options during the launch. All loggers offered at least two of the following activation options:

• immediate launch
• delayed start: user sets date and time in the future
• trigger or push button: user activates logger or uses a magnet to trigger the logging process

Users can determine if the logger stops recording when the memory is full or overwrites earlier data to continue recording. If the former option is chosen, downloads should take place before the memory reaches capacity to ensure that there is no gap in monitoring. If the latter option is selected, download the logger regularly to ensure that data is not lost.

Alerts / Alarms

During the launch process, many loggers allow users to input upper and lower limits of a target range. If the parameters are exceeded, an alarm setting will be activated resulting in a visible alert on either an LCD or LED. This feature indicates that an environmental event occurred and data should be downloaded and reviewed. Alarms negatively impact battery life.

Display

Several loggers offer a LCD for a visible display of real-time data. This is useful for checking current conditions without downloading data. However, if the logger is not in a readily visible or frequently accessed location, or real time readings are not needed, this feature may not be worth the corresponding drain on battery life.

Probes

External probes can be attached to some loggers. Probe sensors are smaller than the loggers and can be placed into hard-to-access locations such as inside a sealed case with the sensor cable leading to the logger on the case exterior where it is accessible for downloading.

Size, Appearance, and Construction

Size may be a determining factor in monitoring inside vitrines, buffered frame enclosures, storage cases, and shipping crates. Casings are normally made of hard plastic butcolors and shapes vary. Small loggers are unobtrusive, but larger loggers are more easily secured in place, a consideration when installed in public spaces. Consider mounting options such as magnetic backing, keyhole mounts, locking eyelet, and mounting brackets before purchase.

Data Retrieval, Viewing and Analysis

In this section factors that impact the user experience are evaluated. A good piece of hardware can be rendered unappealing by poorly designed software. While software evaluation is subjective, good software will be easy to use and provide convenient data manipulation options.

Data Retrieval

Data on standalone loggers is retrieved by connecting in-situ to a laptop or bringing the unit to a desktop computer. If this is impractical, consider loggers that provide alternative download methods such as an option to download using a flash drive or proprietary data collection device. A few of the loggers listed in this COG have wireless functionality or related wireless models which is noted when applicable.

Software Platform

Confirm that the logger software is compatible with your Microsoft Windows platform. Software options for Mac users are limited. Most companies provide free software upgrades over time.

Data Formats

All logger software packages download data into a proprietary format and have basic export abilities. Some format directly to MS Excel and others to Comma Separated Value (CSV) or text format which can be imported into other software. A few allow data to be easily imported into the Image Permanence Institute’s Climate Notebook or www.pemdata.com software. The ability to format a graph in one step for a report or email is a great convenience.

View Raw Data in Table Form

While all loggers allow for graphical representation of the data, several also allow viewing of the raw data in table form without exporting to a program like MS Excel. This is useful to identify specific problem in the raw data.

Download while Logging

This function allows for data backup or retrieval from the logger without re-launching. Monthly downloads on a logger without this capacity will result in 12 short graphs. Loggers that continue logging during the data retrieval process provide a single, long dataset. This avoids spending time organizing or combining multiple data sets.

Combining and Appending Data Sets

The ability to append data to combine short datasets into longer trends is useful if the unit cannot download while logging as discussed above.

Overlay Graphs for Comparison

This key feature allows for easy visual comparison of the conditions of different monitored spaces.

Ability to Modify and Print Graphs

Good software allows the user to modify, manipulate and print graphs. The table notes whether the software offers low, moderate or extensive options, including: changing graphics (line color, appearance, fonts, background), selecting the range of axes, easily zooming in and out, adding or editing graph titles, and annotating the graph or specific data points. The graph should display the time, date, and year of the readings and allow for setting printing options like portrait or landscape view.

Provides Statistics for Data and Subsets of Data

The software should clearly show basic statistics (high, low, mean, average readings, etc.) for the full dataset as well as data subsets.

View Target Ranges for Data

The ability to view graphically represented target ranges for data as noted in the section on Alerts/Alarms.

System Costs

Project budget is a driving force in product selection, but a system lacking needed features will be a poor investment over time. The unit price of the tested loggers ranged from $68 - $740 as of September 2011. Prices are subject to change. Some loggers will incur ancillary costs for a software, cables or a laptop computer or portable download unit, while others have a direct USB connection or free software eliminateing extra charges. Check for volume discounts or kits that bundle loggers with software and accessories for additional savings.

Service and Technical Support

Consider company reputation, reliability, quality of service and technical support when making a purchasing decision. Many companies allow tests of their software via free Internet download.

Conclusion

All loggers listed in this COG are appropriate for monitoring environmental conditions in cultural institutions. Some are better suited to general long-term monitoring and others for special applications. Considering all the variables outlined in this COG Should facilitate choosing a logger appropriate to your project needs.

Products

The products listed here were evaluated as part of the original research that resulted in this text. They are indicated by (2011). It is hoped that there will be subsequent additions to the list as new products emerge. Follow the link to the individual logger page to see how each product performed according to the evaluation criteria above.

• ACR
• EasyLog
• Hanwell
• PEM2
• LogTag
• Onset Hobos
• T&D
• Track-It
• Tinytag
• Veriteq

References

Daly, Gregg M., and Hugh J. Flye. “Dataloggers Deliver.” Engineered Systems 17, no. 8 (2000):84-89.
Morris, Patricia. “Setting Up an Environmental Monitoring Program”. Colorado Libraries (2004): 31-33
Museum Handbook, Part I, Chapter 4: Museum Collections Environment (1999) [1]