Data storage typically involves either proprietary data archives or relational databases. The current leaders in process data historization rely on their own proprietary data archives. Relational databases generally don’t deliver the features or performance of custom or proprietary data archives that have been designed specifically for the storage of time-stamped process data.
Many data historians will allow you to store a subset of the total number of data values scanned for each measurement point. This “data compression” helps to efficiently store your process data within a prescribed deadband. Data compression is one of the least understood mechanisms of data history — yet its intelligent implementation can lead to significant performance improvements. For example, you may want to historize the state of a pump (running/stopped) by scanning the current pump state every second. If the state changes once per hour, storing the one-second pump state values is redundant and unnecessary. By storing only the changed value, you can reproduce the same history with far fewer values and less storage space. This speeds retrieval of the pump state values because fewer values were stored. For continuous readings such as temperatures and pressures, storing data within an allowed deadband can prevent storage of instrument noise or data with more precision than the instrument you are reading.
Accessing Your Historized Data
The tools and applications for accessing your historical data are as important as the data themselves. The ease-of-use, reliability and features of the tools will drive the acceptance and popularity of your historization solution with its users. The majority of users will access your data through three methods: graphical data displays, ad-hoc analysis and reports.
Graphical displays present process data through a collection of historical and real-time objects such as trends, values and bar charts, along with static and dynamic process equipment and pictures. One benefit of a graphical display application with history is the ability to take a real-time HMI display and replay the display over time. Although web browsers aren’t always the best framework for viewing graphical content, many applications have moved to web-based solutions because they don’t require installations and upgrades for each client machine. Newer technologies are allowing companies to produce products that merge the benefits of web applications and desktop applications, providing rich graphical displays with zero or little installation.
Microsoft Excel is the most popular tool for process data analysis and reporting. Most historians allow you to easily retrieve the results of simple and complex data queries directly into Excel. Some tools will perform complex aggregatation or filtering of data before they are retrieved while others retrieve raw or sampled historical data and rely on aggregation and filtering within the spreadsheet.
Although Excel can be used for report creation, its popularity stems from its capabilities as an ad-hoc analysis tool. Applications that concentrate on report generation, such as SQL Server Reporting Services, are far more powerful than standard spreadsheets for report generation.
It’s important that your historical data doesn’t exist within a black box that’s only accessible through the vendor’s applications. This implies that the vendor permits external access through its own programming interfaces as well as supports more open standards such as OPC and OLEDB at reasonable costs. After all, these are your data, and the best solution may involve combining applications from various vendors. So, open access to historical data can be crucial.
Make History Your Future
Access to process history is a must-have in today’s competitive landscape. The maturity of current historization products makes successful implementation an attainable goal. It’s important to chart your long-term objectives for data historization, ensuring that the chosen solution is robust and can handle your various sources of process data. Open standards and a variety of third-party products allow you to customize your solution to maximize user satisfaction and payback.