Historian sampling and deadband: budget storage without losing events

Historian storage planning starts with tag count, sampling behavior and record size. A deadband may reduce recorded values, but it also changes the evidence available to investigate a process event. Decide what must remain observable before using compression to meet a disk target.

FIG. 02From machine to data

Calculate an uncompressed starting point

For fixed-rate storage, records per day = tags × samples per second × 86,400. With 2,000 tags stored once every ten seconds, the estimate is 17,280,000 records/day. At an assumed 24 bytes per record, that is 414,720,000 bytes/day, or about 414.72 decimal MB. Over 30 days, the raw estimate is 12.4416 GB.

This illustrative byte count excludes indexes, metadata, block structure, replication, backups and filesystem overhead. Measure the actual historian's growth on representative tags. Compression ratios from another process are weak planning evidence.

Acquisition and storage are separate decisions

A source may be scanned every second while the historian stores only changes or periodic snapshots. Changing storage deadband does not necessarily reduce PLC polling or network traffic. Check each stage: source scan, gateway publish, historian ingestion and archive retention.

An absolute deadband uses engineering units. A percentage deadband needs an explicit reference span. For a 0–200 °C range, 0.5% of span is 1 °C; that may be too large for a process where a half-degree excursion matters. Preserve the configured range with the tag metadata.

Test against a known event

Select examples that include a slow ramp, a short excursion, a state change and bad source quality. Compare the stored trace with the source observation. Ask whether an investigator can still determine start time, peak, duration and recovery at the required resolution.

A compressed display line can interpolate between points and imply behavior never sampled. Distinguish raw stored points from interpolated or aggregated views in reports.

Make retention a service decision

Define online retention, long-term access and recovery requirements with operations, quality and engineering. Test restoration and query performance as well as disk growth. A copied archive is not useful if the catalog, permissions or software needed to read it are missing.

Pair this plan with the historian naming guide, the industrial data contract and OT restore testing. The calculations above are a vendor-neutral sizing baseline; archive algorithms and limits belong to the selected product's documentation.