Technical Foundation
What Is a Circular Chart?
A circular chart is the printed paper disc used in a circular chart recorder, a mechanical instrument that drives an ink pen across the disc as it rotates at a fixed rate, plotting a continuous record of temperature, pressure, humidity, or flow over time. The chart's printed range and scale must match the recorder's measuring range exactly, since the pen's radial travel corresponds to a specific engineering-unit span printed on the paper.
Rotation period is the other critical fit factor — 24-hour and 7-day charts are the two most common cycles, and a chart's drive holes and center hub must match the recorder's spindle to rotate at the correct rate; using a 24-hour chart in a recorder geared for 7-day rotation compresses seven days of data into one rotation and produces an unreadable, overlapping trace.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Rotation Period | 24-hour, 7-day, or other fixed cycle | Must match the recorder's drive gearing — wrong period produces overlapping or compressed traces |
| Printed Range/Scale | e.g. 0-200°F, 0-100 psi, 0-100% RH | Must match the instrument's calibrated measuring range for readings to be accurate |
| Chart Diameter | Common sizes: 6 in, 8 in, 10 in, 12 in | Must match the recorder model's chart plate diameter to seat and drive correctly |
| Number of Pens | Single-pen vs. multi-pen (2-4 pen) charts | Must match the recorder's pen configuration for multi-parameter recording |
| Recorder Brand Fit | Honeywell, Partlow, Foxboro, Barton and other legacy recorder lines | Hub pattern and print layout are brand/model specific, not universally interchangeable |
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Applications
Where Circular Charts Get Used
Circular charts remain specified anywhere a facility still operates a legacy mechanical chart recorder requiring a continuous, physically archived paper record.
HVAC & Building Automation Legacy Systems
24-hour temperature and humidity charts recording continuous environmental conditions in older mechanical rooms and archives.
Boiler & Steam Plant Monitoring
Pressure and temperature circular charts providing a continuous documented record for boiler operation and compliance.
Food & Cold Storage Compliance
Temperature charts providing a required continuous paper record for refrigerated storage and transport temperature compliance.
Oil & Gas Pipeline Flow Recording
Legacy pressure and flow recorders using circular charts on pipeline metering stations still in service.
Pharmaceutical & Regulated Manufacturing
Continuous temperature/humidity charts supporting validated environmental monitoring records.
Water Treatment Plant Recording
Pressure and flow circular charts documenting plant operation on legacy mechanical recorders.
FAQ
Circular Charts Questions, Answered
How do I know what circular chart fits my recorder?
You need three matching specs: the recorder's brand/model (which sets the hub and drive pattern), the chart diameter, and the rotation period (24-hour vs. 7-day, matching the recorder's internal gearing) — the chart's printed range should also match your instrument's calibrated span. The model number is usually printed on the recorder's nameplate or the old chart currently in use.
What happens if I use a 24-hour chart in a 7-day recorder, or vice versa?
The pen will still draw a trace, but the timing will be wrong — a 24-hour chart in a 7-day recorder only completes 1/7th of a rotation per day, compressing readings into a narrow arc, while a 7-day chart in a 24-hour recorder completes a full rotation daily and overlaps seven days of data on top of each other, making the record unreadable.
Can I use a chart with a different printed range than my instrument's calibration?
No — the chart's printed scale corresponds to the pen's mechanical travel as calibrated to the instrument's actual measuring range. A chart printed for 0-300°F used on a recorder calibrated for 0-200°F will produce readings that don't correspond to the printed scale at all, since the pen movement and the paper's printed gridlines are only in agreement when the ranges match.
Are circular charts still used, or has everything gone digital?
Many facilities have moved to digital data loggers, but a substantial number of legacy mechanical recorders remain in service — particularly in regulated industries where the original recorder is still validated equipment, or where replacing it isn't cost-justified — so circular chart supply remains an active, ongoing need for those installed base recorders.
How long should a completed circular chart be retained?
Retention requirements depend on the application and applicable regulation — food safety, pharmaceutical, and boiler-code compliance programs typically specify a minimum retention period (often one to several years) for the physical chart as the auditable record, so retention should be set by your facility's compliance requirements rather than a generic default.
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