How the Spirometer's Daily Calibration and 3-Liter Syringe Check Works

What's happening

Spirometers are checked daily with a hand-operated 3-liter calibration syringe, and this comes up often enough as an audit question that it's worth explaining plainly. The syringe and the spirometer are calibrated together, as a single linearity check — the syringe isn't a separate certified instrument that gets checked once and trusted forever, and it isn't calibrated apart from the spirometer it's plugged into.

The check itself is a multi-flow linearity test. You drive exactly 3 liters of air through the spirometer using the syringe, but you do it at several different flow rates (fast strokes and slow strokes), rather than one single push. That confirms the spirometer measures volume accurately across the range of flows a real forced exhalation produces, not just at one speed. The CCS-200 displays the resulting volumes both graphically and numerically directly on the printout — the numbers aren't retyped onto a separate calibration log, which avoids a transcription error creeping into the record. The calibration sticker generated from that printout is the certificate of calibration for both the spirometer and the syringe as a pair.

Try these in order

1. Confirm the syringe is fully seated on the connection to the spirometer. A loose or partially seated connection is the single most common cause of a failed or borderline linearity check — it reads as a leak, not as a syringe or spirometer fault.
2. Check the syringe seal and plunger for smooth, even travel. Air escaping around the plunger during the stroke, or a plunger that sticks and then releases, changes the flow profile and can throw off the linearity result even though the total volume looks close to right.
3. Re-run the check with both a fast and a slow stroke. If one flow rate passes and another doesn't, that's useful information — it points toward a flow-range issue with the spirometer rather than a simple leak, which is more consistent across all flow rates.
4. Rule out technique before assuming a hardware problem. The full 3 liters has to be delivered smoothly, without hesitation partway through the stroke. A hesitant or interrupted push can look like a calibration failure that isn't actually one.

If it still won't pass

If you've confirmed a tight seal at the connection, checked the plunger moves smoothly with no leaks, and a careful re-check at both fast and slow flow still doesn't land within range, that points to the spirometer itself rather than technique or the syringe seal — stop testing on that unit and contact us. Continuing to test with a unit that's failing its own calibration check puts every result recorded that day in question.

When to call us

• The linearity check consistently fails after you've ruled out a loose connection, a plunger leak, and stroke technique.
• The check passes at one flow rate but not another, on repeated attempts.
• An auditor is asking for documentation beyond the printed calibration sticker.

You can reach us at (513) 891-0868 or service@fosterinstruments.com.