Calibration vs. Calibration Check vs. External Standard: What's the Difference?

Three terms show up on almost every audiometer or breath alcohol instrument service record, and they get used interchangeably even though they mean three different things: calibration, calibration check, and external standard. Confusing them isn't just a vocabulary problem. It leads employers to believe a daily verification satisfies an annual requirement, or that a passing check proves an instrument is traceable when nothing on the record actually shows that. Getting the terms straight is the fastest way to read a service report correctly and to know whether the paperwork in front of you actually covers what your compliance program needs.

What does calibration actually mean?

Calibration is the process of comparing an instrument's output against a known, traceable reference and adjusting or documenting the instrument so its readings match that reference within an accepted tolerance. For an audiometer, that means measuring the actual sound pressure level and frequency produced at each test tone against ANSI/ASA S3.6 tolerances, using calibrated reference equipment. For a breath alcohol instrument, it means verifying the reading against a known alcohol concentration standard, such as a dry gas standard or wet bath simulator. Calibration produces a record with specific measured values, not just a pass or fail, and it's the step that either confirms an instrument is accurate or corrects it when it isn't.

What does a calibration check actually verify?

A calibration check, sometimes called a functional check or an accuracy check, is a faster verification that an instrument is still performing within expected range, without the full measurement and adjustment process a calibration involves. For audiometers, OSHA's 1910.95 Appendix C functional check is a listening check performed by the operator before each day of testing, confirming the tones sound correct and the equipment is working, not a measurement against a traceable standard. For breath alcohol instruments, an accuracy check run against a known standard between full calibrations serves a similar role: catch drift early, without replacing the periodic full calibration those instruments still require.

How does calibration differ from a calibration check?

Question	Calibration	Calibration check
What it measures	Actual output values against a traceable reference standard	Whether the instrument seems to be working correctly
Frequency	Annually (audiometer acoustic calibration), or per manufacturer/program interval for breath instruments	Daily or before each use
Who performs it	Trained technician using calibrated reference equipment	The instrument operator
Output	Certificate with per-frequency or per-concentration measured values	Pass/fail confirmation, no traceable measurement recorded
Can it replace the other?	Satisfies both requirements	Does not satisfy the calibration requirement

What is an external standard and why does it matter?

An external standard is the known, independently verified reference an instrument is measured against during calibration, and it's what makes a calibration meaningful rather than circular. For breath alcohol testing, that's a dry gas standard or wet bath simulator with a documented, traceable alcohol concentration. For audiometers, it's calibrated reference equipment whose own accuracy traces back through an unbroken chain of comparisons to a national measurement standard, maintained in the US through NIST. If a calibration record doesn't name the external standard used, or if that standard's own traceability can't be shown, the calibration can't actually demonstrate the instrument is accurate. It can only show the instrument agrees with something unverified.

What happens when these terms get confused?

The most common mistake is treating a daily functional check as if it satisfies an annual acoustic calibration requirement. It doesn't, because a functional check never compares the instrument to a traceable external standard, it only confirms the tones sound roughly correct to the operator. An audiometer can pass a functional check every day for a year while drifting out of tolerance on actual output levels, since the functional check isn't measuring output levels at all. This distinction matters most when an audiogram is later disputed. A defensible record needs the annual calibration against a traceable standard on file, not just a stack of daily check logs, because only the calibration record demonstrates the instrument's actual accuracy on the test date in question.

How does this play out for breath alcohol instruments specifically?

Breath alcohol testing devices used under DOT program rules require periodic external calibration checks against a known alcohol concentration standard, on a schedule set by the device's manufacturer approval and the employer's program. An accuracy check performed by an operator between calibrations can catch a device that's clearly reading wrong, but it isn't a substitute for the scheduled calibration itself, and it isn't documented the same way. A calibration record for a breath instrument should show the specific standard used, its concentration, and the instrument's measured response, the same structure an audiometer calibration certificate follows.

How Foster documents the difference for clients

As a CMI authorized distributor and service center, Foster performs full audiometer and breath alcohol instrument calibration, and every certificate we issue names the external standard used and lists the actual measured values against it, not a summary pass or fail. When we train client staff on the daily functional check, we're explicit that it's a different task from the annual calibration and can't replace it on a compliance record. We've walked into audits where an employer had a binder of daily check logs and no calibration certificate at all, assuming the checks covered the requirement. They don't, and that gap is exactly what we flag during a service visit before it becomes an audit finding instead of a fixable oversight.

Frequently asked questions

Does a passing calibration check mean the instrument is calibrated?

No. A calibration check confirms the instrument appears to be functioning, but it isn't measured against a traceable external standard and doesn't produce the documented values a real calibration does. Both are useful, but they answer different questions.

How often does an audiometer need a full calibration against an external standard?

At least annually, following the acoustic calibration procedures in 1910.95 Appendix E, in addition to the daily functional check performed before each day of testing.

What makes a reference standard "traceable"?

Traceability means the equipment used as the reference is itself calibrated against a chain of comparisons that connects back to a national measurement standard, typically maintained in the US through NIST. A calibration certificate should name that chain, not just claim traceability.

Can an employer perform their own calibration check?

Yes, functional and accuracy checks are designed to be performed by the instrument operator as part of routine use. Full calibration against an external standard typically requires a trained technician and calibrated reference equipment.

What should I ask a calibration vendor to confirm they're using a real external standard?

Ask them to name the specific standard used, such as the dry gas standard lot or the reference audiometer model, and to show that standard's own traceability documentation. A vendor who can't answer either question isn't performing a calibration you can defend later.

Foster Special Instruments provides audiometer and breath alcohol calibration and on-site testing programs for employers across our service area, and our BAT technician training covers the difference between daily checks and full calibration. Related reading: our post on what a defensible calibration certificate must contain. Need your calibration schedule reviewed? Request a quote.

Sources

• 1910.95 App C, Audiometric Test Rooms / Functional Check (OSHA)
• 1910.95 App E, Acoustic Calibration of Audiometers (OSHA)
• NIST Calibrations, National Institute of Standards and Technology