What is an intermediate check in calibration?

It is a short, documented test of an instrument between two calibrations, to confirm it still performs as its last calibration showed. You usually measure a stable check standard and compare the result with a limit set in advance. It is a verification, not a calibration, so it does not reset the due date.

Does ISO/IEC 17025 require intermediate checks?

ISO/IEC 17025:2017 clause 6.4.10 requires them whenever they are needed to keep confidence in the performance of equipment, and they must follow a procedure. The standard sets no frequency or method. Clause 7.7.1 also lists intermediate checks, and check standards with control charts, among the ways to monitor the validity of results.

How often should intermediate checks be done?

No standard sets a frequency. Base it on the risk, how much the instrument is used, its environment and its check history, and add a check after any drop, overload, repair or move. For high-precision work the NIST/SEMATECH e-Handbook suggests checking with each measurement sequence, and at least once a day.

How do you set control limits for check standard data?

For one check value per occasion, the NIST/SEMATECH e-Handbook puts the center line at the average of the historical check values. The limits are that average plus and minus k times the standard deviation of the values over time, with k = 2 for tight control or k = 3 to flag only clear departures. Do not take the standard deviation from short-term repeats, because it leaves out the variation from day to day.

What should you do when an intermediate check fails?

Repeat it once to rule out a slip, and take the instrument out of use if it fails again. Check the check standard, have the instrument calibrated or repaired, and decide whether results since its last good check or calibration were affected. ISO 9001 clause 7.1.5.2 and ISO/IEC 17025 clause 6.4.9 both call for that assessment.

Compliance and audits

Intermediate checks between calibrations: what to check and how often

14 min read Published November 8, 2026 Updated November 9, 2026

An intermediate check is a short, documented test of an instrument between two calibrations: you measure a stable check standard and compare the result with a limit set in advance, to confirm the instrument still performs as its last calibration showed. ISO/IEC 17025:2017 clause 6.4.10 requires these checks to follow a procedure whenever they are needed to keep confidence in the equipment.

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What is an intermediate check?

An intermediate check tests whether an instrument still behaves the way its last calibration showed. You measure something whose value you know, usually a stable check standard, and compare the result with a limit you set beforehand. It takes minutes and needs no lab.

It is not a calibration. A calibration relates the instrument's indication to a traceable reference, with a stated uncertainty, and an adjustment often follows it. An intermediate check only confirms that this relation still holds at one or a few points. In the language of the VIM (JCGM 200:2012), it is a verification: objective evidence that the instrument meets a stated requirement. It does not reset the calibration due date.

The benefit shows when something goes wrong. If an instrument fails calibration as found, you have to review what it measured since it was last known to be good. A recent passed check moves that date forward, so the review covers a shorter period.

What ISO/IEC 17025 and ISO 9001 ask for

ISO/IEC 17025:2017 clause 6.4.10 says that when intermediate checks are needed to keep confidence in the performance of equipment, they are carried out according to a procedure. The standard leaves the need, the method and the frequency to the laboratory.

Clause 7.7.1 names intermediate checks on measuring equipment, and check or working standards with control charts where applicable, among the ways a laboratory monitors the validity of its results. The same clause asks for the data to be recorded so that trends can be seen, with statistical techniques applied where practicable. Clause 7.7.3 calls for action when monitoring data fall outside pre-defined criteria.

ILAC-G24:2022 / OIML D 10:2022, the international guidance on calibration intervals, says a laboratory should have a system of intermediate checks. It should confirm that check results fall within predetermined limits before the equipment is approved for further use. The same document lists the frequency, quality and results of intermediate checks among the factors for setting an interval.

ISO 9001 does not use the term. Clause 7.1.5.1 asks you to keep measuring resources fit for their purpose and to retain evidence of it. Clause 7.1.5.2 asks for calibration or verification against traceable standards at specified intervals or before use. A documented check before use, against a traceable standard, is one way to show that fitness.

Tip: Keep the procedure, the limits and the results together. A check that leaves no record cannot be shown to anyone later.

Choosing a check standard

A check standard is a stable item with a known value that you keep for checking an instrument. The VIM notes that a working standard used for verification is sometimes called a check standard or control standard. It can be a gauge block, a check weight, a reference resistor or a part from your own production that you set aside.

The NIST/SEMATECH e-Handbook of Statistical Methods describes a good one. It is close in material and geometry to the items the instrument normally measures. It stays stable, and it is available whenever a check is due. One artifact is usually enough.

Decide what the check has to prove. A check standard with a calibrated value tells you whether the instrument's error is still inside its limit. A check standard without one can only show that the readings have changed. The e-Handbook makes the same point about control procedures: on a process that is not properly calibrated, they can only show that results are comparable with each other.

Treat the check standard as equipment in its own right. Store and handle it so it does not wear or get damaged, and have it calibrated if you rely on its value. A shift in the check results can come from the check standard itself as well as from the instrument.

Metrological traceability explained

What to check on common instruments

A check covers the points where the instrument is used most, or where it is most likely to go wrong. The table shows typical choices. Your own procedure sets the points and the limits.

Typical intermediate checks for common instruments
Instrument	Typical check standard	What the check shows
Caliper or micrometer	The zero, then a gauge block near the middle of the range	A zero shift and the error at one length
Torque wrench	A torque tester, at a setting the wrench is often used at	Whether the readings at that setting are still inside the limit
Balance or scale	A check weight close to the loads you weigh	The error at one load
Temperature probe	An ice point, or a reference probe beside it in a dry block	The error at one temperature
Pressure gauge	A reference gauge on the same pressure source	The error at one or two pressures
Multimeter	A stable voltage or resistance reference	The error on the ranges used most
pH meter	A buffer solution not used in the calibration	The error at that buffer
Infrared thermometer	A stable high-emissivity source, at the same distance and emissivity setting each time	A change in the reading against earlier checks
Sound level meter	A sound calibrator on the microphone, before and after each set of measurements	A shift in sensitivity at one frequency and one level
Reference standard in a lab	A check standard measured as part of the normal routine	Changes in bias and long-term variability, on a control chart

Tip: A check after a drop or an overload belongs to the same system. ISO/IEC 17025 clause 6.4.9 takes equipment that has been overloaded or mishandled out of service until it has been shown to work correctly.

How often to run intermediate checks

No standard fixes a frequency. Base it on the risk of a wrong result going unnoticed, how hard the instrument is used, its environment and its history. ILAC-G24 / OIML D 10 lists the same kinds of factors for calibration intervals. It notes that its methods for reviewing intervals can also be used to review the type and frequency of intermediate checks.

For high-precision work, the NIST/SEMATECH e-Handbook suggests measuring the check standard with each measurement sequence if possible, and at least once a day. More generally it suggests a schedule that samples all the conditions of measurement, such as once a day or twice a week. OIML D 10 describes in-service checks of critical parameters as often as once a day or more, on equipment that allows a quick check.

Add checks at the moments when an instrument is most likely to have changed.

Before a measurement that matters, such as a first article inspection.
After a drop, an overload, a repair or a move.
When the instrument comes back from calibration, before it goes back into use. ISO/IEC 17025 clause 6.4.4 asks laboratories to verify equipment before it is placed or returned into service.
More often while an instrument is new, until you know how it behaves. ILAC-G24 gives the same advice for calibration intervals.
More often when the check results move toward the limit.

How to set acceptance limits

Decide the limit before the first check. ISO/IEC 17025 clause 7.7.3 speaks of pre-defined criteria and ILAC-G24 of predetermined limits. A limit chosen after seeing the result proves nothing.

Limits for intermediate checks usually come from one of two places.

The instrument's own limit. Compare the error found on the check standard with the instrument's maximum permissible error or tolerance. The check has an uncertainty of its own, from the value of the check standard and from the reading. A result close to the limit therefore needs a decision rule, just as a calibration result does.
Statistical limits from the check history. Once you have enough results, set the limits from their own spread. When the check results scatter much less than the tolerance, these limits flag a change in the instrument well before it uses up its tolerance. The next two sections show how.

Decision rules and guard banding

Tip: The two kinds work well together. The statistical limit gives early warning, and the instrument's own limit is the point where it comes out of use.

How to build a control chart for check standard data

A control chart plots each check result against time, with a center line and control limits worked out from earlier results. The method here follows section 2.2 of the NIST/SEMATECH e-Handbook, which was written for check standards in calibration work.

Data case: one check standard value per occasion. That value can be a single reading, or the average of a fixed number of short-term repeats. Take it the same way you report real results. If you report the average of two readings, the check standard value is the average of two readings too.

The center line is the accepted value, the average of the historical check standard values. The e-Handbook asks for at least 100 of them before the accepted value is fixed. The control limits are the accepted value plus and minus k times s, where s is the standard deviation of the check standard values over time.

Do not take s from the repeats within one occasion. Short-term repeats show the precision of the instrument but not the variation from one day to the next, so limits built from them are too narrow. This is the main difference from the subgroup charts used for production processes.

The choice of k sets how many false alarms you accept. With k = 2, about 5 percent of results from a process that has not changed fall outside the limits. The e-Handbook suggests k = 2 for tight control, with more than 100 degrees of freedom behind s. With k = 3, only clearly out-of-control results are flagged. The e-Handbook puts this at about 1 percent of results from an unchanged process.

Type A uncertainty calculator

Which control chart fits your check data?

The right chart depends on how the check data are collected. Each row of the table names its data case.

Look at the whole chart as well as the latest point. The e-Handbook's kilogram example shows a small shift that only the plot over time revealed. The run rules used in process control, such as eight points in a row on one side of the center line, make a Shewhart chart more sensitive to drift, at the cost of more false alarms.

The limits assume the check values come from one normal distribution, with no correlation from one check to the next. A histogram is a quick check of the shape. Two humps or long tails point to a problem in the measurement process. A plot in time order shows whether one check depends on the last.

Control charts for check standard data, by data case
Check data	Chart	Control limits
One value per check: a single reading, or the mean of a fixed number of repeats	Shewhart chart for individual values (e-Handbook 2.2.2.1)	Accepted value ± k × s, with s taken from the check values over time
The same values, when small shifts matter	EWMA chart (e-Handbook 2.2.2)	Limits on an exponentially weighted moving average, narrower than the Shewhart limits
Two or more repeats per check, to watch the instrument's precision	Standard deviation chart (e-Handbook 2.2.3.1)	An upper limit only: s1 × √F, where s1 is the pooled standard deviation of past repeats (J per check over K checks) and F is the upper critical value of the F distribution at J − 1 and K(J − 1) degrees of freedom
A short check history, as a first chart	Individuals and moving range chart (e-Handbook 6.3.2.2, written for production processes)	Mean ± 3 × MR-bar / 1.128, where MR-bar is the average difference between successive check values. Move to the 2.2.2.1 limits once the history is long.

Gage bias and linearity studies

Tip: Until you have a long history, judge each check against the instrument's own limit and plot the results anyway. The chart becomes useful as the points build up.

What to do when an intermediate check fails

A failed check means the instrument may have been measuring wrongly since its last good check. Work through these steps in order.

Repeat the check once, carefully. The e-Handbook lists a chance failure, a slip in setting up and an error in recording among the causes that call for discarding the result rather than acting on the process. Record both results. A repeat that passes does not erase the first one.
If it fails again, take the instrument out of use and label it. ISO/IEC 17025 clause 6.4.9 requires this until the equipment has been shown to work correctly.
Check the check standard. Damage or wear on the check standard, or on a reference standard, can look like a change in the instrument.
Look back to the last good check or calibration and decide whether results since then were affected. ISO 9001 clause 7.1.5.2 asks for this when equipment is found unfit, and ISO/IEC 17025 clause 6.4.9 sends a laboratory to its procedure for nonconforming work.
Have the instrument calibrated, adjusted or repaired, and record the as-found readings before anyone adjusts it.
Review the calibration interval and the check frequency. A drift toward the limit is a reason to shorten one or both.

Tip: An out-of-control signal on a control chart is not the same as an out-of-tolerance result, and the instrument may still be well inside its limit. Treat the signal as a reason to find the cause. The e-Handbook's remedies are to look for drift, repair the instrument or recalibrate the reference standards where needed, and set new limits from recent data if the process cannot be brought back.

What to record for each check

Each check should leave a record that someone else can follow. The NIST e-Handbook suggests one row per check in a single file. ISO/IEC 17025 clause 6.4.13 lists the equipment records a laboratory keeps, including evidence that equipment meets its requirements.

The date, and the time if you check more than once a day.
The instrument's ID and the check standard's ID, with the check standard's value and its certificate where it has one.
The reading or readings, the difference from the check standard's value and the limit it was judged against.
The result and any action taken.
Who did the check, and the environmental conditions where they matter.

Calibration log template

How to set up intermediate checks, step by step

List the instruments whose results matter most and decide which of them need checks between calibrations.
For each one, choose a stable check standard close to the work, and record its value and where that value comes from.
Write the check procedure: the points, the number of readings, the conditions and who does it.
Set the acceptance limit before the first check, from the instrument's tolerance and, once you have the history, from the check data.
Set the frequency from the risk and the instrument's use, and add checks after drops, repairs and moves.
Record each check and plot the results over time.
When a check fails, repeat it once. If it fails again, take the instrument out of use and assess what it measured since its last good check.
Review the frequency and the limits from the check history, together with the calibration interval.

How Axiospec fits

Axiospec is calibration management software. It has no separate record type for intermediate checks. Keep your check log in your quality system. You can attach the check procedure to the instrument as its SOP, and a file to any calibration record.

Axiospec keeps the calibrations on either side of the checks: the as-found and as-left readings, the reference standard used, the certificate and the next due date. When a failed check sends an instrument for calibration and that calibration is logged as Fail, the instrument goes into quarantine and the record asks for the out-of-tolerance impact. On a reference standard, the Reverse Traceability card lists the calibrations it was used for.

The free plan covers up to 50 active instruments. A new workspace starts with a 45-day trial of Professional, with room for 1,500 active instruments and no credit card. We load your existing records for you at no charge on every plan.

Common questions

What is an intermediate check in calibration?: It is a short, documented test of an instrument between two calibrations, to confirm it still performs as its last calibration showed. You usually measure a stable check standard and compare the result with a limit set in advance. It is a verification, not a calibration, so it does not reset the due date.
Does ISO/IEC 17025 require intermediate checks?: ISO/IEC 17025:2017 clause 6.4.10 requires them whenever they are needed to keep confidence in the performance of equipment, and they must follow a procedure. The standard sets no frequency or method. Clause 7.7.1 also lists intermediate checks, and check standards with control charts, among the ways to monitor the validity of results.
How often should intermediate checks be done?: No standard sets a frequency. Base it on the risk, how much the instrument is used, its environment and its check history, and add a check after any drop, overload, repair or move. For high-precision work the NIST/SEMATECH e-Handbook suggests checking with each measurement sequence, and at least once a day.
What is a check standard?: A check standard is a stable item with a known value that you keep for checking an instrument or a measurement process, such as a gauge block or a retained production part. The VIM notes that a working standard used for verification is sometimes called a check standard. The NIST/SEMATECH e-Handbook adds that it should be close in material and geometry to the items the instrument normally measures.
How do you set control limits for check standard data?: For one check value per occasion, the NIST/SEMATECH e-Handbook puts the center line at the average of the historical check values. The limits are that average plus and minus k times the standard deviation of the values over time, with k = 2 for tight control or k = 3 to flag only clear departures. Do not take the standard deviation from short-term repeats, because it leaves out the variation from day to day.
What should you do when an intermediate check fails?: Repeat it once to rule out a slip, and take the instrument out of use if it fails again. Check the check standard, have the instrument calibrated or repaired, and decide whether results since its last good check or calibration were affected. ISO 9001 clause 7.1.5.2 and ISO/IEC 17025 clause 6.4.9 both call for that assessment.

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