Intermediate Check-Ensuring the Confidence of Your Reference Standards During Field/On-Site Calibration

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I received a comment in my other post asking me on what to do about the high readings of environmental conditions (temperature and humidity) that they are experiencing during their onsite or field calibration compared to their defined specs when inside the lab. He is worried that an auditor may find an issue once it is displayed in their calibration certificate.

I believe this is also a concern to some or most Calibration Techs that performs calibration outside their lab so it is worth a topic to discuss. This can become an audit issue if not properly addressed since it is required to be displayed in the calibration certificate.

Also, one of the questions I received during an audit is, how do you ensure that the reference standards you use outside the laboratory are still in confidence or in good condition once it left and/or returned from the fieldwork?

In this post, I will share with you a quality control procedure that we can easily execute to ensure confidence in your reference standard before or after it is being used outside the calibration laboratory. This is what we call a Functional Check or an Intermediate Check.

This is a must in order to detect any possible issue that can be faced during or after an onsite calibration work. This is also applicable to those instruments or reference standards that are sent out for 3rd party calibration.

I will show to you the step by step procedure on how to implement this.


What is a Functionality Check or Intermediate Check?


An Intermediate Check is a quality check performed to ensure that the reference standard used is still in good condition as your “reference standard” where results are verified and analyzed that it is still within defined limits.

Me during my on-site job exposed to a 46.9 deg C environment
Me during my on-site job exposed to a 46.9 deg C environment

This is one type of a calibration quality control that is implemented internally where the resulting data should be recorded and analyzed. This is a planned activity where the frequency of implementation should be laid out.

This is also termed as functionality check, because this is literally what you do, to check and verify the functionality of the reference standard before and after use specific to a fieldwork.

Since we will be focusing on reference standards that are used outside the lab, the frequency is every time it is being used outside the lab. This can fall under a work instruction for all reference standards that will be brought outside the facility.

This is applicable whether the standard is for 3rd party calibration or simply performing an onsite or field calibration.

This is a just a simple cross check so there is no need to perform a full range check. The important thing is to have a program or procedure that is implemented and documented.

It is just a simple way to detect any out of specs reading that may have resulted during the exposure to the different environment or strong vibrations or even mishandling outside the lab.


How to Verify the Environmental Specs of your Reference Standard?

UUT specs
Verify the environmental specifications of the reference standards in its manual to ensure it is suited to the environment that you will be exposed to.

Once this intermediate check has been performed, regardless of what temperature or humidity the reference standard has been exposed to you are confident that the calibration you performed outside of your lab is not compromised.

But before exposing your reference standard to a different, more harsh environment, make sure to check first its environmental specifications in its user manual to ensure that the reference standards are designed for that range of environmental condition to avoid damage.

Also, in order to support the validity of the procedure even if the environmental conditions are different compared when inside the lab, below information should be reflected in your calibration record/report:

  1. The term “on-site” calibration – it is expected that a procedure with regards to onsite calibration is in place in your documentation. This is to differentiate it to the calibration performed inside the lab (bench calibration)
  2. Customer approval of the procedure performed– show evidence of customer or user approval.

7 Steps to Perform an Intermediate Check


As part of our quality control, we should consider or implement on how we ensure that our reference standards are still ok while it is being exposed in a harsh environment that can somehow affect its performance, like:

  • very high temperatures,
  • very high humidity
  • strong vibrations
  • contaminants

The procedure is to have another reference standard that we can use to compare and take a reading with. Both should have a valid calibration status. It is the same as calibrating a simple UUT (Unit Under Test) with a reference standard but it does not necessarily a more accurate standard.

As long as we can detect and verify the desired test point then it is ok. But of course, a higher level standard is better if we have it.

A simple Functionality Check for the RTD using a metrology well.
A simple Functionality Check for the RTD using a metrology well.

One example is an RTD and a Metrology Well. The Metrology Well can be used as a reference standard in performing a functional check with the RTD. It does not necessarily a more accurate standard. I hope you got my point.

Another example of performing an intermediate/ functional check is by using the same reference standard to a previously calibrated UUT. A UUT that is designed for this purpose. See if they have the same or acceptable readings or results comparing it with the previous record.

In performing an intermediate check, below are the steps that I recommend and follow:

  1. The same as performing a calibration, all reference standards that will undergo intermediate check should be left to stabilize in the lab for at least 1 hour.
  2. You should prepare a checklist or a form to document and list needed details about the intermediate check. Below are some details to include (and anything that is significant during checking):
    • The identification of the Reference standard to be used for cross-checking.
    • The name of the standard that will undergo intermediate check,
    • Brand
    • Model
    • Serial No.
    • Temperature and humidity
    • Date of functionality check when released
    • Date of functionality check when returned
    • Who performed the functionality Check
    • The physical conditions including accessories
    • The tolerance to determine pass and fail status.
  3. Choose a test point for the intermediate check as per available functions or nominal value.
  • For example: In an RTD, you can check only at 0 deg C and 100 deg C test point for a 0 to 200 range                                                                  For a Test Gauge, check only on its mid-range (50% of full scale)
  1. Perform at least 3 trials.
  2. Compute for the error and record. Based the tolerance in your procedure or with the manufacturer specs. It can be also on the calibration certificate. How to check?  See this link>> 3 ways to verify the measurement results in a calibration certificate
  3. Have it approved by an authorized person then it is ready for field work.
  4. Then once the reference standard has returned to the lab, repeat steps 2 to 5.
  5. And you’re done



Intermediate checks are part of a laboratory quality control procedure. This can be used as an evidence of a good performance of a reference standard even if you use it outside the defined specs of the lab with respect to bench calibration.

This is how we can demonstrate the validity of results, if ever that you will be audited regarding the uncontrolled outside environment that we and the reference standard are exposed to.

I have discussed in this post the meaning of Intermediate check which is also called a functionality check,  the importance of an intermediate check as a part of a calibration program under quality control, and a simple procedure on how to implement this including the details to be displayed in your report.

Do you have other means of quality control that you perform during a field or onsite calibration for your reference standard? Please comment below.

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2 Responses

  1. Jollin A Doshi
    | Reply

    Other way around is the master is calibrated so u know the uncertainty..

    Calibrate uuc of same accuracy and maximum range and calculate the En ratio if the same is less then 1 the master is perfect

    • edsponce
      | Reply

      Hi Jollin,
      Thank you for the comment. An additional method for quality control involving intermediate check, the same calculation using the interlab comparison method I believe. Very nice.

      Thanks and regards,

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