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

with 10 Comments

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 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.

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.
An example of Intermediate 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.

7 Steps to Perform an Intermediate Check

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 an 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 fieldwork.
  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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Your guide in calibration,



10 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,

  2. Vinod
    | Reply

    Thanks for your post, our auditor also telling to calculate En value but how it possible to calculate En Value as we have only one cmc and one uncertainty for that standard?

    • edsponce
      | Reply

      Hi Mr. Vinod,
      I Appreciate you liked my post. Did you perform an interlaboratory comparison? It is another method for quality control utilizing the process of interlab comparison by calculating the En value. A process of comparing the results of a sample/Device Under Test from a reference lab with your results based on measurement results and its uncertainty. In any case, below is the formula of En. En Formula
      The result should be less than 1 in order to be considered passed. You may check this standard for further reading: ISO/IEC 17043:2005 “Conformity assessment — General requirements for proficiency testing”. This standard will explain more how the formula will be used.

      For any other concern, please comment further.

      Thanks and regards,

  3. beverlyne Sandoval
    | Reply

    How can i performed intermediate check for the dimensional equipment such us gauge blaock set

    • edsponce
      | Reply

      Hi Ms. Beverlyne,

      First of all, thank you for visiting my site.

      We perform an intermediate check to ensure that the reference standard (gauge block) is in confidence while it is still within its calibration interval.

      Gauge blocks have no mechanical or moving parts that can drift or wear-off, therefore, what can we do as an intermediate check is a visual check only.
      During the visual check, this is what you can do:
      1. check if it has a scratch
      2. check if it is deformed in any way
      3. check if it is cleaned and with oil for corrosion resistant

      If a scratch or deformation is observed, that is the time to use a calibrated caliper to verify its reading and parallelism.

      Ensure that the gauge block is stored back to an acceptable and stable environmental condition while performing a verification.

      Hope this helps.


    | Reply

    How can i performed intermediate check for Temperature ? I have 2 temp. sensor (FLUKE) with same model PT 100
    Also for St. weight M1 ?

    • edsponce
      | Reply

      Hi Ibrahim,
      You are welcome. I am glad you liked it.
      There are 2 ways in which you can perform an intermediate check for your temp sensor.

      FIRST is to compare them directly. If you do not have a standard like a metrology well or dry block, you can use the ambient temperature. Our objective here is to have a reference reading before and after the use of the PT100. Do not forget to document or record your readings.

      SECOND, if you have a calibrated dry well, you can use that as your reference for the comparison of temperature. Refer to the procedure I presented in my post above for the execution.

      For std weight, usually, this type of standard can be check through visual only. Just record everything that you observe. Does it have scratches, dents, or a sign of deterioration after it was returned to the lab. Record all observation. If you observe any irregularities like too many scratches, that is the time to perform verification by using a calibrated weighing scale. Or worst, have it recalibrated.

      But if your resources can support, in addition to the visual check, you can use a weighing scale to verify each M1 standard before and after use.

      Hope this helps.

  5. Ansar
    | Reply


    Thanks for your information about intermediate check.If you could help me to know about what criteria we can go for RTD thermometer which we are using as master thermometer.Please reply me


    • edsponce
      | Reply

      Hi Ansar,
      Thank for reading my post.
      If you are referring to acceptance criteria, then below are my suggestions.
      1. You can check the manufacturer specifications, look for the accuracy part. You can use the accuracy as the tolerance for the acceptance criteria.

      2. Another way is to check the calibration certificate and look for the uncertainty results on the range you have chosen. The uncertainty results can also be used as the tolerance for the basis of acceptance criteria.

      3. The final method is to use the computed standard deviation at 95 % confidence level.
      If you are performing an intermediate check for example at 100 deg C using metrology well, measure at 100 degC 20 times.
      a. Record all the 20 results in an excel sheet and use the standard deviation formula.
      b. Compute for the standard deviation,
      c. Then the final result, multiply it by 2 to expand it to a 95% confidence level.

      This can now be used as your tolerance for acceptance criteria.

      There is no wrong choice here, it depends on what method you are most comfortable with. If you want more strict acceptance criteria, you can choose the 2nd option.

      There are other ways to select or determine acceptance criteria, the important thing to do is to document the method you choose.

      Hope this helps, do not hesitate to message me if you still have a question.

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