Tracking and viewing modifications in digital calibration certificates


ACTA IMEKO 
ISSN: 2221-870X 
March 2023, Volume 12, Number 1, 1 - 7 

 

ACTA IMEKO | www.imeko.org March 2023 | Volume 12 | Number 1 | 1 

Tracking and viewing modifications in digital calibration 
certificates 

Vashti Galpin1, Ian Smith2, Jean-Laurent Hippolyte2 

1 School of Informatics, University of Edinburgh, United Kingdom  
2 National Physical Laboratory, United Kingdom  

 

 

Section: RESEARCH PAPER  

Keywords: digital calibration certificates, provenance, temporal databases, XML, prototype 

Citation: Vashti Galpin, Ian Smith, Jean-Laurent Hippolyte, Tracking and viewing modifications in digital calibration certificates, Acta IMEKO, vol. 12, no. 1, 
article 11, March 2023, identifier: IMEKO-ACTA-12 (2023)-01-11 

Section Editor: Daniel Hutzschenreuter, PTB, Germany 

Received November 20, 2022; In final form March 20, 2023; Published March 2023 

Copyright: This is an open-access article distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, 
distribution, and reproduction in any medium, provided the original author and source are credited. 

Funding: This work was supported by ERC Consolidator Grant Skye [grant number 682315] and by an ISCF Metrology Fellowship grant provided by the UK 
government’s Department for Business, Energy and Industrial Strategy (BEIS).  

Corresponding author: Vashti Galpin, e-mail: Vashti.Galpin@ed.ac.uk  

 

1. INTRODUCTION 

Information relating to the calibration of an instrument or 
artefact is captured in documents referred to as calibration 
certificates. These documents are typically provided either as 
physical paper-based documents or in electronic form, for 
example, in archivable Portable Document Format (PDF-A). A 
downside of both approaches is that the information they 
contain is not machine-readable, that is it is not available in a 
form that can be read and processed by computer. Using the 
information therefore requires, to some degree, human 
involvement and as such is prone to error, for example, arising 
from the transcription of numerical information from paper to 
computer.  

Recent initiatives have considered the replacement of paper-
based and electronic calibration certificates by fully machine-
readable calibration certificates, referred to as “digital calibration 
certificates”, often abbreviated to “DCCs”.  

The European Metrology Programme for Innovation and 
Research (EMPIR) [1] funded the “SmartCom” Joint Research 
Project (2018-2021) [2] which developed a framework for DCCs. 

The framework allows the key calibration certificate components 
of administrative data and measurement results to be stored. The 
recipient of a DCC will have, or be able to develop, software to 
ingest and use the information stored therein. 

Ensuring the integrity and longevity of current and historical 
calibration data is essential for calibration laboratories to 
maintain long-term trusted relationships with their customers. It 
is also a requirement of ISO/IEC 17025 accreditation for these 
laboratories [3], along with the preservation of information about 
the methods, processes, software, equipment, and staff involved 
in the calibration task. This accompanying information, which is 
essential to trace and potentially repeat the conditions under 
which the calibration is performed, is more generally known as 
“provenance information” [4]. Laboratories must be able to track 
and compare the differences between successive calibrations of 
the same artefact in data (for example, newly observed 
measurement results) or in provenance (for example, a different 
employee now signs off the calibration results). Since DCCs are 
containers for the trusted exchange of calibration data, their 
potential to increase the productivity of laboratories relies on 

ABSTRACT 
Trust in current and historical calibration data is crucial. The recently proposed XML schema for digital calibration certificates (DCCs) 
provides machine-readability and a common exchange format to enhance this trust. We present a prototype web application developed 
in the programming language Links for storing and displaying a DCC using a relational database. In particular, we leverage the temporal 
database features that Links provides to capture different versions of a certificate and inspect differences between versions. The 
prototype is the starting point for developing software to support DCCs and the data with which they are populated and has underlined 
that DCCs are the tip of the iceberg in automating the management of digital calibration data, activity that includes data provenance 
and tracking of modifications. 

mailto:Vashti.Galpin@ed.ac.uk


 

ACTA IMEKO | www.imeko.org March 2023 | Volume 12 | Number 1 | 2 

automated, integrated, and time-based curation of this data and 
associated provenance. 

The structure of this paper is as follows: we provide 
background on DCCs before we go on to describe our prototype. 
We discuss the choices made in the development of the 
prototype and illustrate its usage with a screenshot. We conclude 
with ideas for further work. This paper is an extended version of 
an earlier conference paper [5]. 

2. DIGITAL CALIBRATION CERTIFICATES 

The structure of a DCC [6] reflects the information that is 
required by ISO/IEC 17025 [3] for reporting calibration results. 
A DCC comprises two compulsory sections. The first 
compulsory section contains the administrative information 
including that which is generally presented on the first page of a 
paper-based certificate. The second compulsory section contains 
measurement results. The measurement results section itself 
relies upon a framework, referred to as the “D-SI”, developed 
specifically for the storage of measurement data. The framework 
ensures that quantity values, units of measurement and 
uncertainty information can all be represented. A DCC may also 
include two optional sections. The first optional section contains 
information presented purely for humans, for example, 
calibration-specific data sheets, and other auxiliary, machine-
interpretable data, for example, relational tables. The second 
optional section contains an encoded version of a human-
readable version of the certificate.  

The D-SI and DCC frameworks specify, for example, how 
numerical values and date and time information should be 
presented and uses the BIPM SI brochure [7] and the siunitx 
package for LaTeX [8] as the basis for the provision of units of 
measurement.  

The D-SI and the DCC may be implemented in a language 
chosen by the user, for example, Extensible Markup Language 
(XML) or JavaScript Object Notation (JSON). The SmartCom 
project has developed and made available an XML schema for 
the D-SI framework [9] while the Physikalisch-Technische 
Bundesanstalt (PTB), the National Metrology Institute (NMI) of 
Germany, has made available an XML schema for the DCC [10].  

Demonstrators have been developed by NMIs using Excel 
and Python [11], [12] and as a web application (GEMIMEG) [13] 
to illustrate the use of these schema. A different approach 
considers embedding calibration data in PDF calibration 
certificates to support machine-readability [14]. NMIs have also 
been investigating good practice for DCCs [15] and usage in 
specific domains [16], [17]. Developers of commercial software 
for calibration are also starting to support the use of DCCs [18], 
[19]. The first IMEKO TC6 M4DConf, held in Berlin in 2022, 
focussed on the digital transformation of metrology and many 
submissions considered DCCs [20]. 

We are aware that the frameworks (and related schemas) may 
be subject to future updates, for example, if the BIPM SI 
brochure is updated, or if the requirements for the contents of a 
calibration certificate change. In the approach we have taken, 
dealing with XML schema changes is straightforward, and in fact 
occurred during our development when a newer version of the 
DCC schema became available, and we discovered that the 
dummy certificate with which we were working was missing a 
compulsory XML element. 

3. THE DCC PROTOYTPE  

Our prototype application brings together concepts from 
metrology (specifically, the XML schema for DCCs) and 
academic research in databases and programming languages and 
enables the transfer of research ideas from academia. On the 
computer science research side, there are two distinct concepts: 
temporal databases and a technique for storing XML documents 
in a relational database, that are combined using the Links 
programming language, a tierless language that supports 
language-integrated query and provides a single language for the 
development of web applications with database back-ends [21], 
[22]. We now consider these three components in detail: Links, 
temporal databases, and storage of XML documents; before 
considering some details of the implementation of the 
application. 

3.1. The Links Programming Language 

Links is a strongly-typed and statically-typed functional 
programming language, that is cross-tier (or tierless): it removes the 
need for the developer to write JavaScript or a particular database 
query language (in this case, PostgreSQL [23]) for the different 
tiers of a web application providing data from a database. In 
particular, Links assists in the writing of correct software by 
providing language-integrated query which allows the programmer to 
write high-level queries that are transformed to correct SQL 
queries with known performance for execution on the database. 
Furthermore, the type-checking of variables and functions 
before program execution reduces the likelihood of run-time 
errors. 

Figure 1 illustrates the Links architecture. The Links 
interpreter takes the Links code and generates correct HTML 
and JavaScript to run in the user’s browser, and to support 
exchange of data between the browser and Links program, thus 
allowing an interactive webpage. The interpreter also generates 
correct SQL queries which are passed to a database server for 
execution which responds with the results. Using this approach, 
data intensive operations can take place on the database server, 
using the fact that database systems such as PostgreSQL employ 
efficient data structures and algorithms for queries. In particular, 
indexes on key values can make search operations very efficient. 

Links provides syntax to specify database tables as well as 
comprehension syntax to iterate over each row of a database 
table. We use the MVU (Model-View-Update) library [24] for the 
development of the web interface as it provides functions to use 
the element of the Bootstrap toolkit for website development 
[25], thereby affording a higher level of abstraction by which to 
achieve an appealing web frontend. 
  

 

Figure 1. The Links architecture. 



 

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3.2. Temporal databases 

Temporal databases capture when a piece of data is valid. In the 
case of a Relational DataBase Management System (RDBMS), 
we are interested in the time period for which a row in a database 
table is valid, and the inclusion of additional fields (columns) 
allows this time period to be recorded [26], [27]. We are 
interested in the provenance of DCCs and hence we want to 
capture when data changes in the database, namely transaction-time 
information. Temporal databases can also record valid-time 
information which is information about when something is true 
in the real world. 

Figure 2 demonstrates the difference between an update in a 
standard relational database versus one in a temporal database 
supporting transaction time. In the standard case, the new data 
overwrites the old data and there is no record of the fact that 
there was a previous value for the field. In a temporal database, 
two additional fields capture information about when the data 
was changed, permitting queries such as “What were the values 
on this day last year?”, “Has this field ever had a different value?”, 
and “Which fields have the most frequent changes?”. In the 
example, the date fields are given informally; however, in the 
database, the fields are of SQL data type TIMESTAMP WITH TIME 
ZONE and cannot be NULL. In terms of performance in the DCC 
scenario, since updates are not frequent occurrences, the 
additional database operations required in the temporal case will 
not impose a significant overhead. 

Links has recently been extended with temporal features that 
have been demonstrated in the context of curated scientific 
databases [28] and we use these features in the development of 
our prototype. 

3.3. XML documents 

For this paper, we have used a synthetic DCC in XML, 
containing dummy administrative data and measurement results, 
formatted according to the XML schema [10] made available by 
PTB.  

XML is a tree-structured language, and an XML document is 
an (inverted) tree of nodes starting from a root node. Each node 
can have any number of child nodes. Nodes without children are 
called leaves. 

Each node in an XML document is either an element or a text 
node (a string of characters). An element consists of an opening 

 

Figure 2. Non-temporal update of a field versus a temporal update. 

<dcc:digitalCalibrationCertificate 

     xmlns:dcc="https://ptb.de/dcc"  xmlns:si="https://ptb.de/si"  

     schemaVersion="3.1.2"  ... > 

  <dcc:administrativeData> 

    <dcc:dccSoftware> ... </dcc:dccSoftware> 

    <dcc:coreData> 

      <dcc:countryCodeISO3166_1>GB</dcc:countryCodeISO3166_1> 

      <dcc:usedLangCodeISO639_1>en</dcc:usedLangCodeISO639_1> 

       ...  

      <dcc:performanceLocation>laboratory</dcc:performanceLocation> 

    </dcc:coreData> 

    <dcc:items> 

      <dcc:item> 

        <dcc:name> 

          <dcc:content>Digital thermometer</dcc:content> 

        </dcc:name> 

         ... 

      </dcc:item> 

    </dcc:items> 

    <dcc:calibrationLaboratory> ... </dcc:calibrationLaboratory> 

    <dcc:respPersons> ... </dcc:respPersons> 

    <dcc:customer> ... </dcc:customer> 

    <dcc:statements> ... </dcc:statements> 

  </dcc:administrativeData> 

  <dcc:measurementResults> 

   ... 

  </dcc:measurementResults> 

</dcc:digitalCalibrationCertificate> 

Figure 3. An example DCC in XML: text nodes are shown by white text on a 
dark blue background, attributes by black text on a grey background and the 
other components are XML elements defined by opening and closing tags.  
The ellipses indicate where elements have been omitted. 



 

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tag and closing tag (or a combined opening and closing tag). The 
child nodes of the element (if any) appear in between a pair of 
opening and closing tags of that element. These child nodes can 
be elements themselves or text (strings of characters). A text 
node has no children (and hence is a leaf).  

An XML schema describes the tags that can be used in a 
document and how they can be nested [29]. Additionally, 
opening tags may contain attributes. Example nodes and 
attributes are given in Table 1 and Table 2. Figure 3 provides an 
example DCC using the XML schema. In the example, 
<dcc:performanceLocation> is an opening tag and 
</dcc:performanceLocation> is a closing tag, together providing an 
XML element, which contains the text node laboratory. In the 
figure, the opening tag for the dcc:digitalCalibrationCertificate 
element contains three attributes which are shown and these 
attributes provide sources for the XML schema and give the 
schema version used in the document. The children of this initial 
element are the elements dcc:administrativeData and 
dcc:measurementResults. 

The temporal features of Links are currently supported by an 
RDBMS, hence it is necessary to map from the XML schema to 
this database. There were two choices here: either to use the 
DCC XML schema to create the appropriate tables in the 
RDBMS or to treat the data in a schema-agnostic manner and 
capture it as an XML document. We choose the latter approach 
because changes to the XML schema will not require changes to 
the RDBMS schema. 

The Dynamic Dewey (DDE) labelling scheme supports the 
labelling of XML documents. It uniquely labels each node in an 
XML document and thereby provides a key for storing the node 
in a relational table. It has good query and insertion performance 
[30]. It is an extension of the Dewey labelling scheme which has 
good query performance but differs from it by it removing the 
need for relabelling (which reduces update performance) when 
the XML document is updated because Dynamic Dewey can 
provide unique labels for new nodes. 

To store an XML document using this approach, only two 
tables are required [30] (although a third table can be added to 
store path information separately – we omit these details here). 
This approach requires the generation of a unique value to serve 
as a key for each node (an element or a text node) in an XML 
document. The DDE algorithm determines this unique label 
(referred to as dde) which consists of a sequence of integers (the 
nodes can be labelled with negative numbers in certain cases) 
separated by dots – so-called dot decimal notation. A total order 
is defined for the labels (which differs from the standard ordering 
over dot-decimal notation), and this ordering can be used to 
reconstruct the nesting of the XML document as well as to 
determine relationships between nodes such as parent, child, and 
sibling.  

The main table stores nodes and is similar to the one 
illustrated in the right-hand side of Figure 2. Its schema is (dde, 
tag, text, path, tt-from, tt-to). Each row has the dde value for the node 
as the key attribute. If the node is an element, the element tag 
will appear in the tag column, otherwise if it is a text node, its text 
string will appear in the text column (one of these two columns 
is always empty). The path column stores the list of the element 
tags followed from the root element to the parent of the node. 
The last two columns tt-from and tt-to store the timestamps that 
describe when the node is valid. Because the dde value captures 
the relationships between nodes, there is no need to explicitly 
refer to other nodes in a row of this table. 

The second table contains attribute information and has the 
schema (dde, attr, value, tt-from, tt-to). In an element, each attribute 
can only appear at most once, hence the attribute name and the 
dde value are sufficient to uniquely identify the attribute’s value. 

Consider the following XML fragment: 

<el1 attr1=”A value”> 

  <el2>Text<\el2> 

  <el3 attr2=”Another value”><\el3> 

<\el1>. 

The DDE algorithm will generate the content shown in 
Tables 1 and 2. For this example, the length of each segment of 
the key is 3 characters. In the implementation, this length is a 
parameter, and it can be set appropriately for document size.  

As can be seen from this explanation, the actual details of the 
XML schema are not used for the database schema – there is no 
table for customer details, for example.  

By using the DDE ordering with the appropriate Links code 
to specify joins over the relations, documents can be stored, 
retrieved and displayed, as shown by our implementation. 

3.4. Implementation 

The prototype provides an interface for viewing a single DCC. 
Straightforward extensions include the ability to edit the DCC 
and to validate that it adheres to the DCC and D-SI XML 
schemas. Further obvious extensions are the ability to work with 
many DCCs, to compare different DCCs, and to support the 
signing of DCCs. We are interested in provenance beyond 
tracking changes in the data, such as recording which person and 
what software made the changes. 

In the longer term, we will require systems that support the 
whole calibration workflow including automated collection and 
processing of machine-readable calibration data.  

Using the interface, it is possible to view the current state of 
the document, the state at a previously specified date and time, 
and to compare two versions at two specified dates and times. It 
is possible to hide or show specific subtrees of the document, 
and when doing the comparison, to see the subtrees where 
changes have been made.  

The Change Analysis option allows the user to see a history 
of all changes over the life of a DCC. Figure 4 shows how 
changes are highlighted as well as cumulative totals of insertions 
and changes for each tag and text node in the document. There 
are various options to view subtrees for a specific tab or to 
control the display of the whole document. The figure also shows 
how details of changes are displayed. (Note that this screenshot 
is of a different DCC to the one presented in Figure 3.) The 
Links code for the prototype is available at 
https://github.com/vcgalpin/dcc-xml-temporal [31]. 

Table 1. Example node table. 

dde tag text path tt-from tt-to 

001 el1  \ … … 

001.001 el2  \el1\ … … 

001.001.001  Text \el1\el2\ … … 

001.002 el3  \el1\ … … 

Table 2. Example attribute table. 

dde attr value tt-from tt-to 

001 attr1 A value … … 

001.002 attr2 Another value … … 

https://github.com/vcgalpin/dcc-xml-temporal


 

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As mentioned above, a more complete application should be 
able to validate against the DCC and D-SI XML schemas. The 
level of validation that is provided by these schemas is very high 
but not complete. In some cases, patterns are used to ensure that 
values are within bounds. For example, the si:probabilityValueType 
[9] has the following associated pattern 

<xs:pattern value="\+?((0(\.\d*)?)|(1(\.0*)?))”/>. 

This pattern specifies that a value of this type is an element 
of the interval [0,1], as required for a probability value. However, 
not all aspects of the input for a DCC are specified in this manner 
and it would be useful to be able to check for correct spelling of 
measurement units and probability distribution names, as well as 
requirements that are specific to a particular domain of 
measurement or instrument. Good practice guides for DCCs in 
specific domains are being prepared [14] and another possibility 
is to provide additional schemas that prescribe requirements for 
specific groups of instruments.  

Moreover, not all functionality needs to be supported directly 
in Links. For example, schema validation, which could be 
provided by different software such as the TraCIM system 2.0 

developed by PTB as part of the SmartCom project [32]. The 
application validates XML documents for the D-SI schema, and 
also undertakes additional compliance checking of the units used 
and the minimum metrological information required by VIM [33] 
and GUM [34]. However, the provision of a suitable interface for 
users will require the integration of the different pieces of 
software. 

3.5. Discussion 

Considering our choice to work with a relational database, 
Links may in future support the storage of XML documents 
directly with temporal features, and hence the translation would 
no longer be necessary. In that case, the schema be used more 
directly to store data, such as the customer details, for example.  

Since the DCC is only one component of the envisioned 
digital calibration workflow and can be seen as output and data 
exchange from some form of data storage, there is no need to 
use an XML storage format. Dealing with temporal data in a 
relational database is better understood and less experimental so 
we believe it makes sense to use a RDBMS for a system for digital 

 

Figure 4. A screenshot of the Change Analysis interface showing an insertion and a change of email – both are highlighted. The insertions and changes columns 
provide information about the number of modifications. The first number shows the number of changes for the particular tag or text node, and the number 
in brackets shows the number of changes that have happened below and including the current tag or text node. 



 

ACTA IMEKO | www.imeko.org March 2023 | Volume 12 | Number 1 | 6 

calibration data. Working in a schema-less fashion with the XML 
document provides the ability to cope with changes in the DCC 
XML schema without affecting the storage in the RDBMS, and 
we have already found this approach to be of benefit when 
changes were made to the DCC schema. 

Another approach to deal with update provenance is 
provided by databases that support versioning such as 
OrpheusDB [35]. This relational database is based on the 
concepts that are used in software versioning systems such as git 
[36] and is motivated by the requirement to track changes in 
curated scientific databases. Temporal databases can be seen as 
providing a linear model of time whereas versioning supports a 
non-linear model of time. In the non-linear model, it is possible 
for separate branches to have different updates that then must 
be merged together at a later time point. It seems unlikely that 
DCCs require this level of complexity. Moreover, versioning 
usually requires users to explicitly create versions, whereas 
temporal database provide an automatic approach to tracking 
changes. Furthermore, querying in the linear model is more 
straightforward than in the non-linear model. 

4. FUTURE WORK AND CONCLUSIONS 

The prototype we have developed demonstrates that the 
combination of DCCs and temporal databases results in 
streamlined curation of calibration data. The prototype leverages 
features of the Links language  

(a) to facilitate the development of a user-friendly web front-
end,  

(b) to integrate relational database queries on a database that 
maps with the DCC XML tree structure, and  

(c) to enable automated tracking of changes made to 
calibration records over time, as well as the explanation 
and timeline for those changes. 

Looking to the future, further advances in the area of 
calibration certificates are in the pipeline, such as Digital 
SchemaX [37] which will include Envelope Digital Certificates 
for grouping related calibration certificates together, as well as 
schema for digital reference materials and digital test certificates, 
and these advances will require appropriate digital solutions. 

Another possibility that results from the digital storage of 
multiple certificates is to link information across certificates, and 
to develop new aspects of traceability, and related to this 
development, certificate validity and frequency of calibration. In 
many cases, NMIs and calibration laboratories might not know 
the exact needs of the users at the end of the measurement chain. 
To meet those needs, a comprehensive traceability chain 
including the specification of the dependence between 
consecutive measurement stages is therefore required [38]. Our 
temporal approach to the curation of calibration data could be 
used to retain detailed information that the end-users would want 
to process in a way that was not anticipated by the NMI and 
calibration laboratories, for example, analysing the risks 
associated with factors that impact multiple stages across the 
measurement chain. 

To conclude, DCCs are the tip of the iceberg (Figure 5) when 
considering the whole lifecycle of machine-readable calibration 
data. Their creation and use depend on software to support 
storage and provenance of the underlying calibration data. The 
development of the prototype has raised interesting questions 
about what such a system should be able to provide, and we will 
continue with our investigations and development. 

ACKNOWLEDGEMENT 

Funding: [VG] This work was supported by ERC 
Consolidator Grant Skye [grant number 682315] and by an ISCF 
Metrology Fellowship grant provided by the UK government’s 
Department for Business, Energy and Industrial Strategy (BEIS). 
[IS, JLH] The National Measurement System of the UK 
Department for Business, Energy and Industrial Strategy 
supported this work as part of its Data Science programme. 

We thank James Cheney for proposing the iceberg analogy, 
and Simon Fowler for his assistance in using the temporal 
extensions to Links. 

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Figure 5. DCCs are the tip of iceberg: software support is needed for the full 
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