take note
• Any step to avoid installation effects in sensors is important.
• Systematically, sources of error are being designed out of
modern sensors.
• About half of all analytical measurement is of pH.
This often presented a special challenge for maintenance personnel
as sometimes the measuring points were installed on boilers several
metres high. In addition, frequent poor weather conditions contrib-
uted greatly to the inaccuracy of the pH calibration. Today, 70 to
80% of pH measuring points is still calibrated this way and the risk
of error is not to be underestimated.
Safety factor 3:Transmitter
Owing to the necessary signal conversion, the sensor and transmitter
became an inseparable pair: this combination must be available for
each calibration. More than 250 000 2- and 4-wire transmitters are
produced annually, with mainly the two-wire transmitter sold to the
process industry. The transmitter is still the most complex and failure-
prone element in the entire measuring chain consisting of electrode,
cable, transmitter and PLC. If you consider the possible error in the
measuring chain according to FMEA or SIL, the transmitter represents
one of the main sources of error based on percentage. The reasons for
this include faulty wire connections during installation, faulty entries
in the complex operating unit and faulty electronics. These errors can
only be rectified with a great deal of maintenance and testing cost,
on average approximately 100 to 300 minutes of maintenance per
year and transmitter were recorded.
Digital pH sensor
Little about these safety factors changed with the birth of the digital
pH sensor in 2006. The idea of inductive data and energy transfer
between sensor head and data coupling solves the problem of high
resistance. However, because of the inductive data transmission,
electronics, memory and processors are required in the sensor head
and in the coupling of the connecting cable. Since there are now
electronics in the connecting cable as well, the overall risk of failure
has not been significantly reduced. At the same time, the system
becomes proprietary which displeases some users who do not want
to be dependent on one manufacturer.
It was only in the years that followed that the revolutionary
options of digital sensor technology in terms of offline calibration
were recognised. This meant that the lifetime of the pH sensors was
extended many times and that by calibrating in the laboratory the
error risk during calibration was reduced. Thanks to the digital tech-
nology, starting in 2006 many customers had the increasing desire to
omit the transmitter and integrate the entire electronics in the sensor
head of the electrode. For economic and technological reasons this
remained a wish – until today.
Intelligent sensors
The sensors described by the author are two-wire sensors featuring
4...20 mA Hart 7 communication – the field bus remains dominant in
2013 due to its widespread installed base – and the IECEx certification
for Ex-Zone 0. With its IECEx II 1G ia IIC T4-T6, Zone 0-2 certification
class, the Smartsens pH sensor can be used in Zone 0. The IECEx
certification is the most comprehensive of all and contains the criteria
for Atex, Nepsi, FM and CSA. The first step includes sensors for the
parameters pH, ORP and conductivity, followed by other parameters.
Especially designed for hazardous environments and hygienic operat-
ing conditions, Smartsens sensors are equally suited to other industries.
Safety along the measuring chain
With the 4…20 mA Hart 7 communication, the sensors discussed
in this article have a true open standard: They can be connected
directly to the master display using standard cable. The sensors are
operated with today's customary handheld devices. The Hart DDs
can be downloaded onto these devices. Smartsens sensors can run
with various common asset management systems. With the existing
sensor base, theses sensors can be used without problem anywhere
the installed base is VP or Sixplug. Handling and installation have
been kept as simple as possible; the connector makes it impossible
to incorrectly connect the wires.
Conclusion
The sensors can also be calibrated offline. In this case, the company,
represented by the author, uses a special open standard: Through
the Smartbridge Hart-USB cable the sensors are provided with power
directly from the PC during calibration and can use this connection
to communicate bidirectionally with the PC. The user interface for
offline calibration is the frame application Pactware, which complies
with the FDT/DTM standard.
FLOW MEASUREMENT + INSTRUMENTATION
Abbreviations
In 1983 Wolfgang Babel received a diploma in Electrical Engi-
neering from Friedrich Alexander University in Erlangen and in
1987, a PhD from Columbia University in the USA. His positions
at Endress+Hauser included R&D director and managing direc-
tor – Analytical, manager of Industry Environmental worldwide
and managing director Process Solutions. He was also a
member of the Board. He is currently the managing director, Krohne Analytics.
Enquiries: Krohne South Africa. Tel. 011 314 1391 or email
.
ATEX – ATmosphere EXplosible
CSA
– Canadian Standards Association
DTM – Device Type Manager
FDT
– Field Device Tool
FM
– Factory Mutual
NEPSI – National Supervision and Inspection Centre for Explosion Protection and
Safety of Instrumentation (Ex for China)
NPT
– Network Point Type
PC
– Personal Computer
SIL
– Safety Integrity Level
USB
– Universal Serial Bus
27
July ‘14
Electricity+Control
