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BWM HL 009
New Dissolved Oxygen Technology
·
At last! a new
technology for measuring dissolved oxygen
·
The LDO™ (Luminescent
Dissolved Oxygen) manufacturer makes startling claims for the performance of
this new sensor
·
LDO™ found to be
“virtually maintenance free”
The launch of a completely new technology for the measurement of dissolved
oxygen is catching the attention of laboratory managers worldwide, and not
least at Chester based Meadow Foods. The company processes seven million litres
of milk and cream a week to produce milk fat and associated products for the
food manufacturing industry. Its customers include all the major dairies and
many of the UK’s leading confectionery manufacturers, and as a consequence it
operates some of the latest process and analytical technology. It is
appropriate, therefore that Meadow Foods should be amongst the first to trial
Hach Lange’s new LDO™ dissolved oxygen meter.
Background
For
more than fifty years galvanic and polarographic sensors have been used to
measure dissolved oxygen. These sensors employ membranes, anodes, cathodes, and
electrolyte solutions that generally require a high degree of maintenance. The
sensors also suffer from drift, and as a result have to be recalibrated
frequently.
Historically, there have been a number of problems associated with galvanic and
polarographic sensors. The membranes are relatively delicate, and can become
contaminated or damaged, in which case it would be necessary to replace the
internal electrolyte. The sensor’s anode is consumed over a period of time and
will require replacement, or it may need replacement if it, or the electrolyte,
becomes poisoned by gases such as hydrogen sulphide.
There are other factors that can affect the accuracy of these traditional
sensors, including variations in pH or the presence of chemicals that induce
voltage, such as iron and aluminium salts, and polymers.
In recent years considerable effort has been expended on the further development and optimisation of the electrochemical measuring technique. The primary feature of all electrochemical measuring techniques is, however, that for every molecule reduced at the cathode, a corresponding oxidation reaction takes place at the anode that results in the degeneration of the anode and in the breakdown of the electrolyte. Both processes inevitably lead to drifting measurements or low readings that can only be kept in limits by means of regular calibration by the user.
The
manufacturer of the LDO™ claims to have solved these long-standing problems
with the launch of a sensor that, in contrast to its predecessors, does not
consume oxygen as part of the measurement process, and does not require
frequent recalibration because it does not suffer from drift (gradual loss of
accuracy). So, how does it work?
The
LDO™ measurement principle is based on the physical appearance of luminescence.
This is defined as the property of certain materials (luminophor) to emit light
that is not produced by heat but as a result of excitation by light. The type
of luminophore and the wavelength of the excitation light, have been carefully
selected, and both the intensity and the decay of the luminescence radiation
over time are dependent on the oxygen concentration that surrounds the
material.
The
LDO™ sensor is comprised of two components:
A
sensor cap with luminophore, applied to a transparent carrier material.
The
sensor body with blue and red LED, a photodiode and an electronic analysis
unit.
In
operation the sensor cap is screwed to the sensor body and immersed in the
water. Oxygen molecules from the sample to be analysed are thus in direct
contact with the luminophore.
The measurement process begins when the blue LED emits a pulse of light. This passes through the transparent carrier material and transfers part of its radiation energy to this material. Electrons from the luminophore are in this way raised from their initial energy state to a higher energy level. This level is left again via a number of intermediate levels (within ms); here the energy difference is emitted in the form of red radiation .
If
oxygen molecules are in contact with the luminophore:
1.
they are able to absorb the energy of the electrons in higher energy
levels and make it possible for the electrons to return to their initial state
without the emission of radiation, and with increasing oxygen concentration
this process results in a reduction of the intensity of the red radiation
emitted, and
2. they cause vibrations in
the luminophore that result in electrons leaving the higher energy level more
quickly. The lifetime of the red radiation emitted is thus shortened.
Both aspects are covered by the term quenching. The pulse of light sent out by the blue LED at time t=0 is incident on the luminophore that then emits red light immediately afterwards. Maximum intensity (Imax) and the decay time for the red radiation are dependent on the surrounding oxygen concentration (the decay time t is defined here as the time between excitation and reduction in the red radiation to 1/e times the maximum intensity).
To
determine the oxygen concentration the life t of the red radiation is analysed. In this way the oxygen measurement is
reduced to a purely physical measurement of the time.
The
sensor is continuously adjusted with the aid of the red LED fitted in the
probe. Prior to each measurement this emits a beam of light, of known
characteristic, that is reflected at the luminophore and passes through the
entire optical system. Changes in the measuring system are thus detected
without delay.
In
summary, the sensor is coated with a luminescent material, called luminophore,
which is excited by blue light from an internal LED. As the luminescent
material relaxes it emits red light, and this luminescence is proportional to
the dissolved oxygen present. An internal red LED provides a reference
measurement before every reading to ensure that the sensor’s accuracy is
maintained.
The Trial
The
LDO™ was assessed during September and October 2003, and found to be very easy
to set up and simple to use. The
instrument was found to be robust in design and performed well under test,
giving reproducible results during the testing period. However, the main
advantage of the LDO™ is the lack of maintenance required; it is not necessary
to replace membranes or electrolyte, or to perform recalibration, which
provides substantial cost savings. The sensor cap is merely replaced once per
year.
The
internal batteries allow the LDO™ to be removed from its docking station and
used in the field to determine DO levels in the receiving watercourse both upstream
and downstream of the factory.
Naturally,
the benefits of the LDO™ will not be confined to the laboratory/portable
versions; on-line measurement of dissolved oxygen is critical to the management
of most wastewater treatment plants. Consequently, an on-line version of the
LDO™ has been on trial at seven different Water Companies, all of which are
delighted with its performance. Previously the staff at these waste treatment
plants have had to frequently remove DO probes from the activated sludge for recalibration,
so it is not surprising that a probe which simply requires a new cap once per
year has become very popular.
Nikki Mellor, Hach Lange’s UK Marketing
Manager is obviously already delighted with customer feedback and reports that
“there has been massive interest in the LDO™ and it would appear that it will
quickly dominate the markets for both laboratory and portable DO meters”
Ends
Words: 1,260
For further information, contact Nikki Mellor, UK Marketing
Manager, Hach Lange
Tel. +44 (0) 1256 333 403
Email Nikki.Mellor@hach-lange.co.uk
