Unique dissolved oxygen analysis probes provide excellent long-term stability, and require very little cleaning and maintenance

Citation

(2000), "Unique dissolved oxygen analysis probes provide excellent long-term stability, and require very little cleaning and maintenance", Sensor Review, Vol. 20 No. 2. https://doi.org/10.1108/sr.2000.08720baf.008

Unique dissolved oxygen analysis probes provide excellent long-term stability, and require very little cleaning and maintenance

Unique dissolved oxygen analysis probes provide excellent long-term stability, and require very little cleaning and maintenance

Keywords: Honeywell, Oxygen, Equilibrium

Advantages in effluent treatment, boiler feedwater monitoring and process water quality in semiconductor manufacture

Honeywell has introduced unique equilibrium probes that measure dissolved oxygen without consuming oxygen or reagents.

Their innovative design provides accurate measurement with long-term stability. They not only are unaffected by inert fouling or changes in flow conditions, but also require very little cleaning and maintenance, compared with conventional probes (see Plate 7).

Plate 7Honeywell's unique equilibrium probes measure dissolved oxygen without consuming oxygen or reagents. Their innovative design provides accurate measurement with long-term stability

Honeywell equilibrium probes can be installed for immersion, in-line or flow-through applications. They are housed in a rugged PVC or stainless steel one-inch NPT pipesized housing.

Dissolved oxygen is measured for many reasons. In wastewater treatment, its concentration must be kept above a critical value to support the microbes that remove bacteria. In power plant boilers, the dissolved oxygen level must be low to prevent corrosion in the boiler feedwater cycle.

The Honeywell probe is unique in that the oxygen consumed to make the measurement is balanced by oxygen produced within the probe. The oxygen within the probe is in equilibrium with the oxygen in the sample, so results are independent of flow rate and inert fouling. Since the chemical reaction, which takes place in the probe, does not create by-products such as lead, which may contaminate the anode, no internal parts of the probe require cleaning or refurbishing. This leads to longer maintenance-free periods of operation.

The fact that no probe materials are being consumed also leads to greater probe stability. Honeywell's unique technology provides the only long-term solution without extensive maintenance or modification of the probe or analyser.

Long-term accuracy depends on both the inherent stability of the probe and analyser and independence from those variables that can interfere with measurement. For instance, some types of dissolved oxygen probes produce signals that depend on a continuous diffusion of oxygen through a membrane. Variations in flow rate or cleanliness produce signal changes that are erroneously interpreted as variations in dissolved oxygen concentration. Furthermore, results can be compromised as a probe ages because parts of the probe are consumed during the measurement. Those probes, as well as others that utilise no membrane at all, can require intensive maintenance, as they require periodic changing of internal electrolytes, etching of internal or external electrodes, or even replacement of the whole probe.

How the Honeywell probe works

Honeywell's unique equilibrium sensor comprises two electrodes, an anode and a cathode, inside a membrane filled with a reference electrolyte. When immersed in a sample, oxygen diffuses through the membrane until the partial pressure of oxygen on both sides of the membrane is equal and balanced. Oxygen is reduced at the probe cathode, while an equal amount of oxygen is generated at the anode. The electrical current through the electrolyte is directly proportional to the oxygen content.

With increasing oxygen content in the sample, more oxygen diffuses in through the membrane and is available for reduction, so the current increases. Conversely, decreased oxygen in the sample results in diffusion of oxygen from the membrane and reduced current.

The reactions are:

At cathode: O2 + 4H+ + 4e- - 2H2O

At anode: 2H 2O - O2 + 4H + + 4e -

It can be seen that no oxygen is consumed, no reagents are consumed and no net reaction occurs. Further, no internal maintenance is required.

By comparison, conventional diffusion-type probes, working on either the galvanic or polarographic principle, require continuous oxygen transfer through the membrane in a one-way reaction. They are highly dependent on sample flow past the membrane to replenish consumed oxygen and on consistent membrane cleanliness to give a constant diffusion rate of oxygen. These probes also require periodic electrode, electrolyte and membrane maintenance, which are not necessary with the Honeywell probe.

Honeywell dissolved oxygen analysers

Honeywell complements the probes with two models of analyser, which monitor and control dissolved oxygen over the parts-per-billion (ppb, 10-9) and parts-per-million (ppm, 10-6) measuring ranges respectively. Features include proportional control, probe bias diagnostics and compensation for pressure and salinity. Temperature compensation is carried out by the Honeywell analyser via a temperature sensor mounted within the probe, alongside the oxygen sensor.

The analysers offer menu-driven software, with a back-lit LCD display having up to six lines of text. They include PID control and up to five relays for alarm, control, automatic cleaning and calibration and/or range identification. Internal diagnostics test and indicate the probe status, calibration, signal level, temperature compensator integrity, power supply and AC noise. Diagnostic and alarm data are retained for up to four hours after loss of power.

For further information on Honeywell dissolved oxygen probes and analysers, please contact: Honeywell Control Systems Ltd, Honeywell House, Arlington Business Park, Bracknell, Berkshire RG12 1EB, UK. Tel: +44 (0) 1344 656000; Fax: +44 (0) 1344 656240; E-mail: info.centre@uk.honeywell.com; Web: www.honeywell.com/uk