FAQ : Dissolved Oxygen Monitors

Polarographic

How does your polarographic dissolved oxygen system work?

Instech's model 125/05 electrode is designed around a style of polarographic electrode developed by Dr. Leland Clark. When a potential of about 0.7V is applied between the anode and the cathode, dissolved gaseous oxygen is reduced at the platinum cathode. This produces a current and consumes the oxygen in the immediate vicinity of the exposed platinum cathode. Oxygen in the sample volume diffuses through the membrane to the oxygen-poor region between the membrane and the electrode. When a steady-state is reached (which usually happens in less than 4 seconds), the electrode current is proportional to the rate of arrival of oxygen molecules at the cathode, which is in turn proportional to the concentration of oxygen outside the membrane.

The model 203 oxygen amplifier does two things: (1) it generates the electrode biasing potential and (2) it measures the electrode current, usually only a few nanoamps, and converts it to a voltage.

The electrode is quite sensitive to temperature changes as temperature affects the speed of the chemical reaction, the ease of diffusion across the membrane and the ambient oxygen concentration of the sample solution. Electrode readings will increase by about 4% for a temperature increase of 1°C.

In order to make an accurate measurement of oxygen levels in the entire sample volume, the oxygen around the outside of the membrane must be replaced. If the sample is not adequately stirred when working in batch cell mode, or if the flow rate is too low when working in flow cell mode, a concentration gradient will be set up outside the membrane and the rate of arrival of oxygen at the electrode will decrease. Slight fluid velocity changes will produce a "motion artifact."

The type of material used as a membrane also critically affects the performance of the system. We have found that 0.001" to 0.0009" thick polyethylene, which is readily available from normal sandwich bags, works best. Membranes that are more oxygen permeable, such as TFE Teflon or silicone, may be required for experiments at low temperatures or when a response rate faster than 4 seconds is desired. However, these materials will cause a greater motion artifact because the concentration gradient extends further into the bulk solution. TFE Teflon will also stretch and thin over time, causing the initial amplifier calibration to become incorrect. FEP Teflon usually works well, but is too stiff to wrap around the small electrode tip. Saran is not sufficiently permeable to oxygen to be useful as a membrane.

Are your miniature electrodes compatible with amplifiers from Yellow Springs Instruments (YSI)?

Yes, with the appropriate cable (be sure to specify SYS600Y or 125/05Y), the electrodes are compatible with the YSI Model 5300 amplifier.
How does your amplifier compare to the one made by YSI?
The performance specs, such as signal-to-noise, are similar. Unlike the YSI Model 5300 amplifier, our amplifier can display the difference between the two channels, and has zero offsets for both channels.

If my signal drifts and is excessively high, what should I do?

The membrane may be perforated or the chloride layer may be contaminated. Clean and rechloride the electrode, and replace the membrane. If this doesn't solve the problem you may have a salt bridge in the cable or connector. To check this, remove the electrode from the chamber, remove the membrane, and dry the tip of the electrode. If the electrode does not read zero pO2, you have a salt bridge and you will have to replace the electrode and cable.

What do I do if my signal wanders at typical levels?

Make sure the temperature of your system is stable (electrode signal varies 4%/degree C). Make sure the solution is stirred sufficiently. Remove any air bubbles on or under the membrane. If you are still having problems, rechloride and/or renovate the electrode tip.

My signal drifts steadily downward. What's happening?

You probably have bacteria in your system. Disassemble and clean thoroughly with alcohol or Clorox®.

What do I do if the electrode does not respond when I turn it on?

The electrolyte has probably dried up; replace the membrane. Or you may be using an impermeable membrane such as Saran. To check for a broken electrode or cable, remove the membrane and dip the tip into deionized water with the sensor turned on. If the amplifier does not register a high reading, you may need to replace the electrode.

Do I need to keep the electrode wet?

No. Store the electrode with the membrane on to protect the tip and replace the membrane and electrolyte before the next use.

Can I use prepared electrolyte solution?

It is best to use KCl solution that has been prepared on the same day. Be sure to use clean glassware.

Where can I find replacement membrane material?

Go to your local grocery store and buy standard polyethylene sandwich bags (e.g., Glad®). Most bags are of the correct thickness - approx. 0.001 inches - and you'll get a nearly unlimited supply for only a few dollars. If you need additional o-rings, installation tools, etc. you may order our electrode maintenance kit, part number IT-125.

Fiber Optic Oxygen Monitoring

How does your fiber optic dissolved oxygen system work?

The sensing end of the fiber in the 125/FO oxygen sensor is coated with a flourophor captured in a sol-gel. As oxygen binds to this material its fluoresence level is quenched. The sensor tip is illuminated with a pulsed blue LED (included in the monitor box) and the fluoresence is measured by a spectrometer, also housed in the monitor box. The signal from the spectrometer is passed to a PC using a serial A/D board. Windows®-based software processes the flouresence information and determines oxygen concentration.