Optical vs. Polarographic DO Sensors: Side-by-Side Comparison
The following table summarizes the differences between optical and polarographic DO sensing technology.
DO Measurement Technology Comparison
| Polarographic Sensor | Optical Sensor | |
Typical Response Time T98% |
<60 s @25°C Air to N2 |
<30 s @25°C Air to N2 |
Typical Measurement Range |
10 ppb to 40 ppm (DO) |
4 ppb to 40 ppm (DO); versions also exist for trace DO measurement |
Power |
None, but the sensor requires connection to transmitter for polarization and conversion of weak nA signal |
7 to 30 VDC max, 1W |
Operation |
Consumes oxygen; requires flow past sensor |
No oxygen consumption or need for flow |
Startup Time |
2-h polarization time required prior to measurement |
Measurement is immediate |
Maintenance |
Requires frequent rebuild and replacement of electrolyte and membrane cap; periodic replacement of cathode |
Periodic membrane cap replacement as indicated by internal quality indicator |
Known Interferences |
CO2, H2S, strong chemicals that can permeate the membrane |
Strong oxidizers such as ClO2 may require special cap design with additional jacketing. Contact Hamilton's Customer support |
A Note on Device Costs
The purchase price of optical DO sensors tend to be higher than that of their polarographic counterparts. However, the price difference is more than made up for by the reduction in ongoing maintenance. The electrolyte and membrane cap for a polarographic sensor are pricey and must be replaced frequently, and the process is labor-intensive. Over the service life of the device, an optical DO sensor has a much lower total cost of ownership than its polarographic equivalent.
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