You addressed galvanic sensors very well but neglected the non-lineraity of the output with respect to O2 changes and the temperature dependence of this type of sensor.
Most galvanic O2 cells will not have a linear ouput with increasing oxygen content (actually partial pressure). I have found that the Teledyne R-17 are more linear than the maxtec/ceramatec 250 sensors. For a R-17 that is on its last leg the output at a PP of 0.21 ATM its output is 8.5 mV @ 70 degF and at a PP of 1.0 the output is 39.4 mV. If it was linear it would be around 40.5 mV. For a Maxtec 250E that is also about 2 years old it is 10.5 mV and 48.5 mV. If linear it would be 50.0 mV
With increasing temperature the oxygen sensor signal rises. This performance is usually compensated with resistors that have a negative temperature coefficient (NTC). The issue of temperature is important for rebreathers and if you are analyzing a tank that was just filled at a high rate (fast fill) and as a result is hot. It is important to have the sensor and gas at the same temperature.
Also the issue of calibrating the unit at an equivalent pressure/flow rate is important and can result in major differences if it is not addressed. In the typical galvanic cell oxygen molecules pass through a membrane and are reduced at a gold-cathode. The higher the O2 partial pressure, the more molecules will pass through the membrane and the current output by the sensor increases. The strength of the current gives the O2 partial pressure in the gas. Oxygen analyzers using galvanic cells (which is what is being discussed here) indicate the partial pressure of the oxygen NOT the percent volume. That is why if you calibrate at one pressure/flow rate and than use a different one for analysis you can end up with an incorrect reading.
And finally, there are a number of other types of O2 sensors that are available as well, ie polargraphic, paramagnetic and zirconia units.
omar
Most galvanic O2 cells will not have a linear ouput with increasing oxygen content (actually partial pressure). I have found that the Teledyne R-17 are more linear than the maxtec/ceramatec 250 sensors. For a R-17 that is on its last leg the output at a PP of 0.21 ATM its output is 8.5 mV @ 70 degF and at a PP of 1.0 the output is 39.4 mV. If it was linear it would be around 40.5 mV. For a Maxtec 250E that is also about 2 years old it is 10.5 mV and 48.5 mV. If linear it would be 50.0 mV
With increasing temperature the oxygen sensor signal rises. This performance is usually compensated with resistors that have a negative temperature coefficient (NTC). The issue of temperature is important for rebreathers and if you are analyzing a tank that was just filled at a high rate (fast fill) and as a result is hot. It is important to have the sensor and gas at the same temperature.
Also the issue of calibrating the unit at an equivalent pressure/flow rate is important and can result in major differences if it is not addressed. In the typical galvanic cell oxygen molecules pass through a membrane and are reduced at a gold-cathode. The higher the O2 partial pressure, the more molecules will pass through the membrane and the current output by the sensor increases. The strength of the current gives the O2 partial pressure in the gas. Oxygen analyzers using galvanic cells (which is what is being discussed here) indicate the partial pressure of the oxygen NOT the percent volume. That is why if you calibrate at one pressure/flow rate and than use a different one for analysis you can end up with an incorrect reading.
And finally, there are a number of other types of O2 sensors that are available as well, ie polargraphic, paramagnetic and zirconia units.
omar
