C-sense in situ pCO2 Sensor
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Equilibration Graph

C-sense in situ pCO₂ Sensor

C-sense™ probes are compact, lightweight sensors for measurement of the partial pressure of gas in liquids. Designed for applications involving immersion in water, oil, or water and oil mixtures, the sensors combine an oil-resistant interface with a compact, temperature-compensated non-dispersive infrared (NDIR) detector. C-sense ships with a copper antifouling guard which minimizes biofouling as well as protects the membrane. Designed for integration,
C-sense enables pCO₂ monitoring at a significantly lower price than
traditional pCO₂ sensors. 

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Frequently Asked Questions

Does the C-sense have to be corrected for hydrostatic pressure?

No. Hydrostatic pressure has no effect.



Does the C-sense need to be corrected for salinity?

No. Salinity has no effect on the C-sense measurement as it is an aqueous measurement.



Does the C-sense need corrections for temperature and total dissolved gas pressure (TDGP)?

The C-sense does not need to be corrected for temperature, it is compensated for in the sensor. However for increased accuracy, it should be corrected for TDGP. Each C-sense is calibrated at 20 degrees C and an atmospheric pressure at or near 101.3kPa (pressure at the time of calibration is stated on the calibration sheet which ships with the C-sense). Corrections can be made, assuming TDGP is known, using the equation below. pCO2 actual = (pCO2 measured x TDGP) / calibration pressure



Under what conditions should a user measure TDGP?

A user needs to measure TDGP when the atmospheric pressure is significantly different than the atmospheric pressure at the time of calibration. This situation arises during extreme weather events and when sampling at elevation. The largest pressure swings caused by weather are on the order of 3% deviation from standard sea level pressure. Independent of atmospheric pressure, TDGP in the ocean is primarily controlled by the amount of dissolved oxygen. Therefore, if oxygen levels at your intended sampling site are anomalous, you are advised to measure TDGP.



What kind of error can be expected if TDGP is not measured?

For every 1% change in TDGP from the pressure during calibration, there results an ~1% error in CO2. To put this into context, the accuracy of the C-sense is 3% of full scale, so depending on the application, you may or may not need to correct for TDGP.



How does the C-sense measure at the specific wavelength absorbed by CO2?

As a proxy to CO2 concentration, absorption of a light source passing through the sample gaseous headspace is measured. In order to limit the wavelengths emitted by the light source to those specific to CO2 absorption, a filter is used.



Does the C-sense sample wet air or dry air?

Because the C-sense does not measure, or compensate for water vapor entering the headspace, it measures a wet sample.



What is the warm-up time for the instrument?

The warm-up time, specific to the operation of the detector, is between 45 seconds and 3 minutes. If the instrument has been powered down and allowed to cool, the user should wait the full 3 minutes. If the instrument is sampling frequently, it won’t cool as much between samples, and therefore, the warm up time is less. When choosing a warm-up time, the ambient temperature of the water being measured should be considered. In order to prevent recording measurements before the detector has warmed, the instrument will not output data for 45 seconds after power is applied.



What it the equilibration time for the instrument?

Equilibration times for pCO2 sensors are often stated as a t63 level. This is defined as the time it takes for the instrument to reach 63% of equilibration. For the C-sense, this value is ~4 minutes.
The equilibration time can be reduced with the use of the Water Pumped Head (P.N. 2400-700).



Can the C-sense be integrated with a SeaBird Electronics CTD?

The C-sense outputs a 0-5 VDC signal proportional to the pCO2 concentration. There are multiple Sea-Bird Electronics CTD’s that can record this signal, convert it to a concentration in ppm, and write it to a file. Information on how to integrate the C-sense to a SeaBird Electronics SBE19 CTD is available here



How often does the C-sense need to be calibrated? If the calibration is a year old, but the instrument has never been deployed, does the instrument need to be recalibrated?

With regular use, the C-sense needs to be calibrated once a year. If the instrument has never been deployed, or deployed infrequently followed by the recommended cleaning techniques, there is no need to re-calibrate.



Is bio-fouling an issue for the C-sense and if so, how can it be mitigated?

Bio-fouling is an issue for most submersible instrumentation measuring natural environments, both fresh and saline. In the case of the C-sense, accumulation of organisms on the membrane has two potential but competing effects. The presence of the accumulation can decrease the equilibration time by impeding the transfer of gasses. Alternatively, the organisms can emit CO2 as a result of respiration, artificially elevating the CO2 concentration. Turner Designs offers accessories for the C-sense that help reduce the effects of bio-fouling. Copper Antifouling Guard P/N 2400-507 can be installed on the sensor head outside the membrane. This protects the membrane and prevents organisms from growing on the instrument head. The Water Pumped Head P/N 2400-700 accessory will inhibit bio-fouling in two ways, first by significantly reducing the amount of available light for photosynthesis, secondly by creating shear stress that prevents organisms from settling on the membrane. The Water Pumped head has the additional advantage of reducing equilibration times. The two accessories should not be used together so it is recommended that the user decide on the necessity of the Water Pumped head first, and plan accordingly.



What material are the screws in the C-sense made of?

The screws are 3-16 stainless steel.



What does the 3% "error" mean in relation to the measurement scale of the C-Sense?

The accuracy is plus/minus 3% of the calibration range, not of the measured value. This would be a possible error of 120ppm for 0-4000 range, or 300ppm for 0-10,000 range.



The dissolved oxygen in our system ranges from 50-150% saturation. Is that going to be a source of error if we do not correct for this?

The CO2 detector works by measuring in the gas phase using an IR detector, this method detection is sensitive to the gas pressure inside the IR cell. In most natural waters the dissolved gas pressure is close to atmospheric and the correction for changes in the gas pressure of the cell are very small. In the case of highly fluctuating O2 saturation, the dissolved gas pressure in the water can change dramatically - up to 10% of the total dissolved gas pressure for 50-150% O2 levels. The level of change is not equal to the change in O2 as there may be physical mixing and exchange with the atmosphere that can change N2 levels when O2 is variable. The bigger concern with trying to correct using O2 data is that O2 is much slower to equilibrate across the sensor’s membrane and this can lead to large lag times for changes in gas pressure in the cell when compared to measured O2 changes.



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