How do you determine when the readings are stabilized?
If one of the readings is still climbing or dropping slightly, but is within the stabilization criteria, is it alright to sample?
Stabilization is based on two things - the criteria for what is "stable" and the precision of the measurement system (i.e., sensors and instrument). Stabilization criteria for purging vary with the parameters but generally should be +/- 3-5% of the reading value for EC/SpC, +/- 0.2 pH units, and +/- 10% or 0.2 mg/L for DO, whichever is greater. (NOTE: regulatory requirements may dictate other criteria.) Usually, you do not use redox/ORP or turbidity as indicators but, if you are required to use these, then +/- 20 mv for redox/ORP and +/- 10% or 1.0 NTU for turbidity, whichever is greater, are reasonable criteria for these parameters.
Three consecutive readings taken about 3-5 minutes apart that fall within those criteria would be considered stable water chemistry. The time between readings is determined by the volume of water in the pump and/or tubing relative to the purging flow rate; you want to purge at least one sampling system volume between readings and preferably 1-2.
The other consideration is the volume of the flow cell; as long as the flow cell volume is less than the sampling system volume it's not a factor, but if the cell volume is greater than the sampling system volume, then you should purge at least 1-2 flow cell volumes between readings.
The key parameters mentioned above should be stable before sampling. If one of them is trending upward or downward, then purging should continue until all are stable within the criteria defined. One important note: since most all field instruments have used digital displays for many years, we often forget that small changes in readings on a digital display are of no significance. For example, a set of pH readings such as 7.23, 7.28 and 7.21 are all essentially the same number; a set of EC readings of 505, 510 and 503 are all within 1% of the reading value and are stable. (If displayed on an analog meter, you would barely notice any movement of the needle.) Again, if one parameter won't stabilize and all others have (other than ORP or turbidity), look for an external reason such as a sensor issue or air leak.
Stabilization is based on two things - the criteria for what is "stable" and the precision of the measurement system (i.e., sensors and instrument). Stabilization criteria for purging vary with the parameters but generally should be +/- 3-5% of the reading value for EC/SpC, +/- 0.2 pH units, and +/- 10% or 0.2 mg/L for DO, whichever is greater. (NOTE: regulatory requirements may dictate other criteria.) Usually, you do not use redox/ORP or turbidity as indicators but, if you are required to use these, then +/- 20 mv for redox/ORP and +/- 10% or 1.0 NTU for turbidity, whichever is greater, are reasonable criteria for these parameters.
Three consecutive readings taken about 3-5 minutes apart that fall within those criteria would be considered stable water chemistry. The time between readings is determined by the volume of water in the pump and/or tubing relative to the purging flow rate; you want to purge at least one sampling system volume between readings and preferably 1-2.
The other consideration is the volume of the flow cell; as long as the flow cell volume is less than the sampling system volume it's not a factor, but if the cell volume is greater than the sampling system volume, then you should purge at least 1-2 flow cell volumes between readings.
The key parameters mentioned above should be stable before sampling. If one of them is trending upward or downward, then purging should continue until all are stable within the criteria defined. One important note: since most all field instruments have used digital displays for many years, we often forget that small changes in readings on a digital display are of no significance. For example, a set of pH readings such as 7.23, 7.28 and 7.21 are all essentially the same number; a set of EC readings of 505, 510 and 503 are all within 1% of the reading value and are stable. (If displayed on an analog meter, you would barely notice any movement of the needle.) Again, if one parameter won't stabilize and all others have (other than ORP or turbidity), look for an external reason such as a sensor issue or air leak.