Mayfly version 0.5b data accuracy/validity

The CTD does not allow an internal calibration, but we do a calibration in the lab with conductivity standards and apply the calibrated data as a calculated variable.

This is maybe semantics, but the Mayfly wouldn’t be a source of quality (or the lack thereof). That’s the sensor’s job. CTDs are usually pretty durable.

One aspect of measurements the Mayfly does manage is the time. If the small battery gets used up, then it will reset to a default time. Another way of checking time is roughly accurate (+/ couple hours)  is when the peak temperature occurs.

DaveG, since you inherited a monitoring station from a previous owner, you didn’t get the full training we give station owners when we teach a workshop and/or install a station for them.  I would suggest looking through all of our EnviroDIY Monitoring Station Manual  since we thoroughly document all of the information about station assembly, installation, and maintenance.  In particular, Section 8 – Monitoring Station Management addresses all of the common issues related to ongoing maintenance and support for a station, including links to quick guides for performing quality control and sensors calibration checks.  There’s also 8 additional sections of appendices in the manual, with sample plots of data with explanations for the most common issues.

But for the quick answers to your questions above:

Battery voltage:  the nominal voltage of the lipo battery is 3.7v.  When fully charged, it will go up to around 4.2v, and can go as low as 3.55v.  Below that, the logger code will prevent the sensors and cell modem from coming on in order to keep the battery from going below 3.5 v.  Stations with a bad or dying battery will usually still operate on sunny days but will then stop at night, so that sort of data pattern would indicate that a new battery is required, but it sounds like your station’s battery is fine, and so is your solar panel.  Occasionally the solar panels or their cables will degrade, causing the station to stop charging properly.

We usually program our loggers to transmit Mayfly board temperature (and now board humidity on v1.0 and above) so that it is some diagnostic info about the board conditions, along with battery voltage and the cellular signal strength.  While the Pelican case looks like it’s an insulated box because of the foam, it definitely is not. Temperatures inside the case at night or in the shade are essentially the same as the surrounding air temperature, with a little attenuation because of the transfer of heat between the inside of the case and outside.  And during the daytime if there’s direct sunlight on the case, board temperatures can get much hotter than the actual air temperature.  We use the light colored tan or silver cases to prevent excess heating by the sun, and also because they visually blend in to the environment better.  So in summary, you can’t really use the Mayfly board temperature as an accurate air temperature measurement, it’s only an indication of the temperature extremes the circuit board is seeing inside the case.  The Mayfly circuit board can theoretically handle the standard industrial operation range of -40C to 85C (-40F to 185F), so it is unlikely your station will exceed those values.  However, it is the lithium battery pack that suffers performance and lifespan issues at either of those temperature extremes so anyone planning to operate their station in the hot desert or the cold arctic might want to keep an eye on the lipo battery performance after a year or more.  I’ve got stations here in Pennsylvania that have been deployed for 6 or 7 years with the same Lipo battery and logger board and have been operating continuously with no issues.  The bigger killer of circuit boards and battery packs is humidity.  Condensation will collect on the battery pack and logger board if there’s humid air in the Pelican case, and that condensation causes corrosion that can kill a Mayfly or battery pack.  We recommend putting a desiccant pack inside the case of stations that have a tendency for humidity and condensation inside the case.

The conductivity range you mentioned seems about right for baseflow in your part of PA, however every stream and watershed is different, so you’ll want to take supplement measurements with a separate handheld conductivity meter to determine if the sensor is reading normally.  There’s no way to recalibrate the sensor, it simply takes a reading and report it to the Mayfly board.  The only time the reading is inaccurate is if the sensor is fouled or damaged.  Fouling can occur on the electrodes of the CTD sensor that will affect the accuracy of the reading, but over-zealous cleaning of the CTD sensor could result in damage to the pressure measurement sensor that’s located right next to the electrodes, so extreme care should be taken when cleaning the CTD sensor.  See section 8 of the Station Manual for more information about cleaning the sensor, and read the manufacturer’s manual (link available on that page, you have Hydros 21 Gen1 version) to learn more about how the sensor operates.  The water depth is measured in millimeters of water above the pressure-sensing part of the sensor.  That disk is about 15mm from the bottom of the sensor housing (on Gen 1 version).  There’s a cutaway diagram in the Hydros 21 manual showing the location of the sensor components, and there’s also great information information in the quick guides (both the Maintenance Quick Guide and also in the QC Quick Guide) that can be found in section 8 of the EnviroDIY Manual.

The turbidity sensor on your station is an optical sensor (OBS3+) that can “see” about 16 inches into the water, so anything that is within that distance will affect the readings.  Things like grass, sticks, fish, rocks, etc will cause the sensor to report a higher turbidity value than is actual, so you’ll need to investigate the area of influence around your sensor to see if there’s anything near the sensor in the direction that the sensor window is pointing.  Sometimes mounting the sensors too close to the bottom of the stream will result in a constant reading that’s higher than normal, but if you’re seeing varying readings, my guess would be something loose nearby that’s waving in the current and causing the noise, or possibly fish activity.   That particular sensor model was discontinued a few years ago by the manufacturer and has been replace by one with a much shorter measurement area, so shallow streams or cluttered channels will be much easier to monitor with the newer sensor (ClariVUE 10).  Your turbidity sensor can’t be calibrated either, so the only way to know it’s working properly is to visually check that the sensor window is clean (refer to the Quick Guides for info about that) and check that you get a low reading when the water is clear.  You can kick up some sediment around the sensor right at the time of measurement (every 5 minutes) to simulate high turbidity and then check that the sensor reported a higher value. You can also place your hand about one inch from the sensor window (while under the water) and you should get the max turbidity reading (around 250NTU on low scale and 1000NTU on high scale).

And as Neil mentioned, the CR1220 battery on the Mayfly board is there to keep the onboard clock synchronized.  Your station is old enough that the backup battery might be getting low if the Mayfly was stored without the main Lipo battery connected for awhile.  Is your station transmitting data to our website, or is it only reporting to the memory card?  Does the timestamp on the data card appear accurate?  For cellular-equipped stations, the logger contacts the internet time server upon initial startup to synchronize the clock, however your station is old enough that it might not have that feature, and of course it only works if you’ve got an active cellular data plan and a functioning cellular module.  I think you’ve got an older 2G model which may not even have coverage in most places nowadays.

The CTD sensor you have is a digital output sensor, meaning all of the measurement happens onboard the sensor and it outputs a string of 3 numbers, all the Mayfly logger board is doing is capturing those numbers and storing them.  If there was a problem with the Mayfly board, then you wouldn’t get the numbers at all, and there’s no calibration or conversion needed with those sensors, so the numbers being recorded on the Mayfly memory card are exactly the same numbers that are being sent directly from the sensor.  If the data from the sensor doesn’t match up to your QA/QC measurements, then the sensor is either fouled or broken.  There are a variety of ways those sensors can fail, sometimes it’s pretty obvious, and other times it’s more subtle and you need to look carefully at all the data and compare it to the real-world, hand-held measurements.

Do you have the continuous data file from the logger with all the data from the CTD sensor?  Your excel chart with weekly reading indicates that something started affecting the depth readings in early January, but if you can graph all 3 CTD parameters with the continuous data, you can probably find a specific event that caused the failure.  Being that time of year, it’s likely that either the sensor was subjected to freezing temperatures (which can cause catastrophic failure of one or more of the parameters) or the sensor cable jacket was damaged, which can affect the depth readings by interfering with the sensor’s atmospheric reference (essentially there’s a small air gap through the cable jacket that goes to the goretex vent on the cable end near the logger box that provides the atmospheric compensation).  Or it could be that the sensor is just fouled with something on the pressure disk and needs a thorough cleaning.  Have you removed it from the stream and examined it carefully and also examined the cable jack along the entire 30-foot length, looking for any nicks or cuts or holes?  What depth and conductivity reading do you get on the logger when the sensor is out of the water?