Can someone help me understand the mechanism behind a particular daily salinity swing that I'm seeing? This is more a curiosity than a concern.
The charts below show a daily swing of about 0.30ppt and I'm fine with that (37gal tank, no sump). The salinity starts dropping every day at 1:00pm when the lights turn on, and then it starts rising when the lights turn off at 10:00pm. This salinity swing follows the same schedule every day, give or take an hour. Temperature is 77F +/- 0.2F. The temp swings about hourly and does not correlate to this salinity swing. The monitoring system is Apex Classic with Temperature and Conductivity probes, supposedly configured to temperature compensate the conductivity probe reading. ATO runs randomly with a fairly tight tolerance and does not seem to correlate to this daily salinity swing. Ambient room temperature is 70F from 9AM-midnight, and as low as 62F overnight. Mechanical filter and protein skimmer run full time. Skimmer removes about 4oz of tank water per day. The only thing that seems to correlate to this regular daily swing in the conductivity probe reading is the light schedule, but the swing is neither exact nor immediate so electric interference from the lights seems unlikely. I'm stumped. Any ideas?
If there is a normal water chemistry reason for this swing, please enlighten me. If this swing is due to characteristics of the measurement system, what the heck explanation fits the data?
FYI - Salinity was running a bit high. Water change today lowered the tank salinity back to peaking around 35ppt according to the probe. Tropic Marin hydrometer says 1.026. Temp is 77F. Conductivity probe calibration seems to be holding well after calibrating it in the tank water, after using hydrometer to adjust the tank water to 1.026 at 77F. Prior attempts to calibrate the probe with calibration packets produced wild results.
Four days of salinity swings showing this regular pattern:
Usually starts to drop around 1:00pm when lights turn on:
Usually starts to rise around 10:00pm when lights turn off:
One-day salinity (orange) and temp (blue):
Four-day salinity and temp:
The charts below show a daily swing of about 0.30ppt and I'm fine with that (37gal tank, no sump). The salinity starts dropping every day at 1:00pm when the lights turn on, and then it starts rising when the lights turn off at 10:00pm. This salinity swing follows the same schedule every day, give or take an hour. Temperature is 77F +/- 0.2F. The temp swings about hourly and does not correlate to this salinity swing. The monitoring system is Apex Classic with Temperature and Conductivity probes, supposedly configured to temperature compensate the conductivity probe reading. ATO runs randomly with a fairly tight tolerance and does not seem to correlate to this daily salinity swing. Ambient room temperature is 70F from 9AM-midnight, and as low as 62F overnight. Mechanical filter and protein skimmer run full time. Skimmer removes about 4oz of tank water per day. The only thing that seems to correlate to this regular daily swing in the conductivity probe reading is the light schedule, but the swing is neither exact nor immediate so electric interference from the lights seems unlikely. I'm stumped. Any ideas?
If there is a normal water chemistry reason for this swing, please enlighten me. If this swing is due to characteristics of the measurement system, what the heck explanation fits the data?
FYI - Salinity was running a bit high. Water change today lowered the tank salinity back to peaking around 35ppt according to the probe. Tropic Marin hydrometer says 1.026. Temp is 77F. Conductivity probe calibration seems to be holding well after calibrating it in the tank water, after using hydrometer to adjust the tank water to 1.026 at 77F. Prior attempts to calibrate the probe with calibration packets produced wild results.
Four days of salinity swings showing this regular pattern:
Usually starts to drop around 1:00pm when lights turn on:
Usually starts to rise around 10:00pm when lights turn off:
One-day salinity (orange) and temp (blue):
Four-day salinity and temp:
