Tony Thompson
Active Member
Hi, I have tested both the Red Sea and DD refractometers. IME the Red Sea Refractometer is a pain to use, It is far too sensitive. I have a number of tanks and found having to re calibrate constantly even between readings. I have a gut feeling that it is the design of the ATC component of the Red Sea instrument that is causing the problem.
The DD on the other hand seems to hold calibration much longer. I agree about the scale being a bit cluttered compared to the red sea but I find it fine enough to read of 35 ppt accurately (considering % of water change and therefore overall difference when mixed with DT water.)
DD state that there refractometer is calibrated at 20c and the red sea at 25 c.. However I think this may lead to some confusion. For the end user the calibration is only apparent when comparing to the Specific Gravity Scale on the eyepiece in relation to the Salinity ppt scale on the eyepiece.
I use a Hanna 4 pole Conductivity meter to measure my salinity when mixing up my salt. Cost me about £50 second hand. I often check my measuring instruments against reference solutions at temp, the hanna is spot on all the time. I just give it a clean after each use. If anyone is wanting a simple and accurate way of measuring salinity I would definitely recommend a good conductivity probe. Just be careful before purchase, check the meter reads within scale for sea water 53 mS/cm. Some conductivity meters will not read in the higher ranges. Also having built in temperature compensation means you don`t` have to make any further calculations.
Monitoring salinity over a prolonged period is a different matter altogether. Probes such as the Apex salinity probe are of a completely different construction.They are made to stand up to prolonged immersion and are usually made of glass similar to a PH robe. This makes them very fragile and without a feedback reference voltage they can be very difficult to calibrate against a reference and your DT due to interference.
That said, the probes such as the Apex probe are more useful at monitoring changes in salinity over a period of time. The accuracy therefore is less important than precision. If you for instance had a problem with your ATO or a leak, the Apex probe should pick up the deviation and send an alert. You don`t` really need to know the exact salinity, only the fact that it is deviating in a particular direction, which would indicate a problem.
If you would like to try out a conductivity probe just do a search on the used lab instruments page of any second hand internet sites. Just make sure it measures within the sea water range.
Nice to see a fellow UK Reefer on here. Best wishes.
