Is there any advantage to either the hydrometer or refractometer?
Hydrometers on eBay are cheap while refractometers cost anywhere from ten to four hundred. Are there any particular features i'm looking for as well as don't need in a refractometer?
Careful, you just cracked open a massively controversial subject! :uzi: People can be wildly varying in their love of hydrometers vs refractometers.
TDS Meter
A TDS meter is used to check that your RODI water is 0 TDS, and to compare it to your raw tapwater, or to the RO output before the DI. The reason for doing this is to determine:
- How bad the tapwater is
- How depleted the RO is before it hits the DI
- When to change the RO
- Ensuring the output is 0 TDS.
Usually you can eyeball the DI and change it when it starts to largely change color.
'Swing Arm' Hydrometer
Can vary wildly from what it tells you. However, many people recommend using a known cal fluid in it then just marking with a permenent marker where that gets you. This way you can ignore the markings that came with it and go off a KNOWN spot.
Requires frequent cleaning, buildup will skew readings.
Airbubbles will skew readings.
Otherwise, you never really need to recalibrate it. It does what it does, and just keeps doing it. The local reef store near me has been using these exclusively for 30 years, and never had an issue. My personal experience is I went and bought one and got 5 different readings with it after watching a video on how to use it, so I said **** that.
Refractometer
Very accurate and easy to read depending on what one you get. However, all the manuals I've read state it
-must be calibrated every single time- you use it. I can vouch that I have used it one day, and picked it up the next only to find it had moved by as much as +/- 0.005! And I have a 'good' one!
Calibrating it is very easy and can be done with RODI water, however ideally you would get a 1.026 cal fluid to use this. People will tell you calibrating with RODI water is wrong. They are wrong. It is not IDEAL, but it will work and is even what is in most manuals. As an instrumentation technologist that works professionally with calibrating instruments, I feel I have the position to say this.
Ideally you will calibrate at your desired measurement point, but a consistent calibration at zero will work. I tested this off the local reef shop and I was bang on the same reading he uses, and bang on to his 1.026 cal fluid.
TLDR: I bought a refractometer for salinity because I tried both and preferred it. TDS meter is for RODI uses only, not salinity.
