Temp and salinity reading not adding up

coppertop

New member
On our most recent mailorder supply order, we bought a digital marina thermometer. the one that has a high and low temp warning and reminder alerts . Up until now all we have ever used was the old fashioned floating thermometer. There is a big difference the readings in both. The digital reads 84.0 F and the old glass one reads 77 F . How accurate are the digital probes that go in the tank? The hydor heater is only reading 73 F so I know that's not that accurate.

Also, we purchased a Vertex refractometer to replace our old hydrometer. The refractometer shows the water is way high .028 . and on the hydrometer it is reading .023. I know the hydrometer is not know for being the most accurate or reliable tool, but we were really surprised by the numbers. Any suggestions on what time span should I bring the salinity back down?

Thanks!

Nicole
 

Rockhead21564

New member
As with all things salty, slolwer is better. I'd do it over 3-4 days, small increments. Did you calibrate the refractometer? I almost positive they all need to be done after you recieve them. I'm not sure on the digital therm. Maybe check with a third source, I use a childs ear therm from wallyworld, about $30 and good back up, also I believe more accurate.
 

coppertop

New member
As with all things salty, slolwer is better. I'd do it over 3-4 days, small increments. Did you calibrate the refractometer? I almost positive they all need to be done after you recieve them. I'm not sure on the digital therm. Maybe check with a third source, I use a childs ear therm from wallyworld, about $30 and good back up, also I believe more accurate.

yip, calibrated it with distilled water. Then checked it again a few times . Took about 5 readings from the tank today and they were all the same. Then rechecked calibration and it was prefect.

That sounds good then. I just want to get the salinity right before I hook up the Auto top off . Hopefully then it will stay much more stable.

I do have an ear therm..never thought of that! THanks!
 

IslandCrow

Reef Monkey
Premium Member
It's best to check your refractometer with a prepared calibration fluid around 35ppt. Checking with distilled water should theoretically get you pretty close, though. As for the digital thermometer, they can certainly be off. One suggestion I've heard is to use a cup of ice water to check its accuracy (it should be right around 32 degrees), but I see a couple problems with that. First, like calibrating a refractometer, it's best to actually calibrate it at least close to the temperature where you'll be using it. Second, not all of those thermometers read that low. Instead, I'll generally just take the probe out of the water and check it against my house thermostat. Certainly not a perfect science, but it should tell you if you're at all close. Also, keep in mind that weak batteries will often times cause erroneous readings, so if it is off, that may be the simple fix.
 

bidny

New member
Definitely check the refractometer against a prepared 35 ppt solution, as IslandCrow suggested. This will provide a much better calibration for you than distilled water. By calibrating with distilled water, you are mostly improving the accuracy for tests near 0 ppt.
 

Randy Holmes-Farley

Reef Chemist
Premium Member
How accurate are the digital probes that go in the tank?

I have a Little Time and Temp device that is off by about 4 degrees (it reads too high).

I also agree with others that refractometers calibrated with Ro/DI will not read accurately in seawater. They are not intended for that, despite many people selling them for it.
They are made to read sodium chloride solutions, which have a different relationship between specific gravity and refractive index than does seawater.
 
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