Error in 3 salt testing devices

Cory · Jan 15, 2015

I got a neptune Apex conductivity probe with an additional temperature probe for automatic temp compensation. My refractometer reads my tank at approximately 35ppt. My swing arm hydrometer reads at 1.028. And now my conductivity after inititial calibration read my water at 33.5ppt. I measured it first in a cup. Then after setting it in the sump it now reads 27.2ppt!

I basically got the conductivity to assure me of the salinity of the water. I find it very picky. Any cables by it interferes, any tiny air bubble interferes. And so does stray voltage which my tank has.

Thus I want something that I can be 99.99% sure what my salinity is. I was thinking of a nist certified floating hydrometer combined with a nist certified thermometer (digital). Do you think those two are foolproof? What would you do?

Randy Holmes-Farley · Jan 16, 2015

Those two are foolproof, yes.

Conductivity should be too. Remember it is temperature compensated so the APEX temp probes needs to be in the conductivity solution (at least, I expect they implement temp corrections that way).

Cory · Jan 16, 2015

Ok thanks Randy! Can you reccomend one online for a reasonable price?

Cory · Jan 16, 2015

How about this one, it seems reasonably priced:

Lamotte Certified Salinity Hydrometer

So nothing but temperature will effect the reading?

Cory · Jan 16, 2015

Sorry for all the questions. Do you think I could calibrate the digital thermometer that comes with the Apex with the freeze slurry method? This one:

Equipment Review: Inexpensive Aquarium Thermometers - Bargains or Reef Killers? Plus Notes on Thermometer Calibration ? Advanced Aquarist | Aquarist Magazine and Blog

Randy Holmes-Farley · Jan 17, 2015

Cory said:
How about this one, it seems reasonably priced:

Lamotte Certified Salinity Hydrometer

So nothing but temperature will effect the reading?

That looks good. I had a Tropic Marin hydrometer like that which was spot on.

Temp impacts it, and if the chloride to sulfate ratio is way off, that may impact it (as it will any method).

Randy Holmes-Farley · Jan 17, 2015

Cory said:
Sorry for all the questions. Do you think I could calibrate the digital thermometer that comes with the Apex with the freeze slurry method? This one:

Equipment Review: Inexpensive Aquarium Thermometers - Bargains or Reef Killers? Plus Notes on Thermometer Calibration ? Advanced Aquarist | Aquarist Magazine and Blog

It is a great method for a glass hydrometer, but I do not know how accurate it is for a thermocouple or thermistor type as that may be software dependent.

Cory · Jan 17, 2015

The ratios of sulphate and chloride should be fine if you use instant ocean? What about when I raised magnesium up a little with Epsom salts. Iirc it was 25-50ppm more magnesium. I don't usually do water changes.

JimWelsh · Jan 17, 2015

When I wanted to calibrate my Apex temperature probe, I took a NIST-traceable digital thermometer, and calibrated it with the ice slurry / boiling water method (do both calibrations, and then derive offset and slope correction for that thermometer from those two calibrations). I then used the digital thermometer, corrected for the calibration offset and slope, to calibrate the Apex temperature probe.

Randy Holmes-Farley · Jan 17, 2015

Cory said:
The ratios of sulphate and chloride should be fine if you use instant ocean? What about when I raised magnesium up a little with Epsom salts. Iirc it was 25-50ppm more magnesium. I don't usually do water changes.

I think it has to be quite far off to be an issue. One can compare my NaCl solution standards to seawater to see the effect, and that is an extreme case with a relatively minor effect of maybe 1-2 ppt.

Cory · Jan 17, 2015

JimWelsh said:
When I wanted to calibrate my Apex temperature probe, I took a NIST-traceable digital thermometer, and calibrated it with the ice slurry / boiling water method (do both calibrations, and then derive offset and slope correction for that thermometer from those two calibrations). I then used the digital thermometer, corrected for the calibration offset and slope, to calibrate the Apex temperature probe.

Thanks Jim! I'm just curious, I thought nist certified meant it was already calibrated and wasn't nescessary. True?

Cory · Jan 17, 2015

Randy Holmes-Farley said:
I think it has to be quite far off to be an issue. One can compare my NaCl solution standards to seawater to see the effect, and that is an extreme case with a relatively minor effect of maybe 1-2 ppt.

Thanks Randy. Just wondering if there is a good glass thermometer with a narrow temperature for reef aquaria? I figure something like 60f to 90f would be good and would be more precise. Can you reccomend a good thermometer for reasonable price?

Randy Holmes-Farley · Jan 17, 2015

Try looking at Cole Parmer for thermometers.

JimWelsh · Jan 17, 2015

Cory said:
Thanks Jim! I'm just curious, I thought nist certified meant it was already calibrated and wasn't nescessary. True?

True, but within specified tolerances. I don't have the specifics of the thermometer I used in front of me right now, but the tolerances were rather loose, as I recall. Something like +/- 1 degree C. My thought was to re-calibrate it in an attempt to get better accuracy than the calibration certificate guaranteed. BTW, it is probably significant that the calibration certificate that came with the thermometer specified a range that spanned at least from 0C to 100C.

Cory · Jan 18, 2015

Randy Holmes-Farley said:
Try looking at Cole Parmer for thermometers.

Randy, thermometers are overwhelming in selection. It doesn't even have a selection for options, just if you want glass or digital. What would you reccomend?

Cory · Jan 18, 2015

JimWelsh said:
True, but within specified tolerances. I don't have the specifics of the thermometer I used in front of me right now, but the tolerances were rather loose, as I recall. Something like +/- 1 degree C. My thought was to re-calibrate it in an attempt to get better accuracy than the calibration certificate guaranteed. BTW, it is probably significant that the calibration certificate that came with the thermometer specified a range that spanned at least from 0C to 100C.

Ok thanks. I'd like a thermometer with a little more precision than that. I'm too lazy to calibrate it.

Randy Holmes-Farley · Jan 18, 2015

Cory said:
Randy, thermometers are overwhelming in selection. It doesn't even have a selection for options, just if you want glass or digital. What would you reccomend?

It's been a long time since I searched thermometers, so I don't really have advice on specific ones or options. But if you elect glass, pick one that has a small range around tank temp unless you intend to check it yourself in ice/boiling water.

Cory · Jan 18, 2015

Ok no worries. The one I found for glass thermometer suitable for reef aquariums seems to be this one:
SAMA Precision Mercury Lab Thermometer 30 to 124Â°F 0.2Â°F (divisions) 76mm (immersion) 460mm (long)

There is a longer model, does that make it easier to read? Is this model the best I could get for glass?