Specific gravity and ppt comparison

[Charles Silva]

Dear all

I have here 3 devices to measure specific gravity and they were calibrated for 25 C (77 F).

My Tank temperature = 27.8 C (82 F)
Room temp = 27.3 C (81.1 F)

My goal is to make sure my refratometer reading is reliable as my goal is to set my water at 35 ppt.

Results:

Sera Hydrometer = 1.020
Tropic Marine Hydrometer = 1.0225
Red Sea Refratometer = 1.028 = 40 PPT


I have some doubts:

- why my refractometer reading is higher ? Should I perform a temp compensation in my hydrometers as they were calibrated for 25 and I'm using at 27.8 C with this calculator ?

https://www.hamzasreef.com/Contents/Calculators/SalinityCorrection.php

Before I use my Red Sea Refratometer I let it for 30 minutes to equalize the optics temperature with my room temp then I put 3 drops and wait for 15 seconds to see the result.

The refratometer result 1.028 = 40 PPT (calibrated for 25 C). Should I convert the reading back to 27.8 or is the ATC able to compensate for 3 deg C ?

 

[Charles Silva]

More info...

My hydrometers standard temp & reference temp are 25/25 deg C (77/77F)

Tank temp is 27.8 C and using this table to convert 1.0225 sg (reading from hydrometer) to +- 28 C

http://www.reefkeeping.com/issues/2004-07/rhf/table1.pdf

I got the final salinity around 31 ppt (hydrometer) but if I use the refratometer I get 40 ppt.

I think my refratometer is not working.

 

[Randy Holmes-Farley Verified Community Expert]

Floating glass hydrometers need correction if they are not used at the exact temp they are designed for. Swing arm hydrometers are generally temperature correcting.

How did you calibrate the refractometer?

 

[Charles Silva]

Hi Randy

Thanks for your answer. This refratometer is brand new and I concluded that this error source is due calibration. I will use a 35 ppt cal. solution. Meanwhile I will use my hydrometer 25/25 and I will correct the sg reading based on the tank temp.

For the refratometer:

Does the external room temp be the same as the tank temp while calibrating ?
What is the temp difference that the ATC can handle ?
Regards

 

[Randy Holmes-Farley Verified Community Expert]

Is the refractometer an ATC refractometer?

In any case, its best to calibrate and measure with the refractometer at the same room temperature.