Hydrometer Test Review

[Jameschen6]

These days I am reviewing result for the proficiency program of year 2010. My hydrometer test results are bad. Percent passing 20 um is 63% (Mean 70.6%), 5um 35% (mean 44.6%) and 2um 21% (mean 30.5%). I have no clue so far what have happened. The only thing seems to be one reason is the Hc reading from the control cylinder. The reading was 9.0 at the temperature of 22.4 and 8.0 at 24.2. Comparing with other years, the Hc readings were normally 5.0-5.0 for such temperature range (The agent was always 48g/L). By applying 5.5 instead of 8.0 for Hc, The results are still not satisfied although getting closer. Any suggestion or clues I can check? Thanks

 

[FixedEarth]

Are you using 152H? Can you attach the lab sheet with the readings and time?

 

[Jameschen6]

Thank you FixedEarth. I did a pair of tests and both of them are similar. So I guess it must a systematic error(s). See the attchment for the data.

 

[FixedEarth]

JS6- I can't download an .exe file. The 8.0 value as you suspect is very high. Try another hydrometer, check water temp, see if the agent label say sodium hexametaphosphate, that you used distilled water, etc.

 

[Jameschen6]

The agent is Sodium Hexametaphosphate. I couldn't find any expired date on it. It was bought at least 4 years ago when I joined the company.

 

[HSIII]

From what I've seen, sodium hex usually has a shelf life of 1-2 years (powdered is approximately 1 year) unopened and stored in favorable conditions.

I would do what Fixed recommends, try another hydrometer bulb, and make sure it was distilled water which was used.

 

[RJP78]

Was the solution of sodium hex made relatively recently (less than a month old)? You could always check the pH of the solution to ensure its still b/w 8 and 9. If not, remake the solution.
Also, isn't it 40 g/L?

 

[Jameschen6]

Thank you RJP78. We do not do the Hydrometer very offten. The solution was always made before the test (Same day or one day before). Honestly I have never checked the pH of the solutions.As required by the MTO, all laboratory should use 48g/L and SG 2.785 for year 2010.