FAQ

What is the recommended frequency of recalibration/re-certification of hydrometers?

As a general rule, we recommend that hydrometers which are used at ambient temperature are calibrated when new, and thereafter at least once every two years. Hydrometers used above or below ambient temperature should be calibrated annually. Follow the guidance as laid down in your Quality Manual if unsure.

Our paper ‘How often should I calibrate‘ also offers some general advice.

What types of hydrometer can you calibrate?

We can calibrate almost any hydrometer made of glass, metal, or plastic, and this includes saccharometers, lactometers, vinometers, brinometers, and marine survey.

What is the turnaround time?

The normal a return of a calibrated instrument is four weeks (UK mainland). Contact us for further information.

Can you visit my premises and calibrate my hydrometers?

No. Hydrometer calibration requires a fixed rig which is situated at our laboratories.

Can you sell me liquids for calibrating my hydrometers?

No. We do not normally supply these for several reasons:

A calibrated liquid only allows you to check a hydrometer at one point.
It is almost impossible to find liquids which match the hydrometer scale lines in terms of density, so you always have to interpolate, and make estimates of where the meniscus is between scale markings.
Although the certificate for a liquid is valid as you open the container, once it has been decanted into another container, such as a hydrometer jar, it effectively becomes invalid.

What is a fiducial mark, and do I need one?

Most hydrometers have their measurement scale printed onto a thin plastic or paper sheet. This sheet is rolled up and inserted into the hydrometer stem, and glued in place. Occasionally, often due to age or the hydrometer being used at unusually low or high temperatures, the scale becomes loose and can slide in the stem. A fiducial mark is a thin horizontal line printed on the scale, which is aligned with a thin horizontal line etched on the outside of the hydrometer stem. As long as these two lines coincide we can be confident that the scale has not moved.

A fiducial mark is therefore essential for calibration.

If we receive a hydrometer without a fiducial mark, we will engrave a line on the outside of the glass, at your risk.

Do you calibrate hygrometers?

A hygrometer is an instrument for measuring moisture content. We do not calibrate hygrometers. See the UKAS website to find a list of laboratories who do.

Can you repair a hydrometer?

Hydrometers cannot normally be repaired.

Should my hydrometer be magnetic?

Until a few years ago, hydrometers were ballasted with lead shot held in place with wax or resin. This was totally non-magnetic. As the use of lead became frowned upon, manufacturers had to find an alternative metal to use as ballast. For high quality hydrometers, many moved to non-magnetic stainless steel or bismuth shot, and these are satisfactory. However, some changed to mild steel shot which can be strongly magnetic.

If you use a magnetic hydrometer, you have a strong possibility of getting a wrong reading. The hydrometer will respond to the vertical component of the earth’s magnetic field, as well as any local magnetic fields arising from steel benches, electric motors, structural steelwork etc.

Why do I need to know the surface tension of my liquid?

Accurate reading of a hydrometer depends on the surface tension of the liquid in which it is floating. The surface tension of the liquid creates a meniscus round the hydrometer stem, pulling the hydrometer down into the liquid slightly. There are several BSI and ISO standards giving surface tension corrections for hydrometers.

S. T. of sample minus that at which hydrometer was calibrated	Correction to be added to hydrometer reading 1000 kg/m	Or	Correction to be added to hydrometer reading 1.000 g/ml
-30	-0.41		−0.00041
-20	-0.27		−0.00027
+10	+0.14		+0.00014
+20	+0.27		+0.00027