Guar gum viscosity is usually measured on a 1% solution in water using a Brookfield-type rotational viscometer. The result depends heavily on the spindle, speed, temperature and how long the solution has hydrated before reading.
So a viscosity figure is only meaningful with its method. Two labs can test the same powder and report very different numbers if their settings differ.
The usual test set-up
The following is a typical trade method, given for illustration. Your own spec, or the supplier's, may differ, and that is fine as long as both sides use the same one.
- Concentration: 1% guar gum on a dry or as-is basis, which must be stated.
- Water: distilled or deionised water at a fixed temperature, often around 25 degrees Celsius.
- Mixing: a set stirrer speed and time so the powder disperses without lumps.
- Hydration time: readings at 2 hours, and often 24 hours, for standard grades.
- Instrument: a rotational viscometer with a named spindle and speed, often 20 rpm.
- Result: reported in centipoise (cP) or millipascal seconds (mPa.s), which are equal.
Why the details matter
- Spindle and speed: guar solutions are shear thinning, so faster speeds give lower readings.
- Temperature: warmer solutions read lower.
- Time: guar keeps building viscosity for hours, so a 15 minute and a 2 hour reading differ widely.
- Dry or as-is basis: moisture in the powder dilutes the gum if not corrected.
- Water quality: salts and pH can shift readings.
- Mixing: lumps that never hydrate lower the result.
Fast-hydrating and industrial methods
For fast-hydrating grades, buyers often ask for a reading after a few minutes as well as the final value. Oilfield users may test on different instruments and in salt water or specific fluids, following their own industry procedures. Agree in writing which method governs the contract.
Some buyers also ask for a viscosity profile: readings at several times or speeds. This shows how the gum behaves in real use, not only its end value. It costs a little more to test, but it helps when you compare two suppliers whose single figures look alike.
Putting it in your contract
Write the full method into your specification: concentration, basis, water, temperature, instrument, spindle, speed and reading time. Ask the supplier to report on that method on each certificate of analysis. Have an independent lab use the same method when you verify. For reading reports, see how to read a lab certificate.
If a dispute arises, a written method lets a third lab settle it. Without one, each side can defend its own number.
How Creston & Co handles this
We agree the test method with you and the supplier before the first order, then use an NABL-accredited lab running that same method for samples and pre-shipment checks. Ask for a free sourcing review.