Evaluation of Wildland Fire Chemicals
Standard Test Procedures
STP 4.2—Refractometer Reading
A hand-held refractometer is a simple to operate, moderately priced instrument
that is suitable for field use. In most cases sample preparation is limited
to suitable dilution of a wet concentrate or blending of a dry concentrate
with water. Although many refractometer are temperature compensated to
allow accurate readings over a range of solution temperatures, they are
sensitive to large temperature changes to which they may be subjected
during field work.
The scale reading on a hand-held refractometer can
be used to provide an estimate of the salt content of a retardant,
provided that a reference table has been prepared for use in converting
reading to salt content.
The conversion from scale reading to salt
content is specific to a particular product or chemical combination;
therefore care must be taken to ensure that an appropriate conversion
Because, there are several different scales used for hand-held
refractometers, caution must be exercised to ensure that the refractometer
and the prepared conversion table use the same scale.
- Handheld refractometer and conversion table
- Test product
- Squirt bottle
or other source of clean water
- Lab wipes or other disposable tissue
- Place a small drop of test product onto the prism glass of the
refractometer using the plastic applicator. Use caution when
using an applicator other
than the one provide with the refractometer as the prism can be
easily scratched. This will affect the accuracy and ease of use
of the instrument.
- Close the cover on the prism.
- Hold the refractometer
with the covered prism pointing to a strong light source.
through the eyepiece and read the number at the interface of
the light and dark
sections of the scale.
- The refractometer scale can sometimes be difficult to read.
- While looking through the eyepiece, slowly move the refractometer
to change the amount of light on the prism.
- Lowering the
refractometer will usually increase the amount of light
and improve readability.
- If the color through the eyepiece is so diffuse that there
is no line (not enough sample) or the entire view is dark (too
much sample), remove the sample from the prism and try again.
the sample from the prism by rinsing with clean water and carefully
drying the prism with a clean lab wipe.
it is not possible to read the sample. This is usually because
it is too concentrated.
- Retardant concentrates must be
diluted before obtaining a reading. Use the mix ratio approved
for the specific product.
- Record the refractometer reading.
- Compare the obtained reading
to the approved range or correlation scale
for the specific product to determine quality or approximate
- National Wildfire Coordinating Group and USDA Forest Service. Lot Acceptance,
Quality Assurance, and Field Quality Control for Fire Retardant Chemicals,
Sixth Edition. 2000.
- Shirley Zylstra, USDA Forest Service. Assessments
of Seven Refractometers for Evaluating Wildland Fire Retardants.
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