Hach DR/2400 - Determining Photometric Accuracy

Getting Started
Getting Started
5940018Getting Started.fm Page 19
1.9 Determining Photometric Accuracy
A verification of photometric accuracy can be performed on the DR/2400 using
Hach DR/Check (Cat. No. 27639-00), NIST SRM 930 neutral density glass, or
NIST SRM 2031 metal-on-quartz standard reference materials.
To verify the photometric performance of the DR/2400 with standards, the
instrument zero should be performed on a “like” standard to obtain the
maximum performance capability of the instrument. Using the DR/Check, a
blank is performed on the blank provided with the DR/Check set and values of
standards are read directly. When using NIST standards or equivalent glass
standards, a blank is performed on the non-absorbing reference standard
provided with the set. If the stated values for the standards include the reflection
loss, then the stated value of the non-absorbing standard is subtracted from each
of the absorbance standards to obtain an adjusted true value for the reference
standards. It is these adjusted values that are used to compare the photometric
accuracy of the DR/2400 when the instrument is blanked on material other than
air. The following example provides a method for checking photometric
accuracy using glass standards with the DR/2400 that most closely matches the
performance obtained when a blank is used in Hach water analysis methods.
Example:
1. Turn the DR/2400 on and set the appropriate wavelength (see How to Use the
Standards on page 18 for setting the wavelength). The wavelength should be
set to the stated value of the reference standard being used.
2. Set the instrument to read in the same units as the reference standards
(usually “Abs” units).
3. Insert the blank or non-absorbing reference standard into the DR/2400 and
zero the instrument. The display should read
0.000 Abs.
4. Remove the blank and insert a reference standard, such as the 0.250 Abs
reference standard.
5. Touch
Read to get the value for the reference standard. This is the “read”
value for the reference standard. An average of ten readings should be used to
obtain the “read” value.
6. Calculate an adjusted value for the 0.250 Abs standard by subtracting the
stated value of the non-absorbing reference standard from the stated value of
the 0.250 Abs standard. If the stated value for the non-absorbing standard is
0.0374 and the stated value for the 0.250 Abs standard is 0.2529, then the
adjusted value for the 0.250 Abs standard is:
0.2529 - 0.0374 = 0.2155 Abs.
7. Compare the “read” values to the adjusted values for the standard.
8. The standard deviation of the ten “read” values is the precision of the
DR/2400.
9. Report the difference between the “read” value and “adjusted” value plus or
minus the standard deviation.
10. Repeat steps 4 through 9 for reference standards of other values or
wavelengths.