Graham Transmissometers like the Model 100V shown below are calibrated by means of two controls on the front of the instrument -- SET 0 and SET 100. With the light path blocked by an opaque object, the SET 0 control is adjusted to produce a reading of 0.00 on the digital readout. In the image below the instrument is being set to read 100.00 with the light path unobstructed. The picture shows a setting of 100.36 which means that the operator must rotate the SET 100 control a little counterclockwise to obtain the 100.00 reading. Both the SET 0 and the SET 100 controls are 10-turn precision potentiometers which permit very precise setting of both the 0.00 and 100.0 readings.
Because electronic components are all affected by their temperature, it is important to allow the instrument to reach thermal stabilization by letting it warm up for at least 20 minutes before attempting to adjust any of the controls.
SETTING THE END POINTS
The Operator should first adjust the SET 0 control to give a reading of 0.00 on the panel meter with the optical path blocked. This should be followed by adjustment of the SET 100 control to show 100.00 on the meter with the optical path unobstructed. It may be necessary to move back to the Set 0 and Set 100 controls for final adjustment, but after two or three iterations the meter will reliably show the two end points ( 0.00 and 100.00)
LINEARITY IS THE KEY
The photodiode used as the instrument's detector, as well as the circuitry employed to measure the Transmittance give extremely linear response over several decades of measurement. This means that if the end points are accurately set, then any reading between those end points will be accurate within the limits of the display. Intermediate readings may be checked with a precision neutral density filter for verification, but the calibration of the average filter is only to accurate to +/- a few percent.
TRANSMITTANCE AND %TRANSMITTANCE
is measured on a scale from 0 to 1.00 with 1.00 being full Transmittance.
of 0.00 is equivalent to a %Transmittance of 0.00%
THE DOUBLE FILTER METHOD OF CHECKING LINEARITY
double filter is a better method of checking for linearity. To do
this requires the measurement of the Transmittance, T with each
of the filters independently to be followed by the measurement of T
the two filters stacked. If the Percent Transmittance of Filter A is 45.0%
(T=0.450) and the Percent Transmittance of Filter B is 62.0 % (T=0.620),
then the Transmittance of Filters and B, stacked, is [ T= 0.450 x 0.620
] = 0.279, i.e. 27.9%
FROM TIME TO TIME
It is perfectly normal for the last digit to fluctuate every few seconds -- the instrument is tuned to provide acute sensitivity to change in Transmittance. If this sensitivity were cut back, the last digit would not fluctuate, but then the instrument would not be following the transmittance and there would be a "Dead Band" where a change in Transmittance did not show up on the readout.
Thereafter, measurements of %Transmittance (%T) are obtained automatically simply by inserting the sample in the optical path. Occasionally -- a few times a day, it is good practice to verify that the readout shows 0.00 with the beam blocked and 100.00 with no obstruction.
All Graham Transmissometers operate in any normal ambient lighting condition. . .no need to operate the instrument in a darkened room!
Further Information, call Gordon Graham today at (818) 700-1263 E-Mail: