5
OPERATION
kVp Measurements
To use the Model 815 X-ray kVp Meter for a kVp measurement, in most
instances it is only necessary to turn on the instrument, and then center the
meter‟s target in the light field and make an exposure.
In order to ensure more accurate readings, the following points should be kept in
mind. Refer to Appendix A for more information.
1. Current (mA)
The Model 815 will operate down to approximately 7 mA if the x-ray is focused
properly on the x-ray target. The best accuracy is obtained at 10 to 50 mA.
2. Height
For half wave x-rays (Most dental x-rays) with 7 to 15 mA output, the cone should
be ½ to 2 inches above the instrument.
For x-rays with 25 to 100 mA output, position the x-ray head 8 to 16 inches
above the instrument.
3. Collimation
Collimate the x-ray to approximately
2” by 2” centered on the target on top of the
instrument.
4. Exposure Time
There are two modes requiring different exposure times.
In the Normal or
“High Speed Mode”, the exposure time must be at least 0.04
sec (40 ms)
. Longer exposure times than 50 or 100 ms are not necessarily or
desirable in order to minimize wear and tear on the head and exposure to the
operator. When the instrument is used to measure exposure time, longer
exposure may be tested.
In
“High Resolution Mode”, the x-ray exposure time must be at least 0.08 sec (80
ms).
5. Waveform
This instrument measures the maximum or peak accelerating voltage that occurs
during any part of the x-ray exposure except for the first 30 ms which we use as
a delay to make sure the x-ray waveform is stable. The instrument will read and
display the maximum value of the accelerating voltage during the rest of the x-ray
exposure.
6. Initial Reading
The x-ray detectors accumulate charge when the unit is not used. When initially
powered on, the instrument will need one x-ray exposure to work correctly.
Disregard the first reading, especially at higher energies i.e. above about 80 kVp.
