Dakota Digital CMX operation manual Chapter Six Making Measurements

CHAPTER SIX

MAKING MEASUREMENTS

The steps involved in making measurements are detailed in this section. The following sections outline how to setup and prepare your CMX for field use.

An automatic or manual zero must always be performed. The auto zero is an off block electronic zero that does not require a zero reference block. This will most always be the zero option of choice, as it makes the zeroing process very easy and convenient to perform. However, If the manual zero option is enabled, the probe zero must be measured on the reference disk (battery disk) attached to the top of the instrument. The zero compensates for variations in the transducer. In all modes the sound velocity must be determined. The sound velocity is used to convert the transit time to a physical length. The sound velocity can be selected from a material chart in the manual, selected from a material list in the CMX, or for greater precision determined from a sample of the test material that has been mechanically measured. To enter the velocity from a table, look up the material on the chart in the appendix of this manual and refer to the section below on Calibration to a Known Velocity. To determine the velocity of a single sample, refer to the Material Calibration section on page 45.

When measuring curved materials, it is more accurate to calibrate from two test points, one at the minimum limit of the target thickness and one at the maximum limit. In this case the reference disk mounted to the CMX is not used. This is called two- point calibration and is described on page 50.

6.1 Auto Probe Recognition & Selecting The Transducer Type

The first step in using the CMX is to plug the transducer into the gauge and power the unit up. The CMX has a special built-in automatic probe recognition feature that will check to see if the probe plugged into the gauge is an auto recognized probe type. If so, the CMX will display a message indicating the transducer type and ask the user for confirmation to use the identified probe. If the transducer is not an auto recognized probe, the CMX will display a message indicating the transducer type has not been recognized, and force the user to select a transducer type from a list of transducers according to frequency and diameter. Whether the transducer is auto recognized or selected from a predefined list, the CMX will recall specific properties about the transducer. Note: Once the transducer has been selected, the CMX will store and recall this transducer type every time the CMX is powered on/off. The type will only change if the user physically selects another type from the list, or selects a previously saved setup. Therefore, if you have previously gone through this section and selected the transducer you are using, proceed to the next section. Use the following steps to select your transducer type. Note: If the transducer is not

identified on power up, be sure the transducer type selected is the same as the transducer plugged into the CMX. Failure to do this will result in erroneous measurements:

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