Target level measuring instruments

NOTE 1 In the case of safety instrumented functions operating in the demand mode of operation, the target failure measure should be expressed in terms of the average probability of failure to perform its design function on demand, as determined by the safety integrity level of the safety instrumented function (see Table 3). 

Basically, the optical method uses equipment such as alignment telescopes, jig transits, and sight levels. Instruments with built-in optical micrometers for measuring displacements from a referenced line of sight enable an accurate determination of target movements, which are mounted on the machine. 

The Zolo laboratory calibration facility is a test cell with a path length of approximately 45 cm and can reach temperatures of up to 2060°F. Controlled levels of target gases can be introduced into the cell. Temperatures are measured using thermocouples and concentration levels are measured using independent conventional laboratory instruments. 

The level of the target analyte should be significantly above the limit of quantification to ensure a good signal-to-noise ratio. With ICP-MS measurements the instrumental response should be 10 -10 counts s (for each ion). For organic IDMS the signal-to-noise ratio should be at least 10 1. 

Fig. 7.3 shows the differences between the heights of target S3, determined by automatic levels and the video theodolite. In the average the differences were about 0.2 to 0.3 mm. Consequently the video theodolite was able to determine the vertical displacements with an accuracy of about 0.2 mm, though the targets were up to 200 m away from the instrument. This was only possible, as the measurements were performed in the interior of the bridge, where the influences of refraction could be neglected. 

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