Siemens SITRANS LUT400 Application Manual page 4

Communications options, connecting to a control system and the available data

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HART and Accuracy

The LUT400 has a stated measurement accuracy of +/- 1 mm plus the additional 0.17% of measured range. You will get this

accuracy when using HART, the display, and the data logs. However, the 4-20 mA channel has a transmit, transmission, and

receive error in addition to the measurement error, but we do not have data on the transmission and receive errors.

The transmission error depends on the wire used, length, and amount of noise. The receive error varies from card to card and is

typically not published.

The transmit error is known for the LUT400 and is under 0.005 mA with a worst-case error of 0.02 mA. At first glance, these

numbers do not appear to be significant. However, upon calculation, it becomes apparent that this small change adds

considerably to the overall error:

Consider a three-meter, high accuracy unit.

Over the three-meter range, a 0.005 mA error is 0.03125 % of 16 mA (range of 4 to 20 mA).

Multiply this by the measurement range of three meters equals an additional 0.9375 mm error – almost the same as the

measurement error.

Doing a similar calculation for the worst case error of 0.02 mA, you will get a transmit error of up to 5.8 mm.

Conclusion: Using 4-20 mA can result in your value being out between 1 mm and 6.8 mm.

LUT400 Communications Options

AG091712

APPLICATION GUIDE

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