LUT400 Communications Options
AG091712 4 of 13
APPLICATION GUIDE
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.
1. Over the three-meter range, a 0.005 mA error is 0.03125 % of 16 mA (range of 4 to 20 mA).
2. Multiply this by the measurement range of three meters equals an additional 0.9375 mm error – almost the same as the
measurement error.
3. 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.
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