Unit codes in the range of 170 - 219 in HART Spec-183/FCG TS20183 Common Table 2, is called Unit Code Expansion range. The meaning of the codes in this range depends on the Device Variable Classification (Common Table 21). In other words, the codes in this range are reused multiple times and their meaning depends on the Device Variable Classification.
Engineering Unit Code Expansion Tables shall only be used by Device Variable declaring a CLASSIFICATION. Only field devices complying with HART revision 6 or later shall use unit code expansion.
Here is an example of combinations that could represent 4 Device Variables for volumetric liquid flow:
| Device Variable | Unit Name | Device Variable Classification Code | Unit Code |
| dev_var_1 | US liquid barrel per second | 103 | 189 |
| dev_var_2 | US liquid barrel per minute | 104 | 189 |
| dev_var_3 | US liquid barrel per hour | 105 | 189 |
| dev_var_4 | US liquid barrel per day | 106 | 189 |
For more detail, please visit the online specification common tables here:
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