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 NameDevice Variable Classification CodeUnit Code
US liquid barrel per second
dev_var_2US liquid barrel per minute
dev_var_3US liquid barrel per hour
dev_var_4US liquid barrel per day

For more detail, please visit the online specification common tables here:

See Table 2: https://library.fieldcommgroup.org/20183/TS20183/25.0/#page=48

See Table 21: https://library.fieldcommgroup.org/20183/TS20183/25.0/#page=92