Dummies send a permissive signal for any missing compartments. For example, in the United States all overfill systems are formatted assuming a six compartment thermistor trailer. Therefore, the terminal controller must receive six permissive sensor signals to allow loading. One dummy can send up to five signals. Consequently a one to five compartment trailer will contain one FT155 dummy (wired to socket). In Canada, where eight channel systems are mandated, two FT155 dummies are needed with one to three compartment trailers and one FT155 for four to seven compartments. The FloTech FT155 thermistor dummy provides instant permissive signal in the coldest weather and is mechanically and electrically compatible with Scully and Civacon models.