NIST no longer sells version 9.1 — they sell version 10.0 (2023). To use REFPROP legitimately:
Cybercriminals know engineers need REFPROP. They embed Trojans, keyloggers, and ransomware into "cracked" installers. A 2022 study by a cybersecurity firm found that 1 in 3 "engineering software cracks" contained a Remote Access Trojan (RAT). refprop 91 free
When you search Google or torrent sites for "refprop 91 free," you will find links to executable files, keygens, and patch files. Here is what you are actually downloading: NIST no longer sells version 9