Review on The Ultimate Solution: T PRO Temperature Sensors Three Wire System – Stainless Steel by Robert Yates

From what I found on the internet the factor is usually type 385. Type 391 is available but is a special order from Temp-Profi. Bought one of these to connect to Omega PID. At 100 degrees Fahrenheit, there was less than 1/4 degree difference compared to a 1/10 degree certified yellow backed mercury thermometer.