Based on what I found on the web the factor is usually 385. Type 391 is available but a special order from Temp-pro. 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.
π Johnson Level and Tool 400EM-S 12-Inch Heavy Duty Metal Combination Square for Professionals, with Inch/Metric Measurements, Silver - 1 Square
8 Review
Kapro 307-08-TMS Try And Mitre Layout And Marking Square With Stainless Steel Blade, 8-Inch Length
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β Stainless Steel L-Square, Size 3-Inch x 4-Inch, Zona 37-434
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π² Johnson RAS-1B Johnny Square: Professional Aluminum Rafter Square, 7'', Silver - Top Quality with 1 Square
8 Review