Atif Ali

It looks as though the Convectron uses a standard KF fitting.

Something like this:

https://www.lesker.com/newweb/flanges/flanges_technicalnotes_kf_1.cfm

Kurt Lesker, MDC Caburn, and others all stock this fitting.

I'd calibrate it in air at 1bar, and then under the best vacuum you can make.

What type of pump do you have?

(rotary, scroll, turbo, etc.)