Magnetrol model P65 modulevel pneumatic liquid level control

I need some advice on calibrating these level transmitters (Magnetrol model P65 modulevel pneumatic liquid level control). They said they found it difficult but language barrier prevented a detailed explanation. They might be different from usual instruments because pressures and temperatures are lower than steam drums.

What's the standard practice for calibrating these types of devices? Do we simulate a water column or what?

Thanks.