Here is one way to do this. Some non-DP pressure transducers are made to read in psia (pounds per square inch absolute) rather than psig (pounds per square inch gauge). I think you can get standard outputs such as 4-20 ma, or 1-5 volts DC. If you are dealing with an existing transducer (transmitter), the part number may tell which type the sensor is. Such transducers are frequently used in refrigeration applications that may run in partial vacuum.

How to calibrate it with ??? for example if the range of the transmitter is -1 to 1 kg/cm2 then what reading should it show when the vessel is empty before the startup ??

I will need a day or two to think more about this. Please stay tuned. (For one thing, I usually work in in British/American units rather than SI, so I will need to do some conversions.) Are you actually measuring pressure here, or are you using pressure to correlate to a liquid level measurement? I ask this because you have mentioned condensate in a steam/water system. What is the maximum pressure this system might encounter?

I once asked the same question (about calibration) to a Foxboro engineer. According to him, you could calibrate the transmitter at 0 to 2kg/cm² and then rerange it to -1 to +1kg/cm².

It is not practically possible to generate -1kg/cm² so there's no sense trying to generate that much vacuum.

regards,

Vulcan