Please provide link to data sheet of the actual sensor you're using. I can only find this one and it connects not to RS232 but to UART (it even explicitly mentions in the documentation that if you want to connect to RS232 you need a level converter).
For the one linked, if that's your sensor: if you have a 3.3V Arduino you can connect it directly (Tx to Rx, Rx to Tx, 3.3V and GND) - if you have a 5V Arduino you'll need level converters.