Pressure transducer

I have a pressure transducer of this type: http://sensing.honeywell.com/product-page?pr_id=144494
The pressure transducer has current-output that ranges from 4-20 mA and pressure that ranges from 0-200 PSI. I want to use arduino to get pressure-values.

I connect the trandsucer to a 220 ohm resistor, and the other end  to (v-) of the powersupply. Then i can use the analog input on the arduino and read the voltage, and  calculate the current and the pressure.

I use a powersupply  of this type: cotek ge-25-12  (i'm powering the arduino from the computer)

The problem is that the voltage before and after the resistor gets negative values (but the drop is positive), and the arduino can only read positive values from 0-5 volt.

Method 1 (connecting v- to arduino ground)

I tried to connect v- on the powersupply to arduino ground(i checked if i could do this  by connecting a 10kohm resistor between v- and ground, and the drop was 0 volt) , in order to set it to zero. Then i could use the analog input on the resistor and read the voltage. I tried to measure the current with a multimeter while i increased the pressure using a air-compressor.  At 0 PSI the current was 4 mA(like the datasheet), and when i increased the pressure, the current increased. But when i reached about 20 PSI, the current stopped increasing, and stayed at 5.47 mA. 

Method 2 (not connecting v- to arduino ground)

Here the opposite happened, the current stayed still in the start (7.5 mA). But at 40 PSI, the current started increasing and gave correct results according to the datasheet.

Why does this happen? How can i solve this problem? Is it possible to get correct current values from 0-200 PSI? If i use method 2, is there any ways to read negative values?
 

Please show how you wired the whole system up, the diagram supplied is theoretical, the transducer requires a power supply(see below), and the way the commons are arranged on the transducer and arduino supplies will effect what the Arduino ADC sees voltage wise.
Since it is a current source, you have to keep the resistance low for the full span(voltage compliance). The max resistance value is specified as (Vs-8)*50 ohms, so for 250 ohms you need a minimum voltage on the transducer of 13V.
A DVM set on current in place of the load resistor is the quickest way of testing the whole system. Once you have it tested and working, put the resistor in place and check the voltage, which will verify you have sufficient voltage on the transducer. If you are not getting the correct results you:
1) Have not wired the unit correctly
2) Have not provided sufficient voltage for the current source to work with the chosen load resistor
3) You have a faulty unit(last resort)
This link provides some useful information.
http://www.devarinc.com/pdf/Troubleshooting a 4-20mA loop.pdf

Cheers,
Richard

Thank you for the answer. I'll try to follow this up.

One more question: Is this sensor measuring pressure relative to atmospheric pressure, or relative to absolute pressure? Out of the results it's obvious that it is relative to normal room pressure, but when i read the datasheet it looked like the sensor worked relative to absolute pressure.

Datasheet: http://sensing.honeywell.com/product-page?pr_id=144494

These transducers measure absolute, sealed gage, or vented gage pressure. The absolute versions have an internal vacuum reference and an output value proportional to absolute pressure, the sealed gage versions have an internal pressure reference of one atmosphere at sea level, and the vented gage versions measure pressure with respect to ambient pressure.

That is straight out of the data sheet, so which model did you order,
cheers,
Richard

I have this type: PX2EN1XX200PSCHX

sealed gage versions have an internal pressure reference of one atmosphere at sea level.
That is the type you have,
cheers,
Richard