24VDC power for 4-20ma loop controll

[tpfunder]

I have a AB 500 plc. and am working on a ph probe. my Question is;
What is the proper way to power the loop. Let me be more specific. the 24vdc power is a switching power supply (sola) the 0vdc or common is not tied to Ground. Should it be tied to ground. The shielding on the cables is grounded. The 0vdc to the ground has a 3vdc reading on my meter. is this a problem. is there a better way to do this? Floating common or grounded common? this loop has been working for years now, I am trying to make it more reliable.

 

[jonesy29847]

What do you mean by more reliable?

 

[TRWServices]

Some 4-20 mA loops are powered from your PLC card, some are externally powered, in any case they should not be "GROUNDED" they would be what you are thinking of as a floating common. If you set your multimeter to read mA, and connect one lead to the 24VDC positive then the other lead to the PH probe, then take your remaining PH Probe lead back to the 24VDC negitive, your meter should read in place of the PLC input, giving you your 4-20 scale. If it does not read 9 times out of 10 the polarity is reversed just swap the leads, here is the appoximate readings you should get depending on your buffer.

PH mA
0 4
1 5.1428
2 6.2856
3 7.4284
4 8.5712
5 9.714
6 10.8568
7 11.9996
8 13.1424
9 14.2852
10 15.428
11 16.5708
12 17.7136
13 18.8564
14 19.9992

If it does not read right usually the probe has aged to the point that the ref buffer inside needs replacing or the salt bridge is old and clogged up. depending on application about 36 months is the max you can get out of one before replacement.