New and old pressure transmitter 4-20 ma output is negative. Company wanted to upgrade to a yokagowa transmitter. Upon confirming proper operation I realized the output was negative. I slammed the old Honeywell (was working fine before upgrade) back together and the output is negative. I'm very puzzed. Thing is, the transmitters 4-20 is sent to a distributed computer system that seems to expect a negative signal. I think it "expects" because it displays a positive pressure, and thus everything seems to be functioning properly. Still, I'm very curious. Why is this negative? I think it's worth while to add that I couldn't find a setting on the Hart controller to switch sign. Oh, and also, the transmitter is a single impulse line with the other side capped, so I "think" that means it's absolute pressure?
Thanks in advance
Sounds like the plugged d/p connection is an attempt to give "absolute pressure" indication, but a negative off-set in the DCS code is used to get gauge pressure.
Similar issue with d/p flow measurements where the square root is done in the transmitter and DCS.
Thank you for your response.
Does it make sense that the ma output is negative. I went to yokogawa web site and they said in no uncertain terms that absolute pressure output was positive. I assume that also means output?
I'm not clear on what is meant by "negative 4-20".
4-20mA is only 'negative' when a mA meter is connected backwards in the current loop circuit (or across the test terminals). Are the meter probe leads plugged in backwards (red in common, black in V/mA?)
Or did you mean low pressure end of the scale is at 20mA and the high pressure end is at 4mA?
Or did you mean the pressure is displayed as negative values in the transmitter's local indicator?
Is the process applying a vacuum pressure, less than atmospheric?
Both Absolute pressure and Gauge transmitters can be the dual process head style with one 'regular' process head tapped with a threaded pressure port and one 'blind' process head that is untapped.
A gauge pressure transmitter (dual process head or in-line style) will display a negative value (in its local indicator) when a vacuum is applied, that's what they're designed to do. Presumably the 4-20mA range is ranged to cover the "negative vacuum range." Then the DCS reinterprets the 4-20mA signal so that negative-with-respect-to-atmosphere is interpreted as positive, absolute values.
Yes, absolute pressure is always an inherently positive value because its reference is absolute zero, but the local displayed pressure value can probably can be represented in the display as a negative value (meaning vacuum with respect to atmospheric pressure) on a modern smart transmitter because the display is displaying whatever it is configured to display (I've never tried, but it might be possible that the display values can be scaled as such).
The stainless ID tag is usually stamped with the 'factory' range and engineering units which will tell you whether the transmitter is absolute or gauge pressure. Or you can decode the model number to determine whether it is an absolute pressure or gauge pressure transmitter.
Typical loops tie +DC out to the transmitter with the return wire connects to the DCS input with conversion resistor to DC common.
The resistance is 250 ohms for 1-5 volt DCS input. The resistor can be integral to the input card or external.
Current is defined as flowing from positive to ground(-) so that a positive 5 volts is monitored by the DCS at 20 ma.
It used to be quite common to find a DP cell measuring backwards because they just couldn't handle a negative pressure.
For example, a DP transmitter measuring the level in a condensate tank. The bottom tank connection is at less pressure than the top connection, because it has filled with condensed water.
Foxboro had a transmitter we used to call a singing wire because, it measured the resonant frequency of a wire under tension.
Also, many of the pneumatic transmitters would only handle the HP on one side. Modern strain gauge instruments don't care.