Capacitance Measurments

[M. Atteya]

Hello everybody,

We are using Type A Cables for Foundation Fieldbus, when we tried to do the segment checkout procedure that is recommended by Fieldbus organization, document AG-181, we found that there are alot of capacitance measuements is not within the acceptance criteria. We are using Fluke 187 True RMS Digital Multimeter for doing the measurements.

There is a note that i don't understand that states "Most capacitance meters measure components only and will not provide the expected results when measuring a complete segment".

1. What does that mean?
2. Is Fluke 187 not recommended? or it can do the right measurements

Your answers will be highly appreciated.

Thank you

[rezabejd]

I have installed a few dozen segments and operated them a lot of years in a fairly harsh environment. I have yet to make a capacitance measurement.

The most critical issue with capacitance, or broadly the "impedance" of the segment (c. 100 ohms), is that the signal zero-crossings (the electrical phenomenon that is read as 1's and 0's) are clear and on-time. I have always found it easier to hook up the segment and look at the waveforms at various spots, or hook up a good fieldbus-specific diagnostic tool like a Relcom FBT-6 or a P+F ADM. I also use a Fluke Scope-meter. The only thing I use a DMM for is checking supply voltage - and then only for cause (like if a device never shows up in the host diagnostic tool despite being wired up).

Is the frequency regular? Does the wave-form show too much capcitance? Is the peak-peak voltage in spec? (somewhere in the vicinity of 1 volt ± 400 mV)

The diagnostic tools like those made by P+F and Relcom, and the 375 communicator by Emerson will identify 99% of the issues, IMO. They will easily identify all the ones you'd detect with a capacitance meter. Maybe AG-181 was too vendor-neutral to recommend or endorse any proprietary solution. You might also want to see if your systems maker (DCS) has any favorite that they have used.

[M. Atteya]

Hello,

We are Using Emerson's DCS, DeltaV v8.3, it recommends exactly the same values and limits which is shown on AG-181. We also use 375 communicator, see attached results for the segment checkout procedure.

Note that we don't have a scopemeter.

Please see the attached results for Capacitance and Resistance and comment/recommend.

Thank you

[rezabejd]

What percentage of your total segments do these represent?

The impedance issues that the capacitance test seems to be indicating may be supported by the low P-P voltages at various addresses. You seem to have quite a few at 500 mV or less.

Did your type "A" cable come with any certification? Some have seen issues with cable that in reality did not meet the spec, or was at the fringes of passing it. Poor quality and / or damaged cable may explain some of the readings you're getting.

Some of our forum's physical layer expert EE guys may have some better suggestions for you, but in the meantime, maybe take your worst segment (the last one?) and start with "trunk only" noting what readings you get. Then add one spur at a time and see if one causes a step-change in the values observed.

[aagostin]

It seems to me as if the measurements were taken under inappropriate conditions.
The idea behind this test is that only cables and the field terminator are installed, no device and no power supply connected.
Today, terminators are built into power supplies and wiring blocks. Wiring blocks are usually not an issue, but the intention at the time of writing AG-181 was to test with a simple (non-fieldbus barrier) type of wiring block installed. As such, can you confirm whether you are using a regular wiring block such as a Megablock, or are you using a fieldbus barrier? If you use a fieldbus barrier, capacitances can be different to AG-181, since a fieldbus barrier is an active device. You should then disconnect the fieldbus barrier to check the cable installation only. Note that the capacitance between + and - conductor should then be some nF only.
So you have to make sure when you test resistance and capacitance, that no instrument and no power supply is connected. Note that instruments and the power supply have to be physically disconnected; if you simply switch off power, you will measure internal capacitances which will falsify your measurement.

Another issue could arise through the use of surge protectors. Depending on the manufacturer and the type model, you could introduce capacitances into your wiring. Best is you post a drawing on your physical setup and the test conditions.
Your segments readings show no capacitance between + and -. You should measure at least one terminator (1uF).
Also, your signal amplitudes seem to be too small. I would have expected something between 650mV and 850mV.
Step number 5 can not be measured with a multimeter. You need a fieldbus multimeter such as the MTL/Relcom FBT-6. A "regular" multimeter can not measure the fieldbus communication and shows you the average of noise and ripple.
FBT-6 will also give you noise readings, which can help you to identify installation and grounding issues. Note that some of the readings given in AG-181 might not be up to date. The signal amplitude for LAS is usually around 650-750mV, but devices typically are slightly higher at 750-900mV. A value of 350mV for a device is not accpetable.
You are welcome to contact me directly.

[smr]

I'm running into a similar issue with a Fluke 87V. All field devices are disconnected and the only terminator set is in a P&F segment protector (F2-JBSC series).

We are measuring capacitance on the home run back at the cabinet which is disconnected as well. If we measure capacitance with the expected polarity + to + - to - we get ~2uF (2 terminators). If we measure it with reverse polarity + to - and - to + we get the expected value of ~1uF (one terminator).

Is there any additional information on how to specify a meter so that we can measure the correct capacitance values, and any possible network configurations that could cause an incorrect measurement?


There is a note that i don't understand that states"Most capacitance meters measure components only and will not provide the expected results when measuring a complete segment".

1. What does that mean?
2. Is Fluke 187 not recommended? or it can do the right measurements

Your answers will be highly appreciated.
Thank you

[mikestrauser]

For what it's worth, Emerson only recommends the Fluke 123 and 124 for making capacitance measurements on Fieldbus segments. This has something to do with the particular algorithm that is used in those meters. Other meters may work OK for certain types of capacitance measurement, but a Terminator has 100ohms in series and this throws some of them for a loop.

The 1 and 2nF in the previous post is incorrect. Fieldbus Terminators have a 1uF (1 microfarad) capacitor in each of them (two per segment).