I need to remove a HART signal from a 4-20ma. I will not be using the HART signal, and it interferes with the 4-20ma input signal. Siemens does not offer the device without HART.

I can convert the 4-20ma to 1-5v with a 250 ohm resistor to ground - will this destroy the HART signal? I am concerned with the HART signal affecting the analog input value.

Any help would be appreciated.

 

I am sorry for my English. You don't have problems with hart signal because the average communication signal value is zero, but the hart address of the device must zero (0) if you want to work with 4-20 mA analog, other device address put fix 4 mA on the out signal.

Jorge

 

First the HART communication will only be present if and when the HART enabled device is being communicated with. You stated that Siemens does not offer a device without HART. I presume that if they did you would be using it, implying that there is otherwise no communication between a "master" and the Siemens device normally in your system.

Second, according to hartcomm.org website, "As the digital FSK signal is phase continuous, there is no interference with the 4-20mA signal." I have to believe that the good people at the HART Communication Foundation might know what they're talking about.

Therefore, I believe your concerns are unwarranted and you should experience no interference with the 4-20mA signaling.

Please advise otherwise.
Best regards,
Steve

 

My advice is to get over your concerns. There's millions of HART devices out there that work just fine with other 4-20ma equipment.

There is no easy way to strip the signal. Adding a resistor most certainly will not. In fact you have to insert a resistor in the circuit to establish HART communications remotely.

 

If your HART signal is indeed interfering with the analog input how about sharing the information i.e. control system Make/Model

I'm pretty sure a small capacitor across the analog input with a resistor in series would dissipate the FSK signal.

Roy

 

I suspect you have excessive noise on the signal that is not HART (as others have explained, there's no HART data unless a HART master asks for HART data), but if you want to strip off any possibility of a HART signal then use any 'loop isolator' that is not specifically HART compatible.

In fact, if your noise is due to a common mode issue, using a loop isolator will solve the common mode problem and might solve the noise problem.

Loop isolator that are HART compatible say so, up front, because they cost more and are specifically designed to pass HART through. The ordinary run-of-the-mill loop isolators are very good at stripping the HART signal off.

 

Thanks for your reply. I do not know for certain that HART would be an issue, but I was warned by someone that it would in the device I am using to read the 4-20ma. The information he had is at least 5 years old, so maybe things have changed. The 4-20 signal is being fed into a mobile controller, similar to a vehicle ECU. It can see very fast changes, down in the 1 msec (1000 Hz) range.

You are correct, we have no need for the HART and are using the 4-20 for level control. It is a radar level sensor. If I understand you another device would need to be online and requesting communications with the HART device for any HART signal to be present, this is reassuring.

I am trying to avoid any issues when the customer goes to start up. They are using 3 of these radar level sensors, and they are quite pricey.

Thanks again, everyone, for your replies.

Phil

 

Thanks for your response, Roy. As I said above, I do not know for sure that it will interfere but was advised it would. However, the information is at least 5 years old.

The 4-20 signal is going into a Danfoss Plus1 controller. Depending on the controller we end up using, it may go in as 4-20 or I may convert it to 1-5V with a 250 ohm resistor to ground. I may also be able to smooth the HART signal out (if its even present) in the software using a exponential or averaging filter.

However, it seems I have nothing to worry about, if I understand correctly. Thanks to Steve's reply, I believe the HART from the level sensor will not be active unless it was requested by a 'master' device. I will have no such device in the system.

Thanks to all for your replies. If I have an issue I will post it. We probably won't know until start up, mid- February.

Phil

 

Update: No issues with the HART signal in a Danfoss controller. Thanks for all your responses!

Phil

 

Most systems have no issues with the HART signals but some do. There are a
number of solutions available such as

HART input filter for transmitters:
http://www.pepperl-fuchs.us/usa/en/classid_1362.htm?view=productdetails&prodid=32778

HART output filters for positioners:
http://www.documentation.emersonprocess.com/groups/public/documents/bulletins/d102798x012.pdf

Cheers,
Jonas