dslreports logo
site
 
    All Forums Hot Topics Gallery
spc

spacer




how-to block ads


Search Topic:
uniqs
1248
share rss forum feed

cmslick3

join:2004-05-24
Joliet, IL

Anyone familiar with HVAC DDC systems

I'm looking for someone who is familiar with building automation DDC systems. I know this is home improvement forum, but I was hopeful someone here would know something, or know someone who does.

I am trying to solve a problem where a DDC system is picking up RF power and therefore reporting false measurements. This is a temperature measurement which is based upon return current from the temperature probe.

If anyone has any information or a person I can contact that would be most helpful.

Thanks...


cowboyro
Premium
join:2000-10-11
Shelton, CT

1 recommendation

First of all install RF filters for the sensor wires. A ferrite choke goes a long way. Also a ceramic capacitor with short terminals on the sensor connections may help.


Red_Menace
poking around since 1978

join:2001-11-03
Littleton, CO
reply to cmslick3
Is the cabling shielded?


whizkid3
Premium,MVM
join:2002-02-21
Queens, NY
kudos:9
reply to cmslick3
So while this is a problem in your DDC system; the problem is not unique to DDC systems. RF interference happens in all sorts of systems. Problem is complicated when the system is used for precise measurements and RF interference is throwing off the precision of the measurements. This is one problem that is frequent and where theory is often as helpful as real-world fixes.

Solving the issue can only start with symptoms & possible causes. Questions include:

1. what type of system is making the measurements (details)?
2. what level of precision is needed?
3. what mitigation techniques are already in place? Is the cable shielded?One end or both ends? What does it connect to at each end? Does it run with other cables? What kind of separation? How long are the wires?
4. what symptoms are being experienced.
5. what is suspected as the interfering source? Induction from other cabling or an actual radio source (antenna)? What can be done to the interfering source to reduce its impact?

Provide us with as many details as you can fathom; and we'll help you solve it.


John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:8
reply to cmslick3
Make sure you're using twisted pair wiring with a shield that is grounded only on the controller end.

shezams
My Other Car Is A Zamboni
Premium
join:2001-08-14
Hyattsville, MD
reply to cmslick3
whizkid3 has most of the causes listed nicely. Other things to consider is the type of signal and what is powering it, and with the run you need to avoid running in the same pipe as higher voltage and avoid running parallel any closer than 6 inches if possible with plenum cable. Using twisted shielded pair will help, as will running in EMT. Here is some good reading »www.bapihvac.com/content/uploads···oops.pdf
--
Simple rules - no offsides, no intent to maim, everything else is all good!

cmslick3

join:2004-05-24
Joliet, IL
SO here are some answers:

The cable between the DDC and the temperature probe is not shielded. It's twisted copper probably 16ga run in conduit. The model of the probe is ACI/TT100.

1. what type of system is making the measurements (details)? Unknown

2. what level of precision is needed? Unknown

3. what mitigation techniques are already in place? Is the cable shielded?One end or both ends? What does it connect to at each end? Does it run with other cables? What kind of separation? How long are the wires? There has been no attempt to mitigate this issue yet. The wire between the DDC and probe is not shielded.

4. what symptoms are being experienced? From the information I've been given the heat sensors are reporting incorrect readings causing the system to not generate heat in certain areas of the building. They are manually setting these probes in test mode and entering bogus readings.

5. what is suspected as the interfering source? Induction from other cabling or an actual radio source (antenna)? What can be done to the interfering source to reduce its impact? The source of the issue is an in building coverage system installed in the duct work by my company on behalf of a major cellular carrier. I don't want to be specific! The RF power from the duct antennas is around 20watts (43dBm). We have tried increasing the space between the temp probe and the antenna and it's not solving the problem. In fact a couple of the impacted probes are in another part of the duct completely and they are still having issues. The building engineers are turning off our equipment and this solves their problem, but creates a problem for us.

As you can tell I'm on the side causing the problems to the building systems. Per FCC rules the cellular carrier has every right to transmit regardless of what interference they cause until the FCC tells them to stop. Unfortunately the building engineers don't like that answer and in the spirit of cooperation I am trying to help come up with a solution for this problem. I was thinking about this yesterday and was wondering what the impact of putting an inductor on the negative side of the wire would be? In theory this would filter any RF current induced on the wire by the antenna, but this is loose theory on my part.


John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:8
reply to cmslick3
So the antenna is installed IN the ducting?

What is the distance from the temp probe to the antenna?

Please elaborate on this.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
reply to cmslick3
OP what kind of measurement device are you using? Thermocouples, resistive temperature devices (RTDs or thermistors) and what type of signal? MV, current (4-20 madc) voltage 1 - 5 volt dc or other.

What kind of controller? P only, P + I or PID? What is the controller output and what is the output controlling? An I/P controlling a pneumatic valve perhaps or just strictly an off/on?

There are a lot of details missing and as Whizkid asks need to be answered.


John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:8
reply to John Galt
Click for full size
Is this what is installed (or similar)?


John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:8
reply to Jack_in_VA
said by Jack_in_VA:

There are a lot of details missing and as Whizkid asks need to be answered.

Reading is fundamental, Jack.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
said by John Galt:

said by Jack_in_VA:

There are a lot of details missing and as Whizkid asks need to be answered.

Reading is fundamental, Jack.

Well I guess Whiz and I flunk "YOUR" class John.


John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:8
said by Jack_in_VA:

Well I guess Whiz and I flunk "YOUR" class John.

Apparently...

Look further up the thread...


John Galt
Forward, March
Premium
join:2004-09-30
Happy Camp
kudos:8

1 edit
reply to cmslick3
Having thought about this for a bit, I think I'd recommend moving the RTD sensor out of the airstream to the exterior of the duct and covering it with a layer of insulation. There will be a loss of instantaneous response, but in the big picture that shouldn't be too much of an issue. I'm presuming that this is normal environmental air.

Another alternative is to put a cover of perforated metal over the probe, essentially shielding the probe from the RF. The holes will have to be small, less than 0.1 of the wavelength of the RF. The down side to this is that there may be clogging of the holes with dust, depending on how religious they are on keep the filters clean. Having it near an access panel would be advantageous (near a fire damper, for example).

Another alternative is to move the RTD a substantial distance from the antenna, but it would still be in the duct and susceptible to the RF.

Generally speaking, I prefer the removal to the exterior of the duct.
--
Don't ask questions -- just do as you're told.


public

join:2002-01-19
Santa Clara, CA
said by John Galt:

Another alternative is to move the RTD a substantial distance from the antenna, but it would still be in the duct and susceptible to the RF.

RTD probes are commonly used inside multi kW power plasma chambers. It is a matter of isolating the output with a separate compartment, feedthrough caps and inductors.

cmslick3

join:2004-05-24
Joliet, IL
John, Yes that's what is installed.

I'm not sure about the possibility of moving the temperature probe outside of the duct. These are in the main air handler locations and are not exactly in the environment which they are monitoring. I do like the idea of trying to isolate the probe with a metalic shield, but this is likely not practical in this particular building.

I wish I knew more about the actual DDC system and components themselves but I am not an HVAC guy and don't have access to the equipment. I also wish we had the permission to take measurements of the voltage and current levels with the RF on and off to compare it's effects.

I'm going to recommend that we try some of the ideas suggested here and see where that goes.

Thanks for your help so far, I'll keep you posted on progress.


PSWired

join:2006-03-26
Annapolis, MD
reply to John Galt
said by John Galt:

Having thought about this for a bit, I think I'd recommend moving the RTD sensor out of the airstream to the exterior of the duct and covering it with a layer of insulation. There will be a loss of instantaneous response, but in the big picture that shouldn't be too much of an issue. I'm presuming that this is normal environmental air.

Another alternative is to put a cover of perforated metal over the probe, essentially shielding the probe from the RF. The holes will have to be small, less than 0.1 of the wavelength of the RF. The down side to this is that there may be clogging of the holes with dust, depending on how religious they are on keep the filters clean. Having it near an access panel would be advantageous (near a fire damper, for example).

Another alternative is to move the RTD a substantial distance from the antenna, but it would still be in the duct and susceptible to the RF.

Generally speaking, I prefer the removal to the exterior of the duct.

I like this idea. Another alternative would be to cut a small hole in the side of the duct and cover it with an RF opaque material such as an ESD bag (the dark translucent kind with a metallic layer embedded within the plastic). Then fasten the sensor to the thin layer. Response will be better than if it were separated by the duct wall.


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
reply to cmslick3
So it's been determined that the RF interference signal is getting in the circuit at the sensor? I would be interested in how that was determined.


cowboyro
Premium
join:2000-10-11
Shelton, CT
said by Jack_in_VA:

So it's been determined that the RF interference signal is getting in the circuit at the sensor? I would be interested in how that was determined.

Me too...

The current output is unsusceptible to EMI

»www.asiconsys.com/en/upfile/2007···6644.pdf
Either the sensor in indeed susceptible to EMI or it's the controller that is at fault.
For some odd reason I have a feeling that RF gets into the controller through the wires. Anyway that is the easiest to check and eliminate with simple LC filters.


leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
·SONIC.NET
reply to Jack_in_VA
I don't see where anybody said anything about a determination. There is still a lot of guesswork involved but those guesses appear reasonable:

- the RF is inside the duct
- the problem sensors are inside the duct
- the wiring to the sensors is likely outside of the duct (unconfirmed assumption)

This makes it more likely for the interference to take place at the sensor then the wiring between sensor and control system (due to the higher exposure to the RF).

You are correct if the purpose of your post was to point out that RF interference at places other then the sensor haven't been completely ruled out yet.
--
Got some spare cpu cycles ? Join Team Helix or Team Starfire!


mattmag
Premium,ExMod 2000-03
join:2000-04-09
NW Illinois
kudos:3
reply to cmslick3


It also appears that the OP is not going to have access to find the answers to many of the questions posed here either. Seems like its a "You need to get rid of that interference but we're not gonna let you touch our stuff" kind of deal. Not always productive to finding the best solution...


Jack_in_VA
Premium
join:2007-11-26
North, VA
kudos:1
Reviews:
·Millenicom
reply to leibold
said by leibold:

You are correct if the purpose of your post was to point out that RF interference at places other then the sensor haven't been completely ruled out yet.

I'm just reading the posts and the "assumptions" that are being thrown out. A quality temperature measurement instrument system properly designed should not have a problem with RF interference.

The OP who really has not been able to provide enough information to determine the problem much less a fix IMO.


leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
·SONIC.NET
said by Jack_in_VA:

A quality temperature measurement instrument system properly designed should not have a problem with RF interference.

Agreed, but 20W of RF energy is sufficient for plenty of mischief.

cowboyro See Profile, thanks for the sensor (thermal probe, temperature transmitter) details.

A 2 wire current-loop over twisted pair (assuming the sensor is really wired with a single pair and not 2 wires from separate pairs) is fairly noise resistant (common mode interference).

I would try a ferrit choke to block RF and a 100nF ceramic capacitor at the controller end of the sensor cable as remedy if the interference is injected somewhere along the route of the cable between temperature transmitter and controller.

Unfortunately the OP is dealing with active electronics inside the temperature transmitter and depending on model perhaps an additional 6ft of cable for a remote temperature probe. The probe itself is just a 100Ohm thermal resistor and therefore a ferrit choke and capacitor may be used if the interference is injected at the thermal probe or cable between probe and temperature transmitter.

If the interference is taking place inside the electronics of the temperature transmitter it gets a bit more complicated.
--
Got some spare cpu cycles ? Join Team Helix or Team Starfire!