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  1. #1

    Sharing a thermistor sensor

    I have a situation where I only have access to one sensor in a pipe. I want to use this sensor in two controllers. Can I run wire from the sensor to the two controllers? Will I get a correct reading? What are everyone's thoughts?

  2. #2
    Join Date
    Jul 2006
    Location
    Southern California
    Posts
    223

    Reading the value of one thermistor with two controllers?

    Many brands of controllers use a wetting voltage to read the thermistor resistance. There is a possibility of having incorrect readings or even damaging the thermistor or controller. I have done parallel thermistors with software to average the final result, but what you have described is something that I do by BACnet or system network broadcasting of the thermistors value. Broadcasting is safe and easy if your system supports it.

    Does your system support a network broadcast of the value from the thermistor wired into one controller over to the other controller?
    "We are what we repeatedly do. Excellence, then, is not an act, but a habit" Aristotle

    Remember to "Pay it Forward"; help out the newer generation of techs, remember someone during our career helped us! ("Pay it Forward" was by someone smarter than me!!)

  3. #3
    When I spoke to tech guy at Kele, I mentioned the same thing about a possible wetting voltage on the sensor but he didn't think that was so. As I explained to him in certain cases I thought the sensor was used as an electrical load in the loop that changes in relation to the temperature. Given that scenario I was worried about two different sources of power.

    Here's the deal. We have a McQuay chiller that uses the Carel (Microtech II) controller. The McQuay chiller had the leaving sensor in a bad location. Since my sensor was in the right location, and the same type McQuay uses, we offered it to the McQuay tech in order to get the chiller to work. The problem is that I now don't have a way to get the reading into my controllers. The Carel controller on this does not seem to have any AO's on it so I can't get anything out of it.

    My controller is a Distech EC-67, so it can take anything (0-10, 4-10, 10kT2). So what are my options without adding a second sensor in the pipe (we don't want to get a hot tap done if we don't have to).

  4. #4
    Join Date
    Nov 2006
    Location
    SE Wisconsin
    Posts
    299
    Is your sensor bing used for control or just a monitor?

    If it's just a monitor can you go with a strap on sensor?

    If it's for control...ummm...I'm open to suggestions.

  5. #5
    Join Date
    Jul 2006
    Location
    Southern California
    Posts
    223

    McQuay LON Comm Card

    Have you thought about putting in a McQuay Lon Communications card?

    They are not expensive and you may be able to grab what ever variable (inputs, status or whatever) the Microtech II is using.

    I have done alot of points mapping to McQuay using their BACnet Card and they integrate very easy.

    Other than that, put a strap on sensor with some thermal paste and then insulate around the strap on sensor. This installation method of a strap on results in about a 1/2 degree loss of accuracy and a delay in temperatures response (lag time to temperature change). If your requirements can live with these issues, do the strap on. Strap ons seem to work better in boiler applications then on chiller applications. Just my experience.
    "We are what we repeatedly do. Excellence, then, is not an act, but a habit" Aristotle

    Remember to "Pay it Forward"; help out the newer generation of techs, remember someone during our career helped us! ("Pay it Forward" was by someone smarter than me!!)

  6. #6
    There are BACnet cards which should have been Lon. Here's the issue. I have a JACE2 but we ordered it all Lon. So now what?

    This is LAUSD and they will not accept any type of strap on sensor.

  7. #7
    Join Date
    Jul 2006
    Location
    Southern California
    Posts
    223

    BACnet McQuay Card issue?

    Get ahold of the rep at DMG and get the LON card, should take two days.

    My guess is that your LON Buss is close to the MicroTech II panel.

    When the LON card gets in, either have Rich switch it out or do it yourself.
    Very simple instructions and not technically difficult to do. All of the tech data on adding this card is down loadable on McQuay's website.

    Sounds to me that you are under a time pressure situation. I would recommend getting the LON card installed. From what I have read,
    the swap between BACnet versus Lon card on the McQuay takes a few minutes in the field. Read the docs at McQuay first, but I think you and your client will be much more satisfied with pulling in all of those McQuay data points, will score points with the owner.

    Making a decision at his point would get you moving in the right direction.
    "We are what we repeatedly do. Excellence, then, is not an act, but a habit" Aristotle

    Remember to "Pay it Forward"; help out the newer generation of techs, remember someone during our career helped us! ("Pay it Forward" was by someone smarter than me!!)

  8. #8
    I am dealing with DMG. That's who I'm trying to help out as I know a lot of the guys there. I don't really need Rich to come up but I have his number if I need help.

    I know what you are saying.

    So after all of this, do we know each other?

  9. #9
    Join Date
    Jul 2006
    Location
    Southern California
    Posts
    223

    Do we know each other?

    I can't tell from your profile. I have been around the souther California controls market for a very long time and have done many many projects all over the state.

    I don't hide my identity on this site. No need to. Check my profile, you may know me. Anyway, If you know me then you know why all my techs and clients call me dracula. Sell during the day and program and engineer during the night. At thats what I used to do.

    The folks at DMG unfortunately sometimes get wacked by the mechanical contractors when the mechanical contractor orders the chillers or AHU's without really knowing how or what to order in the way of controls comm boards for the McQuays.

    Anyway, good luck, I just was involved with the City of LA on some jobs with McQuays interfacing to Alerton, always fun when the mechanical contractor orders the wrong stuff and tries to blame the supplier or controls contractor.

    Same stuff, different address.
    "We are what we repeatedly do. Excellence, then, is not an act, but a habit" Aristotle

    Remember to "Pay it Forward"; help out the newer generation of techs, remember someone during our career helped us! ("Pay it Forward" was by someone smarter than me!!)

  10. #10
    I just sent you an email.

    I know what you mean about the mechanical ordering the wrong equipment. Just once it would be nice if the mechanical would listen to us when we say don't negotiate or get pricing from the equipment supplier until we tell you what to order for us. Never fails. I didn't even know about the chillers even having BACnet cards. It said right in the spec. - BACNet, Lonworks or Modbus - just once I wish the mechanical or equipment supplier would ask us "What do you need?".

    I had a meeting the other day with one of my best customers and was able to meet with the equipment supplier in order to get him to include a BACnet and Lonworks card as part of his package.

    ON this particular job no one asked me what I wanted. The rep from DMG, who I know, told me that the BACnet cards he supplied were worth $ 4,000 !!

  11. #11
    Join Date
    Mar 2006
    Location
    Chicagoland
    Posts
    44
    Quote Originally Posted by outtacontrol View Post
    When I spoke to tech guy at Kele, I mentioned the same thing about a possible wetting voltage on the sensor but he didn't think that was so. As I explained to him in certain cases I thought the sensor was used as an electrical load in the loop that changes in relation to the temperature. Given that scenario I was worried about two different sources of power.

    Here's the deal. We have a McQuay chiller that uses the Carel (Microtech II) controller. The McQuay chiller had the leaving sensor in a bad location. Since my sensor was in the right location, and the same type McQuay uses, we offered it to the McQuay tech in order to get the chiller to work. The problem is that I now don't have a way to get the reading into my controllers. The Carel controller on this does not seem to have any AO's on it so I can't get anything out of it.

    My controller is a Distech EC-67, so it can take anything (0-10, 4-10, 10kT2). So what are my options without adding a second sensor in the pipe (we don't want to get a hot tap done if we don't have to).
    You are correct.
    The thermistor is read by impressing a voltage across it or passing a constant current through it (better way). Connecting a thermistor to two controllers at the same time may cause issues depending upon the design of the controller(s).

  12. #12
    Just as I thought. Thanks. I'll figure something else out with McQuay unless someone can come up with something else.

  13. #13
    Join Date
    Jul 2006
    Location
    Southern California
    Posts
    223

    Transducer splits one thermistor to two controllers?

    By the way, I seem to recall a transducer that used to be on the market that would let you split a single thermistor and send it out to two separate controllers.
    I think it was either at ACT, Triatek, or Kele controls, I will go thru my old manuals to see if I can remember who sold it.

    It was a kick ass transducer because it could do 4-20ma or thermistor and do it without either system controller seeing anything different then one thermistor. I seem to recall using it on a couple of projects in downtown
    LA where we did a LAYOVER control installation (remember those old days
    where the owner wanted you to install the DDC system but have the old system as a back-up). I will post if I can find it.
    "We are what we repeatedly do. Excellence, then, is not an act, but a habit" Aristotle

    Remember to "Pay it Forward"; help out the newer generation of techs, remember someone during our career helped us! ("Pay it Forward" was by someone smarter than me!!)

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