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OT - Low Cost Loop Powered Digital Process Meter

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  • OT - Low Cost Loop Powered Digital Process Meter

    Hoping some of you guys have shopped digital panel meters lately. From a PC equipped with USB I/O devices, I am sending an analog 4-20ma output to a loop powered digital process meter to display the temperature set point for an HVAC zone. This set point temp will only be allowed to range from about 50F to 85F so as to accommodate night set back temperatures and other non-occupied indoor temperatures. Decimals are not needed so even a two digit meter would be fine although 3.5 digits is much more common and perfectly acceptable.

    The lowest cost loop powered process meter I have found is from Omega for $50. See details here

    I need about ten of these and was wondering if anyone knew of a lower cost process meter that would work for my situation. Color of numbers and display size (smaller is better) are not important. Mainly looking for something cheap of reasonable quality.

  • #2
    Are your 10 separate units? or could you use one meter and switch it?

    Switching is trickier with a loop, but is do-able.

    Keep eye on ball.
    Hashim Khan


    • #3
      When I did remote temperature sensing and control, I used a series of LM35 ICs and a Teco PLR. I could not only remote sense but also do all of my logic control using the PLR.

      I encased the LM35 ICs in a tube of aluminum and sealed the ends with something akin to RTV. I had to calibrate each one due to resistor variations but that was easy.

      As far as display of temperature goes, that too was easily done with a little software and a cheap panel mounted volt meter.

      I know this isn't what you are really looking for, but it may help.

      If you need current loop due to line lengths, then you are stuck with the expensive solution.


      Edit: I looked again and the LM35 can be used with a 20ma current loop.
      Last edited by Richard-TX; 05-16-2010, 09:21 AM.


      • #4
        You might be able to make this work with a little simple hacking. For $7 it could be worth the time. If you disable the decimal point it has a display range of 40 to 129.

        Here is a description of the internals:

        Input voltage feeds two circuits, the power supply and a resistive divider, and both can be modded.
        The supply is two-staged. First a 200 Ohm / 5V zener feeds some LCD voltages. The 200 Ohm resistor draws too much current at 13V, but can be increased if don't need to measure voltages as low as 4V. (I'm using 470 Ohm.) For even less power usage, the first stage could be replaced entirely with a 5V LDO.
        Then the 5V feeds a 3.3V linear regulator (or reference). This feeds the microcontroller and doubles as the ADC reference.
        The divider is strait forward outputting in the 0-3.3V range.
        The software will never display more than 12.9V, no matter what feeds into the ADC. It won't signal overvoltage in any way, it just shows 12.9V.
        If voltage drops to less than 4V, the display will flash "LOW", and IT WON'T RECOVER WITHOUT CYCLING THE POWER, it just keeps showing a flashing "LOW".
        Free software for calculating bolt circles and similar: Click Here


        • #5
          The LM-35 is good, but I take it that he already has the sensors......

          of course, the current loop can be sensed with a resistor and voltmeter, and the zero voltage bucked-out by an adjustment for each loop...... easier to switch, and voltmeters are cheap if you can scale to show a sensible voltage-temp relationship.

          The LM 35 has that inherently....

          Keep eye on ball.
          Hashim Khan


          • #6
            Like JTiers said I already have sensors lined up for use in 10 locations of my new office building. Thinking of using this HUM2 model here I will get display of current room temp and relative humidity at the same time 0-10V signals for each are sent to my USB device.

            By displaing the current setpoint temp then I will have three items of information available at every thermostat location with no button pushing required, although I will use momentary push buttons to adjust the setpoint temp.

            Use of a loop powered process meter sure makes it easy to show the set point temps and with no separate power wiring concerns. Some of my HVAC zones (storage areas) are fairly remote in the 4800 sf building and probably don't need user adjustatable temp set points. So maybe I could eschew that feature except for the most occupied zones.

            Regular little panel volt meters are all over the place for $7. Sure seems like it wouldn't be too hard to do as Evan suggests and modify a standard volt meter to display temp values in the narrow range that I need.

            And as Richard and JTiers suggest, although an LM35 is primarily a sensor it has the logic to scale volts to a temperture. But without a lot of direction, I don't think I can separate that feature out for my use. Maybe there is a more direct IC available for that purpose?

            You have got to imagine that if the importer was found for the Omega $50 process meter as seen here that it could be bought for much less.

            I already checked with the Temco Controls importer from China and he cannot obtain a process meter that cheap. Again, such a meter sure makes this job simple. Nobody knows a supplier that can beat $50 on an off the shelf process meter?


            • #7
              There are ICs that measure temp and give a digital output..... I'd have to look them up, some dorks at a prior employer were all hung up on that, and were trying to make an interface very expensive until we suggested the LM35....

              if you didn't have the sensors, you might be able to just read out the digital info on a PC...... Lots of building control runs on various types of bus and ends up at a PC.......
              One with an RS485 would allow you to address each one and write up a VB interface to show them all, log data, even control dampers and etc to keep temps in the acceptable range......

              I see the modbus version is $45........ makes it easy.

              if you don't like that......
              just reading mA to volts and volts as temp is not hard...... you need the scale factor for the sensor, and a reference voltage and pot to null out the bottom-of-scale voltage. A resistor gives mA to volts, and a one time adjustment gives you the zero-out setting to set the scale to the right temp.

              In the case of those, they seem to suggest a particular load, which means you might divide down with a resistor divider to get the voltage variation you want with their suggested load, or make your sense resistor just a part of their total load......

              if you make 0 to 100 deg give 0-1V, you get direct readout. With the scale factor of the sensor, you can pick the resistor or divider.
              Last edited by J Tiers; 05-16-2010, 01:37 PM.

              Keep eye on ball.
              Hashim Khan


              • #8
                Give these guys a call


                They sell some just circuit board products that are very reasonable

                Of course since your doing Building Environmental Controls the quality, versatility, and pre-packaged algorythems of these guys products is impossible to beat

                Last edited by JoeFin; 05-16-2010, 02:00 PM.


                • #9
                  I will be using VB6 to write the complete control program pretty much like you would for an industrial control system. I have a lot of unique things going on that standard hvac oriented controllers don't address. I am assembling a lake loop ground soure heat pump system that uses two Copeland digital scroll compressors that offer 10-100% modulating capacity and two vfd controlled York air handlers. Going to modulate water flow through the heat exchangers and also airflow at many points in the system with modulating dampers. Virtually everything can modulate.

                  It's definitely an overkill approach, but will be a great learning experience and will allow me to play with the system while at work. All modern building control systems are bus based (Modbus, Bacnet, Lonworks, etc.) and I will place wire for future implementation of one of them, most likely ModBus and RS485.

                  But since I will be controlling everything in such detail with my own VB software and will be able to do all the control and tracking of things Jtiers mentioned, I thought I would pass on RS485 and Modbus for now. The controlling PC will be connected to the office network and can be seen from both office and home.

                  Remember for the setpoint temp display, there is no sensor reading involved, the PC through my own logic in VB software and USB will generate the appropriate analog output. I will have 0-5v, 0-10v, and 4-20ma analog outputs available. I just need a loop powered display device that will accept one of those and scale it to a temp reading.


                  • #10
                    Well then, the resistor scaling and voltmeter is your cheapest solution, most likely......

                    I'm not clear how the data return gets to the PC in your setup....... you mentioned a USB device......... datalogger?

                    You should note however, that the modbus is merely the communication to the devices...... and supplants the scaling, display, usb device, etc..... Data is delivered directly to the PC via a modbus driver and a small adapter such as offered by "Blackbox" etc.... which can be USB or RS232.

                    You can grab that data from each device via your VB program and have it right there on demand.

                    if you simply want a local "sanity check" device, the voltmeter setup is the cheapest. But you need some simple parts to go with it. (edit) IF you want it to be direct reading in temp.....

                    You need at least one differential voltmeter (most are), and a temperature compensated reference plus trimpot...... TL431 is a good cheap reference.... plus your "load resistor" that gives the readout voltage You use the pot to "buck out" the voltage that the 4mA end of the scale produces, and choose the resistor to give a suitable range. You can even use another trimpot to scale the read voltage.....
                    Last edited by J Tiers; 05-16-2010, 05:44 PM.

                    Keep eye on ball.
                    Hashim Khan


                    • #11
                      Heck, if you have 0-10 vdc output control then something like the volt meter I posted is definitely the best solution. You don't need to modify anything, just scale the output in software.
                      Free software for calculating bolt circles and similar: Click Here


                      • #12
                        Originally posted by Evan
                        Heck, if you have 0-10 vdc output control then something like the volt meter I posted is definitely the best solution. You don't need to modify anything, just scale the output in software.
                        Remember, my issue is for display of the setpoint temperature for the benefit of the occupant who will stand in front of the voltmeter which will be a component of my thermostat. I can make the PC send 0-10v and a simple volt meter would just display 0-10volt. However, with a process meter like the $50 one from Omega, I can send it 4-20ma and it can take power for itself from that signal and also be set properly to display a temp number scaled from that 4-20ma signal.
                        Last edited by bsmith; 05-16-2010, 06:40 PM.


                        • #13
                          Originally posted by J Tiers
                          I'm not clear how the data return gets to the PC in your setup....... you mentioned a USB device......... datalogger?
                          The PC will have several large USB I/O devices that will provide a large array of analog inputs and outputs along with numerous discrete digital I/O lines. The VB software will continuously loop and monitor all the sensors while also sending output signals as needed.

                          Each of my thermostats will consist of multiple components cobbled (hopefully) into a nice enclosure. The Temco HUM2 device will read and send room temp and room RH as analog 0-10v inputs and also take care of displaying those readings as they are sent to the PC. The process meter will be connected to an analog 4-20ma output and will receive signals from the PC as directed to show the set point temp. The software decides on the setpoint temp based on internal settings and by watching two digital I/O lines connected to momentary buttons. Press one button and the software increases the setpoint one degree. Press the other and the software decreases it. Press either button too many times and the software ignores it. So the momentary buttons are like discrete on/off sensors and the software loops through continuously watching for positive triggers on those buttons.

                          No special commands have to be sent from the PC to watch the buttons and all the other sensors. It's like an industrial control process where all sensors are constantly and rapidly looked at through the looping process. That's why I don't need the modbus option right now. Everything in my system is just connected by sensor wires through the USB devices and seen continuously by the PC software.

                          Internal to the software, I can decide how often I want to update things, but I am seeing all things (every sensor reading) way more rapidly than I really need to. Unlike motion control or machining processes hvac control is not speed sensitive. But this industrial approach is nice in that the programming is easy because you don't have to decide on when to issue special commands to get readings. The software is looping so rapidly you have current readings for everything all the time and you just watch them and take actions as needed and add your own delays and special timing if need be.

                          There is just one PC involved and the only visual output I have for the whole system is the user temp set points at each of the thermostat locations. Of course, besides displaying set point temps for the occupants, the control program is updating trend databases and displaying on a regular computer monitor the control and setting information that the programmer (myself) can view and adjust settings with.

                          I can show temp set points at each thermostat location easily with a process meter. As much money I am putting into this system already, 50 bucks each for 10 of those really isn't so bad.


                          • #14
                            Originally posted by bsmith

                            Remember, my issue is for display of the setpoint temperature for the benefit of the occupant who will stand in front of the voltmeter which will be a component of my thermostat.
                            That says it all right there. Gotcha.

                            Keep eye on ball.
                            Hashim Khan


                            • #15
                              Woo-Hoo! Won some Cheap USB I/O Off Ebay

                              Most USB I/O devices are small pods and it would take tons of them to add up to a significant number of I/O points. But the larger I/O devices like this one at Measurement Computing with 64 analog inputs cost $1,100 and more. A much lower cost per I/O point than the little pods, but still up there.

                              But this is where ebay comes in. Be patient and shop carefully and good deals never fail to come. Just won minutes ago the above 64 analog input device for just $480. See it here

                              Out of 93 ebay purchases I have never had a bad one. It will happen one day, but I won't cry about it. I feel like I am thousands ahead right now and even if a large bad purchase happens in the near future, I will still be way ahead.