{"id":879,"date":"2015-07-14T13:04:28","date_gmt":"2015-07-14T18:04:28","guid":{"rendered":"http:\/\/www.kele.com\/keleblog\/?p=879"},"modified":"2026-02-21T08:39:23","modified_gmt":"2026-02-21T14:39:23","slug":"divide-conquer-those-hard-to-read-flow-meter-pulses","status":"publish","type":"post","link":"https:\/\/www.kele.com\/content\/blog\/2015\/07\/14\/divide-conquer-those-hard-to-read-flow-meter-pulses","title":{"rendered":"Divide & conquer those hard-to-read flow meter pulses"},"content":{"rendered":"

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Editor\u2019s Note – February 20, 2026:<\/strong>\u00a0This article has been reviewed for technical accuracy and remains applicable to current HVAC controllers and pulse output applications. Based on customer feedback, we\u2019ve also linked it directly to relevant\u00a0UPD Series Transducer<\/a>\u00a0product pages for easier reference during design.<\/p>\n

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Some HVAC applications require reading and totalizing pulses from flow<\/a> meters. This sounds simple enough, just take the pulse output from the flow meter, connect it to a Binary Input (BI) on your controller, and set up the program logic to count pulses coming in on the BI. What could go wrong?<\/p>\n

Unfortunately things are not always as simple as they appear. HVAC controllers frequently operate on the \u201cscan\u201d principle where the inputs are not read continuously, but only once per controller scan. Controller scan time could be fast or slow depending on the controller design.<\/p>\n

The controller only \u201csees\u201d a pulse if the input is low on one scan and high on the next scan. This means that:<\/p>\n

    \n
  1. A pulse whose duration is longer than the time between controller scans should always be reliably detected:
    \"\"<\/li>\n<\/ol>\n

     <\/p>\n

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    2. A pulse whose duration is less than the time between controller scans will sometimes be detected and will sometimes be missed:<\/p>\n

     <\/p>\n

    \"\"
    \"\"<\/p>\n

    NOTE:<\/strong>\u00a0 some controllers have one or two \u201chigh-speed counter inputs\u201d in addition to their regular Binary Inputs. High-speed counter inputs have a much higher scan rate and may be fast enough to detect your flow meter pulses reliably with no additional hardware needed. Always check the controller data sheet to see if your controller has any high-speed inputs you can use for connecting your flow meter.<\/p>\n

    \u00a0<\/h3>\n

    What can I do if my flow meter pulses are too narrow to be reliably detected by my controller?<\/strong><\/h3>\n

    Kele sells a Universal Pulse Divider (UPD-2<\/a>) that can divide the incoming meter pulses by a selectable divisor and output a wider pulse that your controller can reliably detect.<\/p>\n

    \u00a0<\/h3>\n

    What kind of pulse signals can the UPD-2 accept on its input?<\/strong><\/h3>\n

    The UPD can accept pulses from a simple contact closure, a transistor switch, or a driven 0-5VDC signal:<\/p>\n

    \"\"<\/p>\n

     <\/p>\n

    \"\"
    Transistor Switch<\/figcaption><\/figure>\n

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    0-5VDC Signal<\/figcaption><\/figure>\n

    In the Low state, the output of the flow meter must be able to sink approximately 2.3 mA of current. This will be compatible with most flow meters.<\/p>\n

     <\/p>\n

    What kind of output does the UPD provide?<\/strong><\/h3>\n

    The UPD output is an optically-isolated electronic switch.\u00a0 It is polarity sensitive. In the High state it is open-circuit, in the Low state it closes the circuit. In the Low state, the output can sink up to 6 mA of current. In the High state, it can tolerate up to 30VDC. The UPD output is compatible with most controller Binary Inputs which are typically configured as an internal reference voltage with a pull-up resistor as shown below:<\/p>\n

    \"\"<\/p>\n

     <\/p>\n

    What is the minimum high or low pulse time the UPD will detect on its input?<\/strong><\/h3>\n

    The UPD will reliably detect any pulse with a high or low time of at least 10 milliseconds. This limit is programmed into the UPD\u2019s microcomputer firmware, the hardware could actually detect shorter pulse durations. But HVAC equipment lives in electrically noisy environments, and we don\u2019t want to start counting noise glitches as flow meter pulses, so we set the minimum valid pulse duration at 10 milliseconds.<\/p>\n

     <\/p>\n

    How do I set the divisor value on the UPD?<\/strong><\/h3>\n

    The UPD has an 8-slider DIP switch for setting the divisor. Each slider has a divisor value assigned thus:<\/p>\n