HFM-401/405 - Teledyne Hastings Instruments

3.4.2. Adjusting ZeroThe pre-conditions required for a zero check must also be followed when making a zero adjustment.The zero adjustment is a digitally controlled “reset” type operation. When commanded, the meterinitiates an internal routine that performs the following sequence: measure the current flow reading,store it in nonvolatile memory as a zero offset, and remove this value from all subsequent readings.NOTEIf the instrument is inadvertently or improperly zeroed, forexample while flow is passing through the instrument, the flowreading is subtracted from all future flow readings. This willproduce large flow indication errors.This offset value can be accessed via the “S40” software query. The reported value is relative to aninternal, un-spanned sensor voltage. As an interpretation guideline, an offset that exceeds 0.15 voltstypically indicates that a faulty zero value is present.There are three different methods to activate the zero reset function--manually, digitally, andelectrically.Manually – With the electronics enclosure cover plate removed, a pushbutton switch on the upperboard is pressed.CAUTIONAccessing the manual zero pushbutton requires removing theenclosure front plate. Care must be taken when reinstalling thisplate to ensure that the sealing gasket is properly positioned andthe fasteners are secure to maintain an IP65 compliant seal.Digitally – A “ZRO” (“*[address]ZRO” for RS485) command is received properly by the flow meter’smain processor.Electrically – An external contact closure generates continuity between pins 8 and 9 of the terminalstrip.3.4.2.3.5. Over-rangeThe thermal mass flow sensor heats a portion of the gas in order to measure the flow rate. As the flowincreases the heated tube is cooled and the slope of the sensor output versus the flow rate decreases.The sensor linearization function corrects for this effect while the flow rate is within the normaloperating region. If the flow exceeds the normal operating region the digital flow indication willcontinue to track this increase with a reduced accuracy. The analog flow will also indicate this overflowcondition until the circuitry reaches its limits (approximately 10 -25% over-range).As the flow continues to increase above the normal operating region the sensor will be cooledsufficiently that the output of the sensor will reach a peak value around 2 – 4 times the full scale flowrate. If the flow continues to increase the sensor output will begin decreasing and the digital flow willindicate a decreasing flow rate even though the flow is actually getting increasing. At approximately 3 –7 times the full scale flow rate the sensor output will drop within range of the normal output and eventhe analog output will record an on-scale flow rate when there is a very large over range flow rate.401-405 SERIES - 17 -