McMILLAN COMPANY

MODEL 100 FLO-SENSOR

FREQUENTLY ASKED QUESTIONS

1. Since the Model 100 Flo-Sensor uses the Pelton Turbine Wheel Principle, how in general, is its behavior when comparing its operation to either a ball and tube rotameter or mass flow sensor?
Turbine Wheel gas flow sensors behave very closely in operation to that of ball and tube rotameters in many ways, but, it , of course, provides a linear D.C. output voltage proportional to flow rate. Unlike thermal mass flow sensors, Model 100, due to its design, has no zero drift problems.

2. Linearity is quoted at + 3% full scale.  At what % of full scale does the maximum non-linearity occur and is this % position the same for all ranges of flow sensors?
+
+ 3% full scale linearity is a safe tolerance for production of flow sensors.  (see figures III thru V).  There will be no absolute pattern, but many units shipped are quite linear.  Others fall near maximum limits.  We offer a $200.00 calibration service for individuals who desire the actual data.  This price is subject to change without notice and excludes hydrogen gas.

3. Are the Flo-Sensors sensitive to mounting orientation?
Yes, the flow sensors, like conventional ball and tube flow meters, are sensitive to mounting orientation and have been tested at factory in horizontal position.   Units can be recalibrated vertically or on their sides but calibration data will differ.  Refer to INSTALLATION information on page 4 of X-100 instruction manual.   Models 100-3 and 100-4 only operate horizontally.

4. Do you recommend using an in-line filter before the sensor?
Yes, an in-line filter is quite beneficial and will insure longer life and minimize fouling us sensor.  We suggest 7 or 10 micron filtering.  Also, when operating with most gases, sensor should be kept slightly warmer than the gases (within limit of its maximum operating temperature) to prevent moisture from condensing so that water droplets get into sensor.  This can stop the operation of flow sensor until all liquid droplets have been baked out of sensor.

5. Quoted maximum operating pressures is 40psi 20 C.  Is this absolute or gauge pressure?  Can unit be operated above 40psi?  Is sensor sensitive to line pressure in reading flow rate?  What is the typical pressure drop across flo-sensor?
40psi is 40psig.  We do not suggest operation at pressures in excess of this value.  Although in proof pressure tests, failure of a plastic tube fitting at 380psig was first failure noted (test was at 25 C.).  It is perfectly okay to operate sensor at 30psig all the time but sensor is sensitive to line pressure.  See enclosed technical data, figure I.  Pressure drop across sensor varies with flow rate.  Model 100-3 pressure drop is 20 inches of water at 100ml/min of dry air flow.   Models 100-6 through 8 have 2 inches of water pressure drop at their rated flows.

6. Below the minimum specified flow rate (say less than 20 ml/min for the 100-3 Flo-Sensor) does the turbine wheel actually stop rotating or is it just that the output voltage is unreliable or non-linear?  Do you intend to supply a flow sensor capable of measuring gas flows below 20ml/min?
For 100-3, the turbine wheel typically stops rotating at about 8ml/min.   Between 8ml/min and 20ml/min, the flow sensor would actually send out at D.C. voltage signal but it would be somewhat lower in voltage than the straight line voltage vs. flow rate curve indicates.  Here again, since these units are mass produced, we cannot give you any specific curve since every unit would be a little different.  We do not currently anticipate producing a gas flow sensor for measurement of gases below 20ml/min.  See figures III through V.

7. Can the Flo-Sensors be damaged by flow rates much higher than their maximum rated flow rates?
Flow sensors can be continuously operated up to 6 VDC without damage.   This is about 20% higher than rated flow rate.  Units can be subjected to 3 to 4 times normal maximum rated flow without damage so long as the unit is only momentarily hit with such a high flow rate.  Life expectancy is increased 9 times longer when operating at 1/3 rated flow.

8. Can the Flo-Sensors be used for corrosive gases such as SO2, NO2, NO, or H2S?
Yes, units are operating on many gases including SO2, NO2 and H2S.

9. What is the expected life of the sensors?  What components are most likely to fail first?
Guaranteed life is one year.  We do not yet have sufficient field experience to tell us expected life.  Bearings and turbine wheel shaft are the only items we expect will fail.  See question 7.

10. You specify two types of tube fittings, Acetal and Kynar.   When would your use one fitting rather than the other? You specify two types of "O" rings, silicone and citon, when would you use one type of "O" ring rather than the other?
See enclosures pertaining to "O" rings and fittings, table I & II.

11. What tubing diameter do you suggest for connecting to the various Flo-Sensors?
All low flow range sensors are designed for operation up to and including 1 liter/minute using 1/8" tubing.  At flow rates in excess of 1 liter/minute, we go the fittings designed to accept 1/4" tubing. (above 20L/min. use 3/8" O.D. tubing.)  Above 50L/min. use 1/2" O.D. tubing.

12. What is the output impedance of the 0-5 VDC output voltage?
Output impedance of flow sensor is virtually 0 ohms, but if a load lower than 2500 ohms is applied across output, the internal circuitry begins to artificially add impedance up to 300 ohms to prevent a short circuit malfunction.  So we suggest that customer not draw more than 1 milliampere from our flow sensor to insure best operation.   This would mean sensor would drive into a load of at least 5000 ohms.

13. When operating Flo-Sensor on gases other than air, does the calibration change?
Yes, like a conventional ball and tube rotameter, model 100 Flo-Sensor needs to be calibrated on the gas type that will be flowing through it.  See curve enclosed showing calibration adjustments for gases other than air.  Customer should specify gas type when ordering.  Otherwise all Flo-Sensors are routinely calibrated using air. (Figure II)

14. Temperature sensitivity is how in relation to increasing temperature or decreasing temperature?
Increasing temperature causes a rise in output voltage at rate of 0.2%/C. or less without any change in actual gas flow rate.  Like wise decreasing temperature causes a decrease in output voltage at rate of 0.2%/C. or less without any change in actual gas flow rate.  A step change in temperature will not cause an instantaneous error to occur in output signal.  Time constant for error change to occur due to step change in temperature is approximately 15-30 minutes.

15. Can Flo-Sensors operate at voltages other than the specified 12.5VDC + 2VDC?
Yes, 100B models operate at 3-6 VDC.  Special units can be ordered to operate from 9 VDC to in excess of 16 VDC.  Contact factory for details.

16. Can you provide a pulse output?
Yes, we can furnish an unbuffered 0-500 pps output that corresponds to flow rate where 500 pps is full scale flow output.

17. Since turbine wheel shaft and/or sapphire bearings are subject to wear and will eventually fail, what is the policy for repair and replacement of these worn components?
Units shipped to us freight prepaid will be repaired at no charge if within our one year warranty.  If out of warranty, a $75.00 repair charge will apply as of January 1, 1991.  Repair charges for out of warranty repairs are subject to change without notice.  This charge of $75.00 includes retest of unit on air to insure its proper operation.  Shipment will normally be U.P.S. and freight charges will be added to invoice charges.  A simple change out of a saddle assembly which takes 5 minutes includes bearings, turbine wheel and shaft and is simple to install.  Current cost is $57.50.  Customer can install these saddle assemblies.

18. Would you show us typical flow rate versus voltage output curves for the various gas flow sensors?
Three curves are enclosed as representative.  Note, however, that each flow sensor shipped could deviate within specification limits from these representative flow rate versus voltage curves.  If it is desired to have the actual calibration data for a particular flow sensor, this can be furnished at added cost.  Contact factory for details and specify gas that will be used.  Actual pressure inside housing that unit will experience during operation

19. What is response time constant of flo-sensor?
Typically 8 - 10 seconds for 0-63% of final value.  Approx.  30 seconds for 98% of final value..

 

TABLE I

Gases Silicone Viton
Acetone vapors poor poor
Acetylene good excellent
Ammonia (cold) good poor
Ammonia (hot) excellent poor
Benzene excellent poor
Butane poor poor
Carbon dioxide excellent excellent
Carbon monoxide excellent excellent
Carbon tetrachloride poor excellent
Chlorine (dry) - excellent
Chlorine (wet) - excellent
Chloroform poor excellent
Denatured alcohol vapors excellent excellent
Diesel oil vapors poor excellent
Dry cleaning units - excellent
Ethane poor excellent
Ethyl alcohol vapors excellent excellent
Ethylene - excellent
Freon 11 poor excellent
Freon 12 poor good
Freon 13 poor excellent
Freon 21 poor poor
Freon 22 poor poor
Gasoline vapor poor excellent
Hexane poor excellent
Hydrogen fair excellent
Hydrogen sulphide (concentrated/wet) fair poor
Mustard gas excellent poor
Naptha poor excellent
Natural gas good excellent
Methane poor excellent
Methyl Ethyl Ketone vapors poor poor
Nitrogen excellent excellent
Nitric oxide poor excellent
Nitrogen dioxide poor excellent
Ozone excellent excellent
Producer gas good excellent
Propane poor excellent
Tolvene vapors poor excellent
Water vapor excellent excellent

 

TABLE II
SUITABILITY OF  TUBE FITTINGS
Acetal Kynar (Polyvinylidene Flouride)
Weak bases and salts excellent excellent
Aliphatic solvents excellent excellent
Esters and Keystones excellent poor
Chlorinated solvents good excellent
Strong bases excellent excellent
Aromatic solvents excellent excellent
Strong oxidents fair good  to excellent
Halogens poor excellent
Strong acids unacceptable excellent
Inert gases excellent excellent

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