One for All HFM-300 User Manual

Size:
792.26 Kb
Download

TELEDYNE

HASTINGS

INSTRUMENTS

INSTRUCTION MANUAL

HFM-300FLOW METER,

HFC-302FLOW CONTROLLER

Manual Print History

The print history shown below lists the printing dates of all revisions and addenda created for this manual. The revision level letter increases alphabetically as the manual undergoes subsequent updates. Addenda, which are released between revisions, contain important change information that the user should incorporate immediately into the manual. Addenda are numbered sequentially. When a new revision is created, all addenda associated with the previous revision of the manual are incorporated into the new revision of the manual. Each new revision includes a revised copy of this print history page.

Revision A (Document Number 151-0599) .................................................................

May 1999

Revision B (Document Number 151-1199) ..........................................................

November 1999

Revision C (Document Number 151-032000) ............................................................

March 2000

Revision D (Document Number 151-072000) ..............................................................

July 2000

Revision E (Document Number 151-092002) ......................................................

September 2002

Revision F (Document Number 151-082005) ...........................................................

August 2005

Revision G (Document Number 151-032007) ............................................................

March 2007

Revision H (Document Number 151-062008) .............................................................

June 2008

Visit www.teledyne-hi.comfor WEEE disposal guidance.

CAUTION

This instrument is available with multiple pin-outs.Ensure electrical connections are correct.

CAUTION

The Hastings 300 series flow instruments are designed for INDOOR operation only.

CAUTION

The Hastings 300 series flow meters are designed for class II installations.

Hastings Instruments reserves the right to change or modify the design of its equipment without any obligation to provide notification of change or intent to change.

 

 

 

 

300-302Series

Page 2 of 31

 

 

Table of Contents

 

1.

GENERAL INFORMATION............................................................................................................................................

4

 

1.1.

FEATURES ....................................................................................................................................................................

4

 

1.2.

SPECIFICATIONS...........................................................................................................................................................

5

 

1.3.

OPTIONAL 4-20 MA CURRENT OUTPUT............................................................................................................................

5

 

1.4.

OTHER ACCESSORIES ......................................................................................................................................................

6

 

1.4.1.

Hastings Power Supplies.....................................................................................................................................

6

 

1.4.2. 300/302 Series Power Supply Interface Cables..............................................................................................

6

2.

INSTALLATION AND OPERATION.............................................................................................................................

7

 

2.1.

RECEIVING INSPECTION ...............................................................................................................................................

7

 

2.2.

POWER REQUIREMENTS ...............................................................................................................................................

7

 

2.3.

OUTPUT SIGNAL...........................................................................................................................................................

7

 

2.4.

MECHANICAL CONNECTIONS .......................................................................................................................................

8

 

2.4.1.

Filtering..................................................................................................................................................................

8

 

2.4.2.

Mounting ................................................................................................................................................................

8

 

2.4.3.

Plumbing ................................................................................................................................................................

8

 

2.5.

ELECTRICAL CONNECTIONS.........................................................................................................................................

8

 

2.6.

OPERATION ..................................................................................................................................................................

9

 

2.6.1.

Operating Conditions.............................................................................................................................................

9

 

2.6.2.

Zero Check ...........................................................................................................................................................

10

 

2.6.3.

High Pressure Operation .....................................................................................................................................

10

 

2.6.4.

Blending of Gases.................................................................................................................................................

11

 

2.7.

OUTPUT FILTER .........................................................................................................................................................

12

 

2.8.

CONTROLLING OTHER PROCESS VARIABLES .............................................................................................................

12

 

2.9.

COMMAND INPUT.......................................................................................................................................................

13

 

2.10.

VALVE-OVERRIDE CONTROL .....................................................................................................................................

13

 

2.11.

GAIN POTENTIOMETER ..............................................................................................................................................

13

 

2.12.

TEMPERATURE COEFFICIENTS ...................................................................................................................................

14

3.

THEORY OF OPERATION ...........................................................................................................................................

15

 

3.1.

OVERALL FUNCTIONAL DESCRIPTION........................................................................................................................

15

 

3.2.

SENSOR DESCRIPTION................................................................................................................................................

15

 

3.3.

SENSOR THEORY........................................................................................................................................................

15

 

3.4.

BASE..........................................................................................................................................................................

17

 

3.5.

SHUNT DESCRIPTION ..................................................................................................................................................

17

 

3.6.

SHUNT THEORY .........................................................................................................................................................

17

 

3.7.

CONTROL VALVE.......................................................................................................................................................

21

 

3.8.

ELECTRONIC CIRCUITRY............................................................................................................................................

21

4.

MAINTENANCE..............................................................................................................................................................

22

 

4.1.

TROUBLESHOOTING ...................................................................................................................................................

22

 

4.2.

ADJUSTMENTS ...........................................................................................................................................................

23

 

4.2.1.

Calibration Procedure .........................................................................................................................................

23

 

4.3.

END CAP REMOVAL ...................................................................................................................................................

23

 

4.4.

PRINTED CIRCUIT BOARD REPLACEMENT..................................................................................................................

24

 

4.5.

SENSOR REPLACEMENT .............................................................................................................................................

24

5.

GAS CONVERSION FACTORS....................................................................................................................................

25

6. VOLUMETRIC VS MASS FLOW.................................................................................................................................

27

7.

DRAWINGS AND REFERENCES ................................................................................................................................

28

8.

WARRANTY ....................................................................................................................................................................

31

 

8.1.

WARRANTY REPAIR POLICY ......................................................................................................................................

31

 

8.2.

NON-WARRANTY REPAIR POLICY .............................................................................................................................

31

300-302Series

Page 3 of 31

1. General Information

The Teledyne Hastings HFM-300is used to measure mass flow rates in gases. In addition to flow rate measurement, theHFC-302includes a proportional valve to accurately control gas flow. The Hastings mass flow meter(HFM-300)and controller(HFC-302),hereafter referred to as the Hastings 300 series, are intrinsically linear and are designed to accurately measure and control mass flow over the range of0-5sccm to0-10slm with an accuracy of better than ±0.75% F.S. at 3σ from the mean (versions >10 slm are ±1.0% F.S.) . Hastings mass flow instruments do not require any periodic maintenance under normal operating conditions with clean gases. No damage will occur from the use of moderate overpressures (~500 psi/3.45MPa) or overflows. Instruments are normally calibrated with the appropriate standard calibration gas (nitrogen) then a correction factor is used to adjust the output for the intended gas. Calibrations for other gases, such as oxygen, helium and argon, are available upon special order.

1.1.Features

LINEAR BY DESIGN. The Hastings 300 series is intrinsically linear (no linearization circuitry is employed). Should recalibration (a calibration standard is required) in the field be desired, the customer needs to simply set the zero and span points. There will be no appreciable linearity change of the instrument when the flowing gas is changed.

NO FOLDOVER. The output signal is linear for very large over flows and is monotonically increasing thereafter. The output signal will not come back on scale when flows an order of magnitude over the full scale flow rate are measured. This means no false acceptable readings during leak testing.

MODULAR SENSOR. The Hastings 300 series incorporates a removable/replaceable sensor module. Field repairs to units can be achieved with a minimum of production line downtime.

LARGE DIAMETER SENSOR TUBE. The Hastings 300 sensor is less likely to be clogged due to its large internal diameter (0.026”/ 0.66mm). Clogging is the most common cause of failure in the industry.

LOW P. The Hastings 300 sensor requires a pressure of approximately 0.25 inches of water (62 Pa) at a flow rate of 10 sccm. The low pressure drop across this instrument is ideal for leak detection applications since the pneumatic settling times are proportional to the differential pressure.

FAST SETTLING TIME. Changes in flow rate are detected in less than 250 milliseconds when using the standard factory PC board settings.

LOW TEMPERATURE DRIFT. The temperature coefficient of span for the Hastings 300 series is less than 0.03% of full scale/°C from15-50°C.The temperature coefficient of zero is less than 0.1 % of reading/°C from0-60°C.

FIELD RANGEABLE. The Hastings 300 series is available in ranges from0-5sccm to0-25slpm. Each flow meter has a shunt which can be quickly and easily exchanged in the field to select different ranges. Calibration, however, is required.

METAL SEALS. The Hastings 300 series is constructed of Stainless Steel. All internal seals are made with Ni 200 gaskets, eliminating the permeation, degradation and outgassing problems of elastomer O- rings.

LOW SURFACE AREA. The shunt is designed to have minimal wetted surface area and noun-sweptvolumes. This will minimize particle generation, trapping and retention.CURRENT LOOP. The4-20mA option gives the user the advantages of a current loop output to minimize environmental noise pickup.

300-302Series

Page 4 of 31

1.2. Specifications

Accuracy ....................................................................................

 

< ±0.75% full scale (F.S.) at 3σ

 

 

(±1.0% F.S. for >10 slm versions)

Repeatability .............................................................................

 

±0.05% of reading + 0.02% F.S.

Maximum Pressure........................................................................................

 

500 psi [3.45 MPa]

 

 

(With high pressure option) 1000 psi [6.9 MPa]

Pressure Coefficient ..................................................

 

<0.01% of reading/psi [0.0015%/kPa] (N2)

 

 

See pressure section for higher pressure errors.

Operating Temperature ....................................................

 

0-60°Cinnon-condensingenvironment

Temperature Coefficient (zero)

.................................... Maximum ±0.1%F.S./°C (from 0 to 60°C)

Temperature Coefficient (span) ..................................

Maximum ±300 ppm/°C (from 15 to 50 °C)

 

 

Maximum ±450 ppm/°C (from 0 to 60 °C)

Leak Integrity ...............................................................................................

 

<1x10-9std. cc/s.

Flow Ranges ..................................................................

 

0-5sccm to0-25*slpm. (N2 Equivalent)

Standard Output ...........................................................................

 

0-5VDC. (load min 2k Ohms)

Optional Output ............................................................................

 

4 -20mA. (load < 600 Ohms)

Power Requirements .....................................................................

 

±(15) VDC @ 55 mA (meters)

 

 

± (15) VDC @ 150 mA (controller)

 

 

Class 2 power 150VA max

Wetted Materials ................................................................................

 

stainless steel, nickel 200

Attitude Sensitivity of zero ..............................................

 

< ±0.7% F.S. for 90° without re-zeroing

 

 

{N2 at 19.7 psia (135 KPa)}

Weight ............................................................................................................

 

1.93 lb [0.88 kg]

Electrical Connector...............................................................................

 

15 pin subminiature “D”

Fitting Options...................

¼” Swagelok®, 1/8” Swagelok®, VCR®, VCO®, 9/16”-18Female thread

Face Seal to Face Seal Length ................................................................

1.88”(47.75 mm) VCR®

(Specifications may vary for instruments with ranges greater than 10 slpm)

1.3. Optional 4-20mA Current Output

An option to the standard 0-5VDC output is the4-20mA current output that is proportional to flow. The 4 - 20 mA signal is produced from the 0 - 5 VDC output of the flow meter. The current loop output is useful for remote applications where pickup noise could substantially affect the stability of the voltage output.

The current loop signal replaces the voltage output on pin 6 of the “D” connector. The current loop may be returned to either the signal common or the -15VDC connection on the power supply. If the current loop is returned to the signal common, the load must be between 0 and 600 ohm. If it is returned to the-15VDC,the load must be between 600 and 1200 ohm. Failure to meet these conditions will cause failure of the loop transmitter.

300-302Series

Page 5 of 31

The 4-20mA I/O option can accept a current input. The0-5VDC command signal on pin14 can be replaced by a4-20mAcommand signal. The loop presets an impedance of 75 ohms and is returned to the power supply through the valve common.

1.4. Other Accessories

1.4.1. Hastings Power Supplies

Hastings power supplies are available in one or four channel versions. They convert 115 or 230 VAC to the ±15 VDC required to operate the flow meter. Interface terminals for the ±15 VDC input and the 0-5VDC linear output signal are located on the rear of the panel. Throughout this manual, when reference is made to a power supply, it is assumed the customer is using a Hastings supply. Hastings PowerPod100 andPowerPod-400power supplies are CE marked, but the Model 40 does not meet CE standards at this time. The Model 40 andPowerPod-100are not compatible with4–20mA analog signals. With the PowerPod 400, individual channels’ input signals, as well as their commands,is become4–20mA compatible when selected. ThePowerPod-400also provides a totalizer feature.

1.4.2. 300/302 Series Power Supply Interface Cables

The Hastings 300 series normally comes with the standard “H” pin-outconnector, which uses theAF-8-AM cable with grey backshells. “U”pin-outversions of the 300 series instruments require a different cable to connect to the Hastings Instruments power supply. This cable is identifiable by black backshell and is available as Hastings Instrument P/N65-791.

300-302Series

Page 6 of 31

2. Installation and Operation

This section contains the steps necessary to assist in getting a new flow meter/controller into operation as quickly and easily as possible. Please read the following thoroughly before attempting to install the instrument.

2.1. Receiving Inspection

Carefully unpack the Hastings unit and any accessories that have also been ordered. Inspect for any obvious signs of damage to the shipment. Immediately advise the carrier who delivered the shipment if any damage is suspected. Check each component shipped with the packing list. Insure that all parts are present (i.e., flow meter, power supply, cables, etc.). Optional equipment or accessories will be listed separately on the packing list. There may also be one or more OPT-optionson the packing list. These normally refer to special ranges or special gas calibrations. They may also refer to special helium leak tests, or high pressure tests. In most cases, these are not separate parts, rather, they are special options or modifications built into the flow meter.

Quick Start

1.Insure flow circuit mechanical connections are leak free

2.Insure electrical connections are correct (see label).

3.Allow 30 min. to 1 hour for warm-up.

4.Note the flow signal decays toward zero.

5.Run ~20% flow through instrument for 5 minutes.

6.Insure zero flow; wait 2 minutes, then zero the instrument.

7.Instrument is ready for operation

2.2.Power Requirements

The HFM-300meter requires +15 VDC @ 55 mA,-15VDC @50 mA for proper operation. TheHFC-302controller requires ±15 VDC @ 150mA. The supply voltage should be sufficiently regulated to no more than 50 mV ripple. The supply voltage can vary from 14.0 to 16.0 VDC. Surge suppressors are recommended to prevent power spikes reaching the instrument. The Hastings power supply described in Section 1.4.2 satisfies these power requirements.

2.3. Output Signal

The standard output of the flow meter is a 0-5VDC signal proportional to the flow rate. In the Hastings power supply the output is routed to the display, and is also available at the terminals on the rear panel. If a Hastings supply is not used, the output is available on pin 6 of the “D” connector. It is recommended that the load resistance be no less that 2kΩ. If the optional4-20mA output is used, the load impedance must be selected in accordance with Section 1.3.

300-302Series

Page 7 of 31

2.4. Mechanical Connections

2.4.1. Filtering

The smallest of the internal passageways in the Hastings 300 is the diameter of the sensor tube, which is 0.026”(0.66 mm), and the annular clearance for the 500 sccm shunt which is 0.006"(0.15 mm) (all other flow ranges have larger passages), so the instrument requires adequate filtering of the gas supply to prevent blockage or clogging of the tube.

2.4.2. Mounting

There are two mounting holes (#8-32thread) in the bottom of the transducer that can be used to secure it to a mounting bracket, if desired.

The flow meter may be mounted in any position as long as the direction of gas flow through the instrument follows the arrow marked on the bottom of the flow meter case label. The preferred orientation is with the inlet and outlet fittings in a horizontal plane.

As explained in the section on operating at high pressures, pressure can have a significant affect on readings and accuracy. When considering mounting a flow meter in anything other than a horizontal attitude, consideration must be given to the fact that the heater coil can now set up a circulating flow through the sensor tube, thereby throwing the zero off. This condition worsens with denser gases or with higher pressures. Whenever possible, install the instrument horizontally.

Always re-zerothe instrument with zero flow, at its normal operating temperature and purged with its intended gas at its normal operating pressure.

2.4.3. Plumbing

The standard inlet and outlet fittings for the Hastings 300 Series are VCR-4,VCO-4or 1/4" Swagelok. It is suggested that all connections be checked for leaks after installation. This can be done by pressurizing the instrument (do not exceed 500 psig unless the flow meter is specifically rated for higher pressures) and applying a diluted soap solution to the flow connections.

2.5. Electrical Connections

If a power supply from Hastings Instruments is used, installation consists of connecting the HFM300/302 series cable from the “D” connector on the rear of the power supply to the “D” connector on the top of the flow meter /controller. The “H” pin-outrequires cableAF-8-AM(grey molded backshell). The “U”pin-outrequires cable #65-791(black molded backshell).

If a different power supply is used, follow the instructions below when connecting the flow meter and refer to either table 2.1 or 2.2 for the applicable pin-out.The power supply used must be bipolar and capable of providing ±15 VDC at 55 mA for flow meter applications and ±15 VDC at 150 mA for controllers. These voltages must be referenced to a common ground. One of the “common” pins must be connected to the common terminal of the power supply. Case ground should be connected to the AC ground locally. The cable shield (if available) should be connected to AC ground at the either the power supply end, or the instrument end of the cable, not at both. Pin 6 is the output signal from the flow meter. The standard output will be 0 to 5 VDC, where 5 VDC is 100% of the rated or full scale flow.

The command (set point) input should be a 0-5VDC signal (or4-20mAif configured as such), and must be free of spikes or other electrical noise, as these would generate false flow commands that the controller would attempt to follow. The command signal should be referenced to signal common.

A valve override command is available to the flow controller. Connect the center pin of a single pole, three-positionswitch (center off) to the override pin. Connect +15 VDC to one end of the three position switch, and-15VDC to the other end. The valve will be forced full open when +15 VDC is supplied to the override pin, and full closed when-15VDC is applied. When there is no connection to the pin (thethree-positionswitch is centered) the valve will be in auto control, and will obey the0-5VDC commands supplied to command(set-point)input.

300-302Series

Page 8 of 31

Fig. 2.1

Fig. 2.2

Figures 2.1/2.2, and Tables2.1/2.2, show the 300/302 pin out.

Table 2.1

"U" Pin-Out

Pin #

1Signal Common

2Do not use

3Do not use

4+15 VDC

5

6Output 0-5VDC(4-20mA)

7Signal Common

8Case Ground

9Valve Override

10

11-15VDC

12External Input

13Signal Common

14Signal Common

15Set Point 0-5VDC(4-20mA)

Table 2.2

"H" Pin-Out

Pin #

1Do not use

2Do not use

3Do not use

4Do not use

5Signal Common

6Output 0-5VDC(4-20mA)

7Case Ground

8Valve Override

9-15VDC

10Do not use

11+15VDC

12Signal Common

13External Input

14Set Point 0-5VDC(4-20mA)

15Do not use

2.6. Operation

The standard instrument output is a 0 - 5 VDC out and the signal is proportional to the flow i.e., 0 volts = zero flow and 5 volts = 100% of rated flow. The 4 - 20 mA option is also proportional to flow, 4 mA = zero flow and 20 mA = 100% of rated flow.

2.6.1. Operating Conditions

For proper operation, the combination of ambient temperature and gas temperature must be such that the flow meter temperature remains between 0 and 60°C. (Most accurate measurement of flow will be obtained if the flow meter is zeroed at operating temperature as temperature shifts result in some zero offset.) The Hastings 300 series instrument is intended for use in non-condensingenvironments only. Condensate or any other liquids which enter the flow meter may destroy its electronic components.

300-302Series

Page 9 of 31

2.6.2. Zero Check

Fig. 2.3

Turn the power supply on if not already energized. Allow for a 1 hour warm-up.Stop all flow through the instrument and wait 2 minutes.Caution: Do not assume that all metering valves completely shut off the flow. Even a slight leakage will cause an indication on the meter and an apparent zero shift. For the standard0-5VDC output, adjust the zero potentiometer located on the inlet side of the flow meter until the meter indicates zero (Fig 2.3). For the optional4-20mA output, adjust the zero potentiometer so that the meter indicates slightly more than 4 mA, i.e. 4.03 to 4.05 mA. This slight positive adjustment ensures that the4-20mA current loop transmitter is not in thecut-offregion. The error induced by this adjustment is approximately 0.3% of full scale. This zero should be checked periodically during normal operation. Zero adjustment is required if there is a change in ambient temperature, or vertical orientation of the flow meter /controller.

2.6.3. High Pressure Operation

When operating at high pressure, the increased density of gas will cause natural convection to flow through the sensor tube if the instrument is not mounted in a level position. This natural convection flow will be proportional to the system pressure. This will be seen as a shift in the zero flow output that is directly proportional to the system pressure.

Fig. 2.4

Span Error vs Pressure

(0.026" Sensor Tube)

 

2.0%

 

 

 

 

 

 

 

 

 

 

 

0.0%

 

 

 

 

 

 

 

 

 

 

 

-2.0%

 

 

 

 

 

 

 

 

 

 

 

-4.0%

 

 

 

 

 

 

 

 

 

 

reading)

-6.0%

 

 

 

 

 

 

 

 

 

 

-8.0%

 

 

 

 

 

 

 

 

 

 

(%

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Span Error

-10.0%

 

 

 

 

 

 

 

 

 

 

-12.0%

 

 

 

 

 

 

 

 

 

 

 

-14.0%

 

 

 

 

 

 

 

 

 

 

 

-16.0%

Mean error

 

 

 

 

 

 

 

 

 

 

max

 

 

 

 

 

 

 

 

 

 

 

min

 

 

 

 

 

 

 

 

 

 

-18.0%

 

 

 

 

 

 

 

 

 

 

 

-20.0%

 

 

 

 

 

 

 

 

 

 

 

0

100

200

300

400

500

600

700

800

900

1000

 

 

 

 

 

 

Pressure(psig)

 

 

 

 

 

300-302Series

 

 

 

 

 

 

 

 

Page 10 of 31

Fig. 2.5

Span Error Vs. Pressure

 

0.017" Sensor

 

5%

 

 

 

 

 

 

 

 

 

 

 

4%

 

 

 

 

 

 

 

 

 

 

 

3%

 

 

 

 

 

 

 

 

 

 

Error (% reading)

2%

 

 

 

 

 

 

 

 

 

 

1%

 

 

 

 

 

 

 

 

 

 

Span

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0%

 

 

 

 

 

 

 

 

 

 

 

 

Mean

 

 

 

 

 

 

 

 

 

 

-1%

Max

 

 

 

 

 

 

 

 

 

 

Min

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

-2%

 

 

 

 

 

 

 

 

 

 

 

0

100

200

300

400

500

600

700

800

900

1000

Pressure (psig)

If the system pressure is higher than 250 psig (1.7 MPa) the pressure induced error in the span reading becomes significant. The charts above show the mean error enveloped by the minimum/maximum expected span errors induced by high pressures. This error will approach 16% at 1000 psig. For accurate high pressure measurements this error must be corrected.

The formulae for predicting mean error expressed as a fraction of the reading are:

Error

= (9.887 *1011 )P3

(3.4154 *107 )P2

+(8.3288 *105 )P,

(0.026" Sensor)

26

 

 

 

 

Error

= (1.533*10-10 )P3

(3.304 *107 )P2

+(1.8313*104 )P,

(0.017" Sensor)

17

 

 

 

 

Where P is the pressure in psig and Error is the fraction of the reading in error.

The flow reading can be corrected as follows:

Corrected = Indication(Indication* Error)

Where the Indication is the indicated flow andError is the result of the previous formula (or read from charts above).

2.6.4. Blending of Gases

This section describes two methods by which to achieve a controlled blending of different gasses. Both methods use the flow signal (Output) from one flow instrument as the Master to control the Command signal (Input) to a second unit.

The first method requires that the two controllers use the same signal range (0 to 5 VDC or 4 to 20 mA) and that they be sized and calibrated to provide the correct ratio of gasses. Then, by routing the actual flow Output signal from the primary meter/controller through the secondary controller’s

300-302Series

Page 11 of 31

External Input pin (See Tables 2.1 & 2.2), the ratio of flows can be maintained over the entire range of gas flows.

EXAMPLE: Flow controller A has0-100slpm range with a 5.00 volt output at full scale. Flow controller B has0-10slpm range with a 5.00 volt output at full scale. If flow controller A is set at 80 slpm, its output voltage would be 4.00 volts (80 slpm/100 slpm x 5.00 volts = 4.00 volts). If the output signal from flow controller A is connected to the command Set Point of flow controller B, then flow controller B becomes a slave to the flow signal of controller A. The resultant flow of controller B will be the same proportion as the ratio of the flow ranges of the two flow controllers.

If the set point of flow controller A is set at 50% of full scale, and the reference voltage from flow controller A is 2.50, then the command signal going to flow controller B would be 2.50 volts . The flow of gas through flow controller B is then controlled at 5 slpm (2.50 volts/5.00 volts x 10 slpm = 5 slpm).

The ratio of the two gases is 10:1 (50 slpm/5slpm). The % mixture of gas A is 90.9090 (50slpm/55 slpm and the % mixture of gas B is 0.09091% (5 slpm/55 slpm).

Should the flow of flow controller A drop to 78 slpm, flow controller B would drop to 3.9 slpm, hence maintaining the same ratio of the mixture. (78 slpm/100slpm x 5v = 3.90v x 50% = 1.95v; 1.95v/5.00v x 10 slpm = 3.9 slpm; 78 slpm: 3.9 slpm = 20:1)

In the blending of two gases, it is possible to maintain a fixed ratio of one gas to another. In this case, the output of one flow controller is used as the reference voltage for the set point potentiometer of a second flow controller. The set point potentiometer then provides a control signal that is proportional to the output signal of the first flow controller, and hence controls the flow rate of the second gas as a percentage of the flow rate of the first gas.

2.7. Output Filter

The output signal may have noise superimposed on the mean voltage levels. This noise may be due to high turbulence in the flow stream that the fast sensor is measuring or it could be electrical noise when the flow meter has a high internal gain. i.e. 5 sccm full scale meter. Varying levels of radio frequency noise or varying airflow over the electronics cover can also induce noise.

Noise can be most pronounced when measuring the flow output with a sampling analog/digital (A/D) converter. When possible, program the system to take multiple samples and average the readings to determine the flow rate.

Fig. 2.6

JP-1

If less overall system noise is desired, a jumper may be installed over the pins of JP-1on the flow measurement card. See Figure 2.6. Covering the pins closest to the “D” connector will activate aresistor-capacitor(RC) filter that has a time constant of one second. This will increase the

settling time of the indicated flow rate to approximately 4 seconds. Covering the other two pins will lower the response time to approx. 1 second. This adjustment will not affect the calibration of the flow meter circuit or the actual flow response to change in command signal (flow controllers). This will only slow down the indicated response (output voltage/current).

2.8. Controlling Other Process Variables

Normally, a flow controller is setup to control the mass flow. The control loop will open and close the valve as necessary to make the output from the flow measurement match the input on the command line. Occasionally, gas is being added or removed from a system to control some other process variable. This could be the system pressure, oxygen concentration, vacuum level or any other parameter which is important to the process. If this process variable has a sensor that can supply an analog output signal proportional to its value then the flow controller may be able to control this variable directly. This analog output signal could be 0-5volts,0-10volts (or4-20ma for units with 4- 20 ma boards) or any value in between.

300-302Series

Page 12 of 31

On the controller card there is a jumper that sets whether the control loop controls mass flow or an external process variable. See Figure 2.7. If the jumper is over the top two pins, the loop controls mass flow. If the jumper is over the bottom two pins, the loop controls an external process variable. This process variable signal must be supplied on pin 12 of the D connector (for U pin out units) of the measurement card. When the controller is set for external variable control it will open or close the valve as necessary to make the external process variable signal match the command signal. The command signal may be 0-5volts,0-10volts(4-20ma for4-20ma input/output cards) or any value in between. If the process variable has a response time that is much faster or slower than the flow meter signal it may be necessary to adjust the gain potentiometer.

2.9. Command Input

The flow controller will operate normally with any command input signal between 0-5volts(4-20ma for units with4-20ma input/output cards) If the command signal exceeds ±14 volts it may damage the circuit cards. During normal operation the control loop will open or close the valve to bring the output of the flow meter signal to within ± 0.001 volts of the command signal. The command signal will not match the flow signal if there is insufficient gas pressure to generate the desired flow. If the command signal exceeds 5 volts the controller will continue to increase the flow until the output matches the command signal. However, the flow output does not have any guaranteed accuracy values under these conditions.

If the command signal is less than 2% of full scale (0.1 volts or 4.32 ma) the valve override control circuit will activate in the closed position. This will force the valve completely closed regardless of the flow signal.

2.10.Valve-OverrideControl

The valve override control line provides a method to override the loop controller and open or close the valve regardless of the flow or command signals. During normal operation this line must be allowed to float freely. This will allow the loop control to open and close the valve as it requires. If the valve override line is forced high (> +5 volts) the valve will be forced full open. If the valve-overrideline is forced negative (<-5volts) the valve will be forced closed.

2.11.Gain Potentiometer

On the top left of inlet side of the flow controller there is a hole through which the gain potentiometer is accessible (Fig 2.3). This gain potentiometer affects the gain of the closed loop controller. Normally this potentiometer will be set at the factory for good stable control. It may be necessary to adjust this potentiometer in the field if the system varies widely from the conditions under which the controller was setup. Turning this gain potentiometer clockwise will improve stability. Turning the potentiometer counter-clockwisewill speed up the valve reaction time to changes in the command signal.

Fig. 2.7

Gain Potentiometer

Control Loop Jumper

300-302Series

Page 13 of 31

2.12.Temperature Coefficients

As the ambient temperature of

Fig. 2.8

the instrument changes from

 

the original calibration

 

temperature, errors will be

 

introduced into the output of

 

the instrument. The

 

Temperature Coefficient of

 

Zero describes the change in

 

the output that is seen at zero

 

flow. This error is added to the

 

overall output signal regardless

 

of flow, but can be eliminated

 

by merely adjusting the zero

 

potentiometer of the flow

 

meter/controller to read zero

 

volts at zero flow conditions.

 

The Temperature Coefficient of

 

Span describes the change in

 

output after the zero error is

 

eliminated. This error cannot

 

be eliminated, but can be

 

compensated for

 

mathematically if necessary.

 

The curve pictured in Figure 2.8

 

shows the span error in percent

 

of point as a function of

 

temperature assuming 230C is

 

the calibration temperature.

 

300-302Series

Page 14 of 31

3. Theory of Operation

This section contains an overall functional description of the Hastings 300 series of flow instruments. In this section and other sections throughout this manual, it is assumed that the customer is using a Hastings power supply.

3.1. Overall Functional Description

The Hastings 300 meter consists of a sensor, base, and a shunt. In addition to the components in a meter, The 300 controller includes a control valve and extra electronic circuitry. The sensor is configured to measure gas flow rate from 0 to 5 sccm, 0 to 10 sccm, or 0 to 20 sccm, depending on the customer’s desired overall flow rate. The shunt divides the overall gas flow such that the flow through the sensor is a precise percentage of the flow through the shunt. The flow through both the sensor and shunt is laminar. The control valve adjusts the flow so that the sensor’s flow measurement matches the set-pointinput. The circuit board amplifies the sensor output from the two RTD’s (Resistive Temperature Detectors) and provides an analog output of either0-5VDC or4-20mA.

3.2. Sensor Description

A cross section of the sensor is shown in Figure 3.1. The sensor consists of two coils of resistance wire with a high temperature coefficient of resistance (3500 ppm/oC) wound around a stainless steel tube with internal diameter of 0.6604 mm and 7.62 cm length. Each coil is 1.372 cm in length, and they are separated by 1.27 mm distance. These two identical resistance wire coils are used to heat the gas stream and are symmetrically located upstream and downstream on the sensor tube. Insulation surrounds the sensor tube and heater coils with no voids around the tube to prevent any convection losses. The ends of this sensor tube pass through an aluminum block and into the stainless steel sensor base. This aluminum block thermally shorts the ends of the sensor tube and maintains them at ambient temperature.

There are two coils of resistance wire that are wound around the aluminum block. The coils are identical to each other, and are symmetrically spaced on the aluminum ambient block. These coils are wound from the same spool of wire that is used for the sensor heater coils so they have the same resistivity and the same temperature coefficient of resistance as the sensor heater coils. The number of turns is controlled to have a resistance that is 10 times larger than the resistance of the heater coils. Thermal grease fills any voids between the ambient temperature block and the sensor tube to ensure that the ends of the sensor tube are thermally tied to the temperature of this aluminum block.

Aluminum has a very high thermal conductivity which ensures that both ends of the sensor tube and the two coils wound around the ambient block will all be at the same temperature. This block is in good thermal contact with the stainless steel base to ensure that the ambient block is at the same temperature as the main instrument block and, therefore, the same temperature as the incoming gas stream. This allows the coils wound on the aluminum block to sense the ambient gas temperature.

Two identical Wheatstone bridges are employed, as shown in Figure 3.2. Each bridge utilizes an ambient temperature sensing coil and a heater coil. The heater coil and a constant value series resistor comprise the first leg of the bridges. The second leg of each bridge contains the ambient sensing coil and two constant value series resistors. These Wheatstone bridges keep each heater

temperature at a fixed value of dT = 48°C above the ambient sensor temperature through the application of closed loop control and the proper selection of the constant value bridge resistors.

3.3. Sensor Theory

Consider the sensor design shown in Figure 3.1. The heat convected to or from a fluid is proportional to the mass flow of that fluid.

Since the constant differential temperature sensor has 2 heater coils symmetrically spaced on the sensor tube, it is convenient to consider the upstream and downstream heat transfer modes separately. The electrical power supplied to either of the heater coils will be converted to heat, which

300-302Series

Page 15 of 31

can be dissipated by radiation, conduction, or convection. The radiation term is negligible due to the low temperatures used by the sensor, and because the sensor construction preferentially favors the conductive and convective heat transfer modes. The thermal energy of each heater will then be dissipated by conduction down the stainless steel sensor tube, conduction to the insulating foam, plus the convection due to the mass flow of the sensed gas.

Because great care is taken to wind the resistive heater coils symmetrically about the midpoint of the tube, it is assumed that the heat conducted along the sensor tube from the upstream heater will be equal to the heat conducted through the tube from the downstream heater. Similarly, the heat conducted from the upstream and downstream coils to the foam insulation surrounding them is assumed to be equal, based on the symmetry of the sensor construction.

Since the sensor tube inlet and outlet are linked by an aluminum ambient bar, the high thermal conductivity of the bar provides a ‘thermal short’, constraining the ends of the sensor tube to be at equal surface temperature. Moreover, the tube ends and the aluminum ambient bar have intimate thermal communication with the main flow passageway prescribed by the main stainless steel flow meter body. This further constrains each end of the sensor tube to be equal to the ambient gas temperature.

Further, since the length of each heater section is nearly 21 times greater than the inside tube diameter, the mean gas temperature at the tubes axial midpoint is approximately equal to the tube surface temperature at that point. Recall that the outside of the sensor tube is well insulated from the surroundings; therefore the tube surface temperature at the axial midpoint is very close to the operating temperature of the heater coils. The mean temperature of the gas stream is then approximately the same as the heater temperature. Assuming the mean gas temperature is equal to the heater temperature, it can be shown that the differential pressure is:

(3.1)

Pμ Pd= 2 mCp(TheaterTambient)

The value of the constant pressure specific heat of a

gas is virtually constant over small changes in Fig. 3.1 temperature. By maintaining both heaters at the

same, constant temperature difference above the ambient gas stream temperature, the difference in heater power is a function only of the mass flow rate. Fluctuations in ambient gas temperature which cause errors in conventional mass flow sensors are avoided; the resistance of the ambient sensing coil changes proportionally with the ambient temperature fluctuations, causing the closed loop control to vary the bridge voltage such that the heater resistance changes proportionally to the ambient temperature fluctuation.

The power supplied to each of the 2 heater coils is easily obtained by measuring the voltage across the heater, shown as V2 on Figure 3.2, and the voltage

across the fixed resistor R1. Since R1 is in series with the heater RH they have the same current flowing

through them. The electrical power supplied to a given heater is then calculated:

(3.2)

Ρ =

(V1V2)V2

 

 

R1

With a constant differential temperature applied to each heater coil and no mass flow through the sensor the difference in heater power will be zero. As the mass flow rate through the sensor tube is increased, heat is transferred from the upstream heater to the gas stream. This heat loss from the

300-302Series

Page 16 of 31

heater to the gas stream will force the upstream bridge control loop to apply more power to the upstream heater so that the 48oC constant differential temperature is maintained.

The gas stream will increase in temperature due to the heat it gains from the upstream heater. This elevated gas stream temperature causes the heat transfer at the downstream heater to gain heat from the gas stream. The heat gained from the gas stream forces the downstream bridge control loop to apply less power to the downstream heater coil in order to maintain a constant differential temperature of 48oC.

The power difference at the RTD’s is a function of the mass flow rate and the specific heat of the gas. Since the heat capacity of many gases is relatively constant over wide ranges of temperature and pressure, the flow meter may be calibrated directly in mass units for those gases. Changes in gas composition require application of a multiplication factor to the nitrogen calibration to account for the difference in heat capacity.

The sensor measures up to 20 sccm full scale flow

rate at less than 0.75% F.S. error. The pressure Fig. 3.2 drop required for a flow of 20 sccm through the

sensor is approximately 0.5 inches of H2O (125 Pa).

3.4. Base

The stainless steel base has a 1.5" by 1.0” (38.1 mm by 25.4 mm) cross-sectionand is 3.64"(92.5 mm) long. The length from face seal fitting to face seal fitting is 4.88” (124.0 mm). The base has an internal flow channel that is 0.75"(19.1 mm) diameter. Metal to metal seals are used between the base and endcaps, as well as the base and sensor module. Gaskets made of nickel 200 are swaged between mating face seals machined into the stainless steel parts. All metal seals are tested at the factory and have leak rates of less than1x10-9 std. cc/s. Because of this corrosion resistant, all metal sealed design, the Hastings 300 can measure corrosive gases, which would damage elastomer sealed flow meters.

3.5. Shunt description

The flow rate of interest determines the size of the shunt required. As previously indicated, 9 separate shunts are required for the range of flow spanning 5 sccm to 10 slpm full scale. These shunts employ a patented method of flow division, which results in a more linear flow meter. As a result, the Hastings 300 flow meter calibration is more stable when changing between measured gases.

For the 5 sccm, 10 sccm, and 20 sccm flow rates a solid stainless steel shunt is used. The shunt uses a close tolerance fit to block the main flow passage thereby directing all flow through the sensor tube. The 50 sccm flow range uses a stainless steel shunt which has been machined flat on an edge. The gap between the main flow passage and the flat machined on the shunt creates an alternate laminar flow passage such that the overall gas flow is split precisely between the sensor and the shunt. By increasing the number of flats and the size of the laminar shunt passageway, flow rates up to 200 sccm are accommodated.

For flow rates above 200 sccm, the shunts are made so that an annular flow passage is formed between the shunt cylinder and the main flow passage. A stainless steel plug with an annular spacing of 0.006"(0.15 mm) accommodates the 500 sccm flow range. Increased flow rates require larger gap dimensions. Eventually, a maximum annular gap dimension for laminar flow is obtained (~0.020"(0.5 mm)). This patented shunt technology also includes inboard sensor ports which ensure laminar flow without the turbulence associated with end effects. This unique flow geometry provides an exceedingly linear shunt.

3.6. Shunt Theory

A flow divider for a thermal mass flow transducer usually consists of an inlet plenum, a flow restriction, shunt and an outlet plenum. (See Figure 3.3) Since stability of the flow multiplier is

300-302Series

Page 17 of 31

desired to ensure a stable instrument, there must be some matching between the linear volumetric

flow versus pressure drop of the sensor and the shape of the volumetric flow versus pressure drop of

the shunt. Most instruments employ

 

Poiseuille’s law and use some sort of

 

multi-passagedevice that creates

 

laminar flow between the upstream

 

sensor inlet and the downstream outlet.

 

This makes the volumetric flow versus

 

pressure drop curve primarily linear, but

 

there are other effects which introduce

 

higher order terms.

 

Most flow transducers are designed such

 

that the outlet plenum has a smaller

 

diameter than the inlet plenum. This

 

eases the insertion and containment of

 

the shunt between the sensor inlet point

 

and the sensor outlet point. If the shunt

Fig. 3.3

is removed, the energy of the gas must be conserved when passing from the inlet

 

plenum to the outlet plenum. From Bernoulli’s equation, the sum of the kinetic

 

energy and the pressure at each point must be a constant. Since all of the pressure

 

drops are small, it can be assumed that the flow is incompressible.

 

The pressure drop over the shunt can be shown to be:

 

 

 

 

 

 

 

 

4

 

(3.3)

 

 

= 1

 

 

Di

 

 

ΔΡ

a

ρV2

 

 

 

1

 

 

 

 

2

i

 

 

 

 

 

 

 

 

 

 

Do

 

 

 

 

 

 

 

 

 

 

 

We can see that even with no effect from the shunt there will be a pressure drop between the sensor inlet and outlet points. This pressure drop will be a strong function of the ratio of the two diameters. Since the drop is a square function of the flow velocity the differential pressure will be non-linearwith respect to flow rate. Note also that the pressure drop is a function of density. The density will vary as a function of system pressure and it will also vary when the gas composition changes. This will cause the magnitude of the pressure drop due to the area change to be a function of system pressure and gas composition.

Most of the shunts used contain or can be approximated by many short capillary tubes in parallel. From Rimberg1 we know that the equation for the pressure drop across a capillary tube contains terms that are proportional to the square of the volumetric flow rate. These terms come from the pressure drops associated with the sudden compression at the entrance and the sudden expansion at the exit of the capillary tube. The end effect terms are a function of density which will cause the quadratic term to vary with system pressure and gas composition. The absence of viscosity in the second term will cause a change in the relative magnitudes of the two terms whenever the viscosity of the flowing gas changes.

(3.4) ΔΡa =128πDμLQ4 +π82ρDQ4 2 (KC +Ke )

The end effect for a typical laminar flow element, in air, account for approximately 4% of the total pressure drop. For hydrogen, however, which has a density that is about 14 times less than air and has a viscosity that is much greater than air, the second term is completely negligible. For the heavier gasses, such as sulfur hexafluoride which has a density 5 times that of air, the end effects will become 10% of the total. This fundamental property makes it a difficult to maintain accuracy specifications when calibrating an instrument using one gas for use with another gas.

The pressure drop is linear with respect to the volumetric flow rate between a point that is downstream of the entrance area and another point further downstream but upstream of the exit region. From Kays & Crawford, that entrance length (L ) of a capillary tube in laminar flow is a function of the Reynolds number and the tube diameter. It can be shown that:

300-302Series

Page 18 of 31

(3.5)

Le=

Qρ

5πμ

 

 

For a typical flow divider tube the entry length is approximately 0.16 cm. From this it can be seen that if the sensor inlet pickup point is inside of the flow divider tube but downstream of the entrance length and if the sensor outlet point is inside the flow divider tube but upstream of the exit point then the pressure drop that drives the flow through the sensor would be linear with respect to volumetric flow rate. Since the pressure drop across the sensor now increases linearly with the main flow rate and the sensor has a linearly increasing flow with respect to pressure drop, there is now a flow through the sensor which is directly proportional to the main flow through the flow divider, without the flow division errors that are present when the sensor samples the flow completely upstream and downstream of the flow divider.

Unfortunately, a typical shunt has an internal diameter on the order of 0.3 mm. This is too small to insert tap points into the tube. Also, the sample flow through the sensor is approximately 10 sccm

while the flow through a shunt is approximately 25 sccm. This means the sample flow would be

affecting the flow it was trying to

 

measure. If the sensor tube is

 

made large enough, and with

 

enough flow through it to insert

 

the sensor taps at these

 

positions, then the pressure drop

 

would be too small to push the

 

necessary flow through the sensor

 

tube.

 

 

 

 

The solution is to use a different

 

geometry for the flow tube. It

 

must be large enough to allow

 

the sample points in the middle

 

yet with passages thin enough to

 

create the differential pressures

Fig. 3.4

required

for the

sensor.

An

annular

passage

meets

these

 

requirements.

The basic operation is similar to the operation of the tubular shunt but the equations for the entry length and pressure drop will be different.

If we assume that the annular region is very small, ( r << r):

(3.6)

C f =

24

Re

 

 

Then it can be shown that the pressure drop is:

− = 12QLμ

(3.7) Pi Po πτ( τ)3

The shunt must generate a pressure drop at the desired full scale flow which drives the proper flow through the sensor tube to generate a full scale output from the sensor. Since the full scale flow of the sensor is the same for all of the different full scale flows that may pass through the shunt, the geometry must vary for the different full scale flows in order to generate the same pressured drop for all of them. From Equation 3.7 it can be seen that if the width of the annular ring is varied slightly it can correct for very large changes in the full scale flow rate (Q).

Below is a graph showing how the thickness of the annular ring must be changed to create a passage that will properly divide the flow for various full scale flows. This graph is based on the 75 Pa pressure drop required to push full scale flow through a particular sensor that has 2 cm spacing between the inlet and outlet taps. The flow divider has an outside diameter of 0.95 cm.

300-302Series

Page 19 of 31

Fig. 3.5 Thickness of the annular ring as a function of flow rate for a sensor with a 75 Pa drop and a 2 cm spacing.

 

0.18

 

 

 

 

 

 

 

 

0.16

 

 

 

 

 

 

 

 

0.14

 

 

 

 

 

 

 

 

0.12

 

 

 

 

 

 

 

(cm)

 

 

 

 

 

 

 

 

Thickness

0.1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Passage

0.08

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.06

 

 

 

 

 

 

 

 

0.04

 

 

 

 

 

 

 

 

0.02

 

 

 

 

 

 

 

 

0

 

 

 

 

 

 

 

 

0

5

10

15

20

25

30

35

Flow (liter/min)

Each shunt must have a section of the annular region upstream of the upstream sensor tap to allow the flow to become fully developed before reaching the first tap. The entry length for the annular passage is then:

(3.8)

Le=

Qρ(r)

40πτμ

 

 

Below is a graph that demonstrates the entry length that would be required to design a flow divider for various full scale flows. The parameters on the sensor that the flow divider must match are the same as the ones on the previous graph.

Fig. 3.6

Entrance length as a function of flow rate

for an annular ring of the size specified in figure 3.5.

 

4.5

 

 

 

 

 

 

 

 

4

 

 

 

 

 

 

 

 

3.5

 

 

 

 

 

 

 

 

3

 

 

 

 

 

 

 

(cm)

2.5

 

 

 

 

 

 

 

Length

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Entrance

2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1.5

 

 

 

 

 

 

 

 

1

 

 

 

 

 

 

 

 

0.5

 

 

 

 

 

 

 

 

0

 

 

 

 

 

 

 

 

0

5

10

15

20

25

30

35

Flow (liter/min)

300-302Series

Page 20 of 31