Frequently Asked Questions

All GFM mass flow meters can be fed with universal 12 to 24Vdc power and have both 0-5Vdc and 4-20 mA analog outputs available simultaneously.

GFC17/37/47 models of mass flow controllers can be fed with universal 12 to 24Vdc power. GFC57/67/77 models of mass flow controllers can be ordered with fixed 12Vdc or 24Vdc power supply option (has to be specified during the order) and have both 0-5Vdc and 4-20 mA analog outputs available simultaneously. All models of the GFC controllers have jumper selectable 0-5Vdc or 4-20 mA analog inputs.

Turndown ratio define the range in which a flow controller can accurately control the flow rate. It is the ratio of the controllers high and low control range and is calculated using a simple formula:

Turndown Ratio = Maximum controllable flow / Minimum controllable flow

For example, if GFC mass flow controller has a 40:1 turndown ratio the flow controller is capable of accurately control down to 1/40th of the maximum flow or down to 2.5% of controller full scale range. In contrast the DPC mass flow controller has a 200:1 turndown ratio and capable to accurately control flow rates down to 1/200=0.5% of controller full scale range.

Measurements of volumetric flow are not as reliable as mass flow measurement due to the effects that changes in temperature or pressure have on the density of a fixed volume of gas. Unlike volumetric flow measurement instruments such as variable area meters or turbine meters, thermal mass flow instruments are relatively immune to fluctuations in temperature and pressure of the flowing fluid. The mass flow instrument is capable of providing direct measurement of mass flow, as opposed to most other method that measure volumetric flow and require separate measurements for temperature and pressure in order to calculate density and, ultimately, the mass flow.

As the name of the instrument states many of you would think that measurements of mass flow provided by mass flow instrument would be expressed in true units of mass, such as grams/minute, milligrams/second etc. In most industries however, it is convenient for users to think and work with units of volume. It is OK to use volumetric based units as far as we are talking about the same reference conditions.

In order to use density in converting mass flow to volumetric flow, we must use the specific pressure and temperature conditions at which we use the density value for the gas. These conditions may contain various references: normal reference and standard reference which usually are different in European or American notation. According to the AALBORG definition the prefix ‘s’ in sml/min or sl/min units of measure refers to “STANDARD” conditions which are: 101.325 kPa absolute (14.6959 psia) and temperature of 21.1°C (70°F). Prefix ‘n’ in nml/min or nl/min units of measure refers to “NORMAL” conditions which are: 101.325 kPa absolute (14.6959 psia) and temperature of 0°C (32°F).

Please be aware of the reference conditions when ordering a mass flow instrument. “Normal” and “Standard” can be different to each customer.

The GFM and GFC instrument have optional 3.5 digits LCD display. A 3.5 digit display has actually four segments: one half digit and 3 full digits. It means that the most significant (left) digit can be only either 0 or 1. This will allow to display numbers from 000 to 1999. In general the number of the digits after decimal point will depend on the instrument full scale range value expressed in particular engineering units. If we wanted to display 20.0 sml/min on a 3.5 digit display we would have to “throw out” the leading half digit as we cannot make use of it because it can only be a “1”. We are limited to using the three full digits, so the display full scale reading would be 200. The decimal point is manually placed via a jumper, so the final display would be 20.0 sml/min and if user need 200 sml/min full scale there are will be no decimal point available. In contrast if instrument full scale range is 100 sml/min the LCD display can utilize first digit and display will be able to provide 100.0 sml/min reading with one digit after decimal point.

DFM, DFC, XFM, ZFM series instruments are equipped with RS232 or RS485 digital communication interface. The interface type: RS232 or RS485 must be selected during the order and cannot be changed on the field by user.

DPM and DPC series instruments are equipped with universal RS232/RS485 digital interface controller which can be configured by user on the field to be ether RS232 or RS485. In addition these instruments support optional Modbus RTU interface based on galvanically isolated RS485 transceiver with high common-mode transient immunity.

GFM and GFC series instruments do not have native built in digital communication interface. However users may order supplemental TIO Flow Totalizer, I/O, Monitor & Control which are equipped with RS232 or RS485 digital communication interface.

The warm-up time interval for AALBORG thermal mass flow instruments is 5 minutes for accuracy of ± 2% of full scale range and at least 15 minutes for specification stated accuracy.

The important differences between the two interfaces are: length of the communication cable, number of slave devices being used, data transmission speed, level of electrical noise, and network topology. Generally RS232 is intended to be used as point to point communication. So if you have only one instrument and it is located not too far (< 50 m) from master computer or PLC it is the best choice. However if you have multiple instruments which have to be connected to single master PC/PLC or distance of the communication cable is more than 50 meters the RS485 is the logical choice. RS485 interface can work with up to 64 other devices on a single network segment.

Care must be taken setting up a RS485 network. Unlike RS232, the RS485 interface requires the use of a daisy chain topology and may cause issues if setup in a star. Termination resistors are also required for long networks (> 100 m) at the first device in the chain and on the last device in the chain to help with reflections.

Compare to AALBORG DPM/DPC laminar flow differential pressure mass flow instruments, thermal bypass mass flow instruments are slow (1 to 3 seconds typical response time) and require lengthy warm-up time (up to 15 minutes) to reach thermal equilibrium and stated accuracy. Thermal bypass mass flow instruments has to be calibrated with the actual gas used in the end use application. Because every gas has a different thermal capacity, if they are not calibrated with the same gas used in installation, they will need to have their flow reading values to be corrected by a correction factor or K-factor. Typically K- factors introduce a degree of uncertainty (up to 5 or sometimes 10%) to the measurement which leads to decreasing of instrument accuracy.

DPM/DPC series mass flow meters are based on the measurement of the differential pressure across specially designed restrictor flow element. The restrictor flow element is designed to establish laminar flow across instrument entire range of operation from 0 to 133% of full scale range. High accuracy and high resolution differential pressure sensor is utilized to measure pressure drop across laminar flow channel, which is linearly proportional to volumetric flow rate. Based on data from the sensors and Gas Properties from the instrument built-in data base, the microcontroller calculates the volumetric flow rate using Poiusuelle’s equation. To convert volumetric flow in to the mass flow, high accuracy and high resolution absolute pressure and temperature sensors are utilized.

Pros and Cons of the DPM/DPC

For more information see product overview pages :
DPC Mass Flow Controller
DPM Mass Flow Meter

If GFM instrument LCD display reading is more than ±0.5% of full scale after 15 minute warm up time and no flow condition (pipes are disconnected from instrument fittings), more likely it can be corrected by adjusting ZERO potentiometer R34 through the access hole on the side of the instrument (see page 14 of the GFM operating manual for details). Adjusting R34 potentiometer to clock wise direction will increase reading and adjusting R34 potentiometer to counter clock wise direction will decrease reading.

If your GFM instrument does not have LCD display zero adjustment has to be done via analog 0-5 Vdc interface. Since GFM is using unipolar power supply it output voltage cannot swing below 8 mV and it is possible not intentionally over adjust zero reading to negative direction. To prevent this use following adjustment techniques:

1. Adjust R34 potentiometer to clockwise direction until voltmeter reads 100 mV.
2. Start slowly adjust R34 potentiometer to counter clockwise direction until voltmeter reading stops decreasing. Once voltage reading stop decreasing immediately stop potentiometer adjustment. It is true instrument zero.

GFC17/37/47 mass flow controllers utilize normally closed proportional solenoid valve. This valve kept in close position by compression spring and adjustment screw. The adjustment screw, located under the hex nut, is adjusted on the factory to the point that stem viton insert is fully covering orifice, which makes valve to be closed. Note that proportional solenoid valve is not a shutoff valve. It is normal for proportional solenoid valve to have internal leak up to 0.5%FS. One common reason for proportional solenoid valve to be out of adjustment: keeping control set point even very small (2% for example) while disconnecting inlet pressure or installing plug on the instrument downstream pipe. In this case because differential pressure across the valve is zero, the valve becomes overheated within 15 minutes and mechanical characteristics of the seat insert and compression spring are compromised. There are two possible scenarios which required different approach to valve adjustment:

Indication #1: With “no flow conditions” (gas pipes are not connected to the GFC) and valve closed (pins 3 and 12 on the GFC 15-pins D-Connector are connected together) LCD reading is zero, but when 20 PSIG inlet pressure is applied the LCD reads more than 0.5% of full scale.

Likely reason: Valve is out of adjustment and leaking.
Remedy:

1. Adjust control set point to zero. Set Valve mode to “CLOSE” position (connect pins 3 and 12 on the 15 pins D-connector together). This step is very important!
2. Apply 20 PSIG inlet pressure.
3. See operating manual page 17 (Figure 6-1). Unscrew hex nut cover on the top of the solenoid valve.
4. Using a screwdriver readjust adjustment screw on the top of the valve to CW (clock wise) direction until zero reading on the display. Be very careful during adjustment: make only 15 degree turn each time and wait one minute due to the sensor’s response time. If reading is still high make another 15 degree turn. Do not over adjust valve. If you made more than 5 complete (360 degree) turns and leakage still exists stop adjustment. In this case unit has to be returned to the factory for servicing.
5. This is not a shut off valve. It is normal to observe up to 0.5 % of F.S. leakage.
6. Install hex nut cover on the top of the solenoid valve.
7. Disable Valve “Close” mode, apply 100% control set point and check if instrument can reach 100%FS reading.

Indication #2: Differential pressure across the GFC controller is stable and within specification but LCD reading and actual flow are not stable (oscillate 1 to 4 times per second).

Likely reason: Valve compression spring is over adjusted and instrument PID control cannot handle stable flow.
Remedy:

1. Make sure differential pressure across the GFC is within specification.
2. Install control set point to 100% F.S. This should recreate the oscillation conditions.
3. See operating manual page 17 (Figure 6-1). Unscrew hex nut cover on the top of the solenoid valve.
4. Using screwdriver readjust adjustment screw on the top of the valve to CCW (counter clock wise) direction until reading on the display will be stable. Be very careful during adjustment: make only 15 degree turn each time and wait about 15 seconds due to sensor’s response time. If reading oscillates make another 15 degree turn. Do not over adjust valve. If you noticed that flow rate is constant and more than 105% of full scale, it means you over adjusted valve and it has leakage. In this case make adjustment to CW (clock wise) in order to fix this problem until reading will go back to 100% full scale.
5. Adjust zero set point (or valve close command), wait about 3 minutes and check if valve is able to close.
6. This is not a shut off valve. It is normal to observe up to 0.5 % of F.S. leakage.
7. Install hex nut cover on the top of the solenoid valve.

Maximum pressure drop, or pressure loss, is the difference in pressure as measured between inlet and outlet fittings of the instrument while flowing 100% of full scale rate. The unit of measurement is typically pounds per square inch differential (PSID). Essentially this parameter determines minimum required differential pressure across the instrument to reach 100% full scale flow.

Maximum differential pressure (also in PSID) is the differential pressure limit across controllers inlet and outlet fittings at which controllers loosing ability to optimally control flow rate. Exceeding maximum differential pressure limit usually leads to flow rate oscillations.

All stated above means that controller operating pressure window is difference between “Maximum differential pressure limit” and “Maximum pressure drop” parameters. When selecting controller for particular application the user must verify that differential pressure for controller in the installation is always inside of the operating pressure window. For example for GFC37 mass flow controller calibrated up to 50 sl/min of Air the “maximum pressure drop” is 8 PSID. The “maximum differential pressure limit” for this controller is 50 PSID. That means that operating pressure window for this controller is 50 – 8 = 42PSID.

If a flow of more than 10% above the maximum flow rate of the Mass Flow instrument is taking place, a temporary condition known as “swamping” may occur. Readings of a “swamped” instrument cannot be assumed to be either accurate or linear. Flow must be restored to below 110% of maximum instrument full scale range. Once flow rates are lowered to within calibrated range, the swamping condition will end. Operation of the instrument above 110% of maximum calibrated flow will not damage the instrument but may significantly increase instrument recovery time.

Before trying to adjust GFC NR7 local set point potentiometer make sure your controller was ordered for “LOCAL” set point control option (check NJ1 jumpers location according to FIGURE 2-3 of the GFC operating manual). NOTE: if NJ1 jumpers are installed for “REMOTE” set point option the NR7 local set point potentiometer will not work.

For LOCAL flow control, use the built-in NR7 set point potentiometer located at the same side as the solenoid valve of the GFC transducer (see FIGURE 2-2 of the GFC operating manual). While applying inlet pressure to the transducer, adjust the NR7 set point potentiometer with an insulated screwdriver until the flow reading is the same as the desired control point. [Display will only show actual instantaneous flow rates. There is no separate display for set points].

NOTE: NR7 set point potentiometer has 24 turns span. Each turns equals 180 degree. It does not have mechanical limits for zero and 100%FS values. Potentiometer will produce mechanical clicking sound when user try to adjust potentiometer outside of the nominal span (below zero or above 100%FS set point value).

GFM and GFC series instruments are analog instruments and do not have native built in digital communication interface. However users may order supplemental “TIO Flow Totalizer, I/O, Monitor & Control” which is equipped with RS232 or RS485 digital communication interface.

Follow the link below for more details:
TIO Flow Totalizer, I/O, Monitor & Control

Yes, if you calculate and apply proper K-Factor. In this situation use following formula to calculate K-Factor:

K-Factor = Kn2[Actual used Gas] / Kn2[Calibrated Gas] = Kn2[Argon] / Kn2[CO2] = 1.4573 / 0.7382 = 1.9741

Where:

• Kn2[Actual used Gas] is K-Factor relative to N2 for gas which actually used in the installation (Argon in our case);
• Kn2[Calibrated Gas] is K-Factor relative to N2 for gas this instrument was originally calibrated for (CO2 in our case);

In our example in order to get flow rate of Argon gas we have to multiply instrument reading by calculated above K-Factor. NOTE: K-Factors at best are only an approximation. K factors should not be used in applications that require accuracy better than +/- 5 to 10%.

Each thermal mass flow instrument utilize special restrictor flow element (RFE) hardware which designed to provide proportional division of the flow between main body and capillary tube sensor based on instrument required full scale flow range and gas. In order to change instrument full scale flow range value this RFE hardware has to be replaced and instrument must be recalibrated. This procedure can be done as far as instrument capable to handle newly requested flow rate with the same lower block size. In our example above we have GFM17 calibrated for 0-100sml/min of AIR. That means we can reconfigure this instrument for any flow rate up to 10 sl/min of AIR because GFC17 model (see link below) can handle flow rates up to 10.0 sl/min of AIR.

GFM Thermal Mass Flow Meter catalog pages [PDF]

Please contact AALBORG for instrument reconfiguration procedure prices.

Generally all thermal mass flow instrument exhibit attitude sensitivity which affects instrument accuracy. However it depends on against which axis instrument is rotated in user installation and how it was oriented during initial factory calibration (can be found in the calibration data sheet). AALBORG thermal mass flow instruments can be rotated against horizontal axis (when instrument flow pass is parallel to the horizontal field) without any accuracy degradation. However if instrument was factory calibrated horizontally but installed that flow pass (imaginary line between inlet and outlet fittings) is perpendicular to the horizontal axis, the instrument accuracy will be affected because of the sensor zero shift. This zero shift is proportional to the internal instrument pressure. The more pressure the more zero shift in the instrument. This zero shift may be adjusted by readjusting instrument zero potentiometer, however as soon as internal pressure changes zero will change as well again. That is why AALBORG recommends install thermal mass flow instrument with flow pass parallel to the horizontal axis. This way the instrument will preserve calibrated accuracy regardless the pressure changes in the installation.

The following two products have LabVIEW support using Aalborg DPM/DPC LabVIEW Driver [ZIP] :
DPC Mass Flow Controller
DPM Mass Flow Meter

For more details please refer to the DPM/DPC LabVIEW Driver Library User Guide [PDF].


Other AALBORG digital instruments that do not have dedicated LabVIEW drivers provide ASCII commands set published in operating manual, which can be used with LabVIEW communication control API.

All AALBORG mass flow controllers except models GFC57/67/77 utilize normally closed proportional solenoid valve. That means when controller loses power the valve is left in closed state, meaning there are no flow.

For GFC57/67/77 models with motorized valve when power is lost the valve will be left in exact the same position where it was on the moment of power interruption, meaning it may be open, closed or somewhere in the intermediate position. However on the power up event motorized valve will be temporally closed for short period of time to find “home closed” position and then will execute flow rate according to the currently requested set point.

All AALBORG digital mass flow instruments shipped with free WINDOWS communication software which allows users simultaneously monitor and perform data logging Process Information (PI) parameters including: Mass Flow, Totalizers and Alarms status for up to 64 instruments on the RS-485 bus. User may set required data log interval and select which Process Information parameters has to be recorded. Please follow link below to download software corresponding to your instrument model number.

Software Downloads

Minimum flow rates are approximately 10% of the maximum.

Valves are positioned at the outlet for vacuum and liquid applications.

You can double the range of the meter.

High resolution valves give more precise adjustability, particularly with very low flow rates.

The main advantage of the 65mm meter is if you have limited space, and also a slightly lower cost. The advantage of the 150mm unit is that you get better readability with the taller scale.

1. First try bursts of clean dry air at the inlet and outlet. (If the meter has a valve it works best if you remove the valve). Simply unscrew it from the frame, and use the valve opening as the inlet.
2. While blowing air through the valve opening, cover and then uncover the outlet with your finger. The float should move up when the fitting is uncovered, and then move down from back pressure when you cover it.
   Note: You may need to bang the top and/or bottom ends of the meter on a counter top, (with the meter held vertically) to initially free the float.
3. If you have not freed the float, the next thing will be to remove the tube and clean it and the float, with alcohol. You will need to remove the upper float stop. A pipe cleaner can be used to clean the tube. Blow out the tube with clean air to dry the alcohol.
4. After cleaning the float, do not touch it with bare fingers during reassembling.

If you cannot free the float contact Aalborg for a return number, so we can service it.