Frequently Asked Questions

Positive displacement measurement is based on dividing a flowing stream into known, repeatable portions. In a perfect flow meter, the displacement chambers would form and collapse without any liquid seeping between the moving parts. In a Max meter we have nearly achieved this level of perfection. First, we build our meters with a 0.0002” gap between the moving parts so that liquid cannot escape from the measurement chamber. Next, we support all of the moving parts on ball bearings so they turn smoothly without requiring a large pressure drop to make the meter turn. Finally, we seal the measurement chambers with a no-friction liquid seal that does not create any drag in the meter’s rotation. The sum of all of these refinements is a very low pressure drop which boosts the volumetric efficiency of the meter to more closely approximate the perfect meter.

See our sample calibration sheet (pdf).

Low-flow is a relative term, but in all cases it means measuring at a rate well below the norm. To detect and report flow at 1/1000th of a meter’s full scale is a phenomenal feat. Max makes several meters with this turndown range and many more with 200:1 and higher ranges. What gives Max meters this ability? The meters are built so that they will continue to turn smoothly and repeatedly as low as 1 displacement per minute. Close tolerance machining specifications provide precision fits and simultaneously reduce drag. This, combined with very small displacements, creates a meter that can measure below 1 ml/min.

Our signal processing approach really sets us apart. Instead of counting a gear tooth or rotor blade when it passes by, we continuously monitor an internal shaft position to determine flow meter rotational speed and direction. Our sensors detect rotation to within one one-hundredth of a degree, and coupled with the microprocessor’s “learning mode” technology and unique signal damping process, provides a useable output signal resolution 20 to 50 times larger than the competition. The payoff is increased measurement data for intermittent flow streams, low-flow rate measurements, and applications where accuracy is critical.

For most situations, the meter can be mounted in any orientation. The special cases are:

  • High temperature where you should mount the meter so that the transmitter is to the side or below the meter. Transmitters with heat dissipating fins should be mounted sideways so that air can rise up between the fins. For other high temperature applications, mounting the transmitter underneath the meter will reduce the amount of heat which migrates into the transmitter housing.
  • If there is possibly entrained bubbles in the fluid or system it is best to place the meter on its side with fluid flowing up through the meter, or upside down so that bubbles do not collect in the measuring chambers.

Positive displacement meters are not affected by turbulence in the flow stream. It is permissible to install the meter near pumps, valves or plumbing changes. The displacement per revolution, of the meter will not change. No upstream or downstream straight runs are required.

There is not a specified calibration interval; however, many customers have adopted a 1 year interval as part of their Quality Management program. To avoid unnecessary down time, others have established sampling techniques, or the use of Master meters to test the meter in-place. These customers only remove the meter when their test data shows a drift beyond their manufacturing tolerances.

Yes, we calibrate all of the meters we manufacture to a NIST traceable standard.  We can also provide refurbishment services for worn or damaged meters.

For transmitters with a plug in the lid, please refer to the standard installation instructions
For transmitters with a 1/2" conduit hole connection, please refer to the Ex-Proof installation instructions.

No. There are no serviceable parts in the meter. Opening the meter may damage the close-fitting, precision parts. Please consult the factory before attempting any trouble shooting of a Max meter.

Signal dither is a potential source of measurement error. Dither occurs when a meter generates an output when there is no net reportable flow. This is usually observed at low flow or zero flow conditions. If the meter misinterprets vibration or pulsing liquid motion it will over report the actual flow rate. To prevent this error, the flow meter must detect flow direction and buffer or ignore the change in flow. Common techniques are to use a quadrature output which will produce an alternating A pulse then B pulse. If the meter starts to dither, this output will reverse the pulse phase or possibly create a sting of “A” pulses. Special electronic displays that accept two pulse inputs can ignore the erroneous signals. For simplicity, Max transmitters use a sophisticated sensing system that tracks the meter’s position and detects the dither before producing its frequency output.

Our recommended filtration for the Piston series is 5 or 10 micron, for the Gear series it is 15 and 30 micron and for the helical rotor series the recommendation is 150 micron (100 mesh). For high viscosity fluids, you may have to loosen this specification to maintain an acceptable pressure drop in your system. Please consider the tight tolerances within the meter and possible sources of contamination. If dirt is present in your process, the recommended filtration level should be maintained.

Max offers filter assemblies with replaceable mesh kits. See our filter and filter element section

Almost all of the Max meters have metal-on-metal contact in the flow stream. By running a non-lubricating fluid or compressed gas you run the risk of galling the mated surfaces. There is also high-carbon, stainless-steel ball bearings submerged in the flow and they are susceptible to rust if exposed to water and air. The Model 234 meter is the exception to this restriction as it has graphite-like pistons and no ball bearings in the fluid stream, so neither galling nor rust are an issue.

The turndown ratio is a measure of the meter’s working range. By dividing the upper most flow rate by the minimum, measureable flow rate you determine a value that is usually presented as 3:1, 10:1 or maybe 100:1 (pronounced one hundred to one). The larger the ratio the wider the operating range of the device. Meters that cannot hold their accuracy are often specified to work in a 3:1 range. Higher precision devices that hold their accuracy may be specified as 200:1 or 1000:1 devices. Such a large range makes it possible to use the same meter for both 10 L/min measurements and 10 ml/min.

No, the analog signal is generated directly by the transmitter’s software. The Max 290 Series of transmitters are based on a high resolution position sensor. Thousands of times per second, the position sensor tracks the rotation of the meter. Based on the displacement of the meter and this small amount of forward motion, the transmitter can calculate an instantaneous, volumetric flow rate.

We are often asked to provide an analog output signal for a wide operating range project. While we are happy to accommodate this request, there are certain realities that must be considered. When you select an analog signal, you are setting the span to the project’s full scale value and the voltage or current level produced at each flow rate will then have a fixed magnitude of resolution. No matter how long you hold a flow rate and observe the analog level, you cannot improve on the resolution or reduce the uncertainty of the information.

In contrast, if you are using a pulse generating system, you can extend the observation period to increase the certainty. For example, if you are collecting a 100Hz signal and wait for 1 second, you can calculate your flow rate to 100 pulses +/-1. This is based on the fact that you may have just missed the next pulse, or caught the lead edge of the next pulse. This +/-1 count of the right hand digit cannot be overlooked. Now, if you wait for 10 seconds and collect 1000 pulses, your +/-1 count is a 0.1% uncertainty. If you would like to further reduce the uncertainty, you could wait for 100 seconds and be able to calculate the rate with a 0.01% uncertainty.

Returning to the fixed uncertainty that exists in an analog signal. When you set 10.00 volts equal to 100% of the flow rate, the uncertainty in the right hand digit is equal to 0.1% at the full flow rate ( 1 part in 1000). When you drop to 10% of full flow, the output is 1.00 volts and your uncertainty is now 1%. At 1% of full flow, the uncertainty rises to 10%. The meter may be accurately parsing up the flow and spinning at exactly the right speed, but the output resolution cannot report the flow beyond the right hand digit.

Valve testing and real time data gathering may require that you pass rapidly through a range of flow rates. Stopping at each flow rate and collecting a statistically valid sample may not be practical. So there is a place for analog signals in flow testing, but their answers can only be as good as the resolution of the data. Please keep this in mind when setting pass/fail criteria.

Max offers a variation of their linearized transmitter which has a transistor output such that when triggered by the software, the signal line is connected to Common. Please refer to the installation sheet offered on the Technical Documents page to review the use of this circuit. Current sinking devices are used to provide pulses whose voltage is equal to the power supply voltage.

These two claims are only equal at the maximum capacity of the measuring device. If a meter’s accuracy is based on a percentage of its full reading capability, the error is a fixed value. For example; an error of 0.5% of full scale, in a 100 gal/min device is +/- 0.5 gpm. This is the uncertainty all of the time, so as you move away from the full scale capability, the 0.5 gpm error becomes a much larger percentage. At 50 gpm, you are risking a 1% error. At 10 gpm you have a potential 5% error. On the other hand, if the device has an error expressed as a percentage of the actual flow, then a 0.5% error of 10 gpm is only +/-0.05 gpm; a 10 times better result.

Linearization is a process of taking a repeatable event and offsetting the anticipated errors, usually in real time, to adjust the data. In the case of a flow meter reporting the liquid passing through it, several mechanical factors will result in a calibration curve that is not a flat line (i.e. you do not get the same number of pulses per volume for all flow rates).  The meter’s output curve will typically show more pulses per volume in the middle ranges and lower pulses per volume at the extremely low and high flow rates.  Fortunately this response curve is repeatable, so the non-linearity of the curve can be taken into account and smoothed out through compensating software.

All Max meters are calibrated over their entire operating range and the RPM and corresponding pulse output values are stored in the transmitter.  This initial calibration guides the transmitter’s software to apply the appropriate factor to convert the natural output to a corrected output.  Once this adjustment is set, the output will truly be the same pulses per cc value (K-factor) at all flow rates.  This compensation greatly simplifies the use of the Max meters as each meter of a type can be set to the same K-factor; making them interchangeable.

Typical K-Factors by Model:
Model P213 (1000 pulses/cc)                Model G004 (500 pulses/cc)                  Model H241 (15000 pulses/liter)
Model P214 (90 pulses/cc)                      Model G015 (200 pulses/cc)                  Model H242 (5000 pulses/liter)
Model P215 (20 pulses/cc)                      Model G045 (70 pulses/cc)
Model P001 (12,000 pulses/cc)             Model G105 (25000 pulses/liter)
Model P002 (1000 pulses/cc)                 Model G240 (7000 pulses/liter)

For the Piston Meters: 21X-311-000

  • First 3 digits: Meter size: 213, 214, 215
  • 4th digit: Pressure rating: 3 or 4 for 1000 psi, 6 for 3000 psi, 5 for 7250 psi
  • 5th digit: Plumbing connection: 1 or 6 for NPT, 8 for manifold base, 9 for SAE ports
  • 6th digit: Transmitter link: 1 for 270 and 290 Series, 0 for Model 284 (obsolete) 3 for Model 286 (obsolete)
  • Last 3 digits: O-Ring material: 000 for Viton, 72x for Teflon, 71X for Neoprene

(Other variations exist; please call Max Machinery if you have a question regarding a particular part number)

For the Helical rotor meters: 241-221-000

  • First 3 digits: Meter size: 241, 242
  • 4th digit: Materials of construction: 2 for Stainless steel, 3 for added electric heater ports
  • 5th digit: Pressure rating: 2 for 500 psi, 3 for 500 psi with liquid heat trace, 6 for 3500 psi, 7 for 3500 psi with heat tracing
  • 6th digit: Transmitter link: 1 for 270 and 290 Series transmitters, 4 for Model 289 transmitters
  • Last 3 digits: O-Ring material: 000 for Viton, 72x for Teflon

(Other variations exist; please call Max Machinery if you have a question regarding a particular part number)

For the Transmitters: 295-000-000

  • 295-050-000 is the sending portion of a high temperature transmitter (refer to its attached grey box to apply the numbers shown below)
  • The 4th digit: Output type: 0 for Pulse-one phase, 1 for Pulse-two phase, 6 for pulse-curent sinking, 2 for Analog-current, 3 for Analog-voltage
  • 5th digit: Housing: 0 for single piece, standard temperature rating, 8 for receiving part of a two piece, high temperture rating
  • 6th digit: Connection: 0 for five pin-Turck® style, M12 connector, 1 for 1/2" conduit hole
  • Last 3 digits: Power requirement: 000 for all pulse and 24VDC powered analog transmitters , 100 for 12VDC powered analog transmitters

(Other variations exist; please call Max Machinery if you have a question regarding a particular part number)

For the P, G and H Series meters please refer to the part number matrix


It is best to think of the sensor as a combination of a meter and transmitter. Servicing and calibrating them as a set is always the best idea. However, if it becomes necessary to replace a transmitter in the field, it is best to confirm that you have a proper match of meter to transmitter. There are several alternatives for the rotating piece within the meter and each family of transmitter will only respond to the proper meter type. In addition, there are phasing and scaling changes which should be discussed with a Max technician to maintain the optimal, meter performance.

Max Machinery has replaced RMA numbers with formal quotes for the return of Max products for service (calibration or repairs).

Max will now be issuing a Quote Number and formal quotation for each service request. The Max Quote Number must be clearly visible on the outside of the box containing your goods.

Each quote will include the cost and lead time for the requested service type.

Max will not process your service order without the following:

  • Receipt of valid Purchase Order (PO) or Credit Card.
  • Receipt of Material Safety Data Sheet (MSDS) for the process and/or flush fluid used in your meter.  “Process” and/or “Flush” must be clearly identified on each MSDS.

MSDS’ should be included in the box with your equipment. They can also be emailed to service [at] or uploaded on the service form. Please reference your Quote Number when emailing your MSDS.

The Material Safety Data Sheet is an important health and safety document which protects both your and our employees from dangerous chemical exposure.  Whenever a meter is sent to our factory, we need a copy of the MSDS for the fluid which you have run through the meter.  If you do not have these on file, you should get one from your chemical supplier or their web site.  

No, all assembly, testing or repairs are done at our Factory in Healdsburg, California USA

Rather than I.S., we sell a higher level protection and provide explosion proof housing, read more. 
Max Machinery has chosen to use a sophisticated, microprocessor based encoder on its flow meters. The current draw required for these devices prevents our 290 Series of transmitters from being certified as I.S.

The price of a Max positive displacement flow meter will vary based on the size of pipe or line being measured. The various output options and other flow specification considerations are unique to each order. This graph shows the price position related to comparable flow measurement technologies.