Welcome to the October 2014 issue
of fLowdown - a quarterly Newsletter from
Titan Enterprises Ltd. written to keep you
informed about the latest technological developments, applications advances and
breaking news in the field of flow measurement.

If a particular feature interests you, do not hesitate to contact us or follow the link for further information. We welcome your feedback.

Trevor Forster (Managing Director)

 

Trevor's
Technical Tip

Trevor Forster is Managing Director of Titan Enterpises. His experience in fluid handling dates back to the mid 1960's when he started working on rotating seals and flowmeter design for third party clients. Drawing upon over 40 years of using innovative design and production techniques to produce elegant flow metering solutions for organisations around the globe, in this feature - Trevor offers you a useful technical tip.

Selecting the Optimum Output for Your Flow Meter

Deciding what output best connects to a particular type of flow meter is typically not a problem for an experienced engineer. However for the non-specialist or occasional flow meter user this decision may pose a problem as there are a wide array of flow meter output pulse types and each has its advantages and disadvantages. In essence these are all on/off switches in various different guises.

The input circuitries of flow indicators often have configuration routines to accommodate various device outputs. Correctly configuring these connections is crucial for efficient system operation.

Today flow meters are offered with output pulse types including Reed Switches, Transistors (NPN and PNP junctions), Logic devices, Namur sensors (current pulse) and Magnetic coils.

Here are some tips to help you make an informed decision when selecting the optimum output pulse type for your flow meter.

1. Reed switch


This is the simplest form of pulse output. It is literally a magnetically operated switch in a glass capsule. Typically Reed switches are inexpensive and easy to understand but limited to lower frequencies. They can be used in hazardous areas providing the site engineer is happy with simple apparatus as there is no danger of extraneous voltages or currents being produced. The device merely switches the voltage offered to it and if this is suitably protected the reed switch is perfect. The logging flow meter “sees” the voltage change when it closes. However there are a few limitations with using Reed Switches. They are mechanical switches and have a finite life as fatigue and breakage is likely after around 109 operations under ideal conditions. The contacts should be protected by a current limiting component to prevent a supply being directly switched causing them to weld together. For hazardous areas this extra protection can stop an intrinsically safe instrument reading the contact closures. There is also a potential problem with contact bounce which causes some very high frequency pulses which can be seen by many modern electronic instruments. There are two solutions to this. First is to use a slow input to the connected equipment. This may be a "slow speed" input on a PLC or a small capacitor and resistor may be used to prevent the bounce being seen by the instrument. A second method is to utilise the software to ignore fast pulses, there are several well established methods of doing this.

2. Transistors

Transistors are simple solid state switches which are usually configured in Open Collectors. This simply means that you have to add some external circuitry, such as a resistive load configured for NPN or PNP.

a. NPN. An Open Collector NPN output is configured like a switch to the 0V supply of the flow meter. It requires a 'Pull-Up' resistor to a positive voltage. The transistor is then said to 'sink' current from the external resistor back to 0V when it is activated. The advantage is that this voltage might not necessarily be the supply voltage of the sensor as shown below but can be any chosen (within specified limits) to suit the rest of the instrumentation network e.g. 24 volts for safer transmission over longer distances. The exact value of the resistor is not critical, but needs to be chosen to be large enough to limit the current (within the specified limit) at the chosen voltage, but small enough to provide enough current to activate the switch. Typical pull up resistor values are between 1K and 50kOhm.


b. PNP. An Open Collector PNP output is the mirror image of the NPN counterpart, providing a switch to the sensor supply voltage. It has to be connected externally via a 'Pull Down' resistor to 0V, this output switch is then said to 'source' current to the external resistor. Although more popular with many PLCs; in practice the PNP output is less flexible than that of the NPN as its voltage range is predetermined. Typical pull down resistor values are also between 1K and 50kOhm.


3. Logic output

A logic output is an output which switches (conditions described as logic 0 and logic 1) between predetermined voltage levels. The most common logic output devices are TTL and CMOS. TTL is defined as a logic 0 (output below 0.4V) and a logic 1 (above 2.4V). Usually a 0 is a few mV and a 1 is close to 5V. CMOS is defined relative to an internal supply voltage (usually 3.3V or 5V) as having a 0 at below 33% of the supply voltage and a 1 at above 66%. In practice CMOS output will be have a 0 output at virtually 0V and a 1 output close to the internal supply.




4. Namur Sensor.
(EN 60947-5-6)

A Namur sensor is a two wire sensor which is supplied with a constant voltage. With a Namur sensor, as the flow though the flowmeter varies the resistance of the sensor changes. Typically the current cycles from 2.1 to 1.2mA as the target passes the sensor. These devices are normally used with specialist flow meters in hazardous areas as the power consumption is very low and the change in resistance/current can be easily monitored remotely. Converters to standard outputs are available.

Continued in column 2

How do I measure ?

In this newsletter feature we bring you some of the more interesting and topical application challenges posed to Titan Enterprises and the solution we provided. Don't hesitate to send your flow metering application requirements to dee@flowmeters.co.uk to see how we can help you.

How do I measure pulsating flows ?

Last month we covered pulsating flows. This has made people ask what types of meter give the best results with changing flow rates. Pulsations in a flow line are usually caused by the pumping element. Piston, diaphragm, peristaltic and solenoid pumps can all induce quite serious pressure and flow pulses. Very few flow meters are able to measure these fluctuations accurately. To get best results it is therefore recommended to remove the pulsations near the pump with a pressure regulator and a pulsation damper. The pulsation damper is likely to be most effective at only one set of operating conditions so it is advisable to adjust this at your required operating flow. Should the pump speed change the system will be less effective. If it is not practical to install the damper, a length of flexible hose may reduce the problem.

How different types of flow meter compare when faced with pulsating flow conditions.

Flow Meter Type Effect of pulsating flow conditions
Mechanical flow indicators Variable area flow meters are likely to be unreadable as the float will bob up and down with the flow.
Low inertia turbines The lower mass of the turbine will allow the meter to accelerate and decelerate quickly. The Titan 800 series turbine has a step start-up time of less than 20 milliseconds and over a period of time will quite successfully give a reasonably accurate total.
Positive displacement meters Positive displacement flow meters such as the Titan oval gear meter range will give accurate readings as they take discrete pockets of fluid and transfer them through the meter. Care should be taken with some types of PD meters as hydraulic shock can damage the product.
Coriolis meters Coriolis meters are typically tolerant to pulsations as they measure the mass of the product passing through the device.
Vortex shedding Flow pulses in line will often render vortex shedding meters highly unreliable in pulsating flow conditions.
Ultrasonic Most ultrasonic flowmeters have a set cycle time and should this coincide (alias) with the flow pulsations errors will be recorded.
Electromagnetic Most electromagnetic flowmeters have a set cycle time and should this coincide (alias) with the flow pulsations errors will be recorded.

From column 1

5. Magnetic coil

If you move a magnet in front of a coil, you get a voltage. Implemented in a flow meter the magnet and the coil are stationary in the sensor and a magnetic turbine blade changes the magnetic coupling sufficient to induce a voltage swing. Though induced voltages are typically low levels (c. 10millivolts) they are cyclical and quite easy to detect. If the environment in which the sensor operates is electrically “noisy” the signal should be amplified or converted to a more robust level prior to transmission. As indicated earlier in the NPN section low voltage pulses do not travel well over long distances.


The table below provides a useful summary to guide your decision on the optimum output for your flowmeter.



New Products

Flexible Flow Meter Platform for OEM Product Development

The Hedgehog is a new electronic development platform for OEM applications based upon Titan’s Atrato ultrasonic flow meter. The instrument has the capability of running up to 4 flow sensors simultaneously with real time analysis. The software interface permits a wide operating envelope allowing OEM products to be developed for 0.5 to 20mm pipes including in-line and clamp-on devices. The structure is such that software changes can be simply “dragged and dropped” into a box in the interface permitting remote on-board logic updating and system re-programming. A full array of inlets and outlets are available including both analogue and high speed logic signals for fast response system requirements. Smaller single channel versions `Hoglets` have also been produced to enable testing of single pre-production meters where space may be at a premium.



Pre-Scaler Enables Faster Commissioning of OEM Equipment

All Titan 800, 900 and 1000 series flowmeters can be fitted with a pre-scaler which is programmed to suit customer requirements. Any K factor less than the meters calibrated value can be used. The pre-scaler was designed for an OEM customer who wanted all his production electronics to be pre-programmed and each flowmeter to be identical. The miniature circuit sits inside the standard flowmeter body and is programmed after flow calibration. Titan can re-program the meter at any time.



Humour

Some humorous asides for the Engineers amongst us:


Normal people believe that if it ain't broke, don't fix it. Engineers believe that if it ain't broke, it doesn't have enough features yet !


To the optimist, the glass is half-full. To the pessimist, the glass is half-empty. To the engineer, the glass is twice as big as it needs to be.


Three engineering students were gathered together discussing who must have designed the human body. One said, "It was a mechanical engineer. Just look at all the joints." Another said, "No, it was an electrical engineer. The nervous system has many thousands of electrical connections."The last one said, "No, actually it had to have been a civil engineer. Who else would run a toxic waste pipeline close to a recreational area?"

Flow Technology Spotlight

In each issue of fLowdown we will review a particular flow metering technique, its benefits, shortfalls and the applications to which it is best suited.

Comparison of oval gear and standard gear meters

These two types of gear meter are superficially the same but operate in very different ways.    A standard gear meter usually has a few very large gear teeth which are meshed in a chamber with close clearances on all surfaces.  The teeth themselves form a seal along their length so the only possible leakage path is around the outside of the meshed cogs to the chamber walls. The movement of the liquid causes a differential pressure across the meter and more importantly across the gears themselves.  The pressure imbalance on the gear faces produces a small force, the difference in the area caused over the gear height by the pink and green lines in the drawings below.  This force imbalance between the gears causes the gears to rotate displacing a volume of fluid approximately equal to one gear tooth volume, shown in red in the illustration below.  The direction of rotation is counter intuitive as the gears rotate towards the incoming stream.    Usually a sensor is used to count the passing of each tooth generating a high resolution pulse train. 

Oval gear meters rely on an entirely different theory. The teeth are still used to drive the gear but the differential force is developed by the shape of the ovals not the gear teeth on the lobe. The teeth form a fluid seal and drive the lobes. Oval Gear meters from different manufacturers include gears of varying oval shapes depending on the resolution and flow requirement of the target application. By using an oval shape a much greater driving pressure can be generated resulting in a wider flow range and lower pressure drop compared to a standard gear meter. The displaced volume is a product of the oval shape not the gear profile shown in blue below. The sensor is usually magnetic with a detector at the face of the gear. The resolution is lower than the standard gear meter.

In conclusion - for high resolution applications the gear meter is the flow meter of choice as they have a very small displaced volume and give a large number of pulses per revolution at high accuracy. However the downside of standard gear meters are their pressure drop and flow range. A lot of force is required to drive and this results in a reduced dynamic range.

By comparison the oval gear flow meter has a far greater dynamic flow range and a much lower pressure drop making these devices suitable for low pressure systems. For the vast majority of applications the lower number of output pulses per unit volume is not an issue. Further the choice of gear materials is greater as the forces they have to deal with are considerably lower, which can even permit all polymer oval gear flow meters to be made.


Bulletin Board

To enable you to make informed decisions about the flow metering challenges facing your organisation this regular newsletter feature keeps you up-to-date on the latest literature, web, video and social media initiatives from Titan Enterprises.

Product Range Catalogue

Titan's overview catalogue detailing its complete range of off-the-shelf flow meters is finally completed ! Please click here to download a pdf copy.


Top 10 Tips for Selecting
a Flow Meter

Drawing upon over 40 years of experience, Trevor Foster – an internationally recognised authority in flow meter technology and applications offers sound, non-commercial advice on what to consider when selecting, implementing and maintaining a flowmeter system. To download a copy please click here.

Click here for a printable version of Flowdown.

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Titan Enterprises Ltd, Unit 2, 5A Cold Harbour Business Park, Sherborne, Dorset, DT9 4JW
Telephone: +44 (0)1935 812790 - Fax: +44 (0)1935 812890 - Email: sales@flowmeters.co.uk