APLISENS PEM-1000 ELECTROMAGNETIC FLOWMETER

1. INTRODUCTION

This manual covers typical scenarios of interfacing pulse output of the PEM-1000 flowmeter to various types of external input circuits and configuration of the output. Please note, that the output can also be used in frequency mode, with no hardware differences. For the simplicity of this document, the output will be called “pulse output”, regardless of the configuration.

This manual assumes, that the user has basic skills in operating the flowmeter, such as navigating the menu and entering numerical values using the local keyboard.

2. ELECTRICAL CONNECTIONS

The pulse output terminals (5, 6) are accessible in the rear chamber of the flowmeter's case as shown at the picture below:

Figure 1. The pulse output terminals: Diagram showing the pulse output terminals on the flowmeter, labeled 1 through 15. Terminals 5 and 6 are specifically indicated as the pulse output connection points.

Internal structure of the output will be explained, to avoid problems with interfacing to external circuits of the PLCs, test rig counters, pulse counters etc.

Pulse output is a galvanically isolated, passive optocoupler-like circuit, equipped with additional overcurrent protection and diode bridge, to allow any polarity connection. The simplified schematics of this output is as follows:

Figure 2. Simplified schematics of the pulse output: Simplified schematic of the pulse output circuit. It features a protector component, an optocoupler (LED and phototransistor), and a diode bridge, indicating galvanic isolation and polarity-insensitivity.

The protector circuit is low voltage drop passive current source with current limit of about 20 mA. Connecting (mistakenly) low impedance voltage source to the output is not destructible, thanks of this protection. Guaranteed output current of the output is 10 mA. The most important feature of the above circuit is voltage drop of about 1-1,5 V at the terminals 5-6. This voltage drop is higher than on-state voltage drop of standard open-collector optocoupler or relay output, which negatively affects interfacing to external circuits requiring low voltage drop output.

There are three basic cases of interfacing the pulse output to an external input circuits:

• discrete inputs with moderate detection threshold or current detection,
• passive discrete inputs with low detection threshold (~0,7 V) intended for NPN drive,
• low voltage, active discrete inputs intended for open collector or reed (relay) contacts only.

Inputs with moderate detection threshold are typically active current sources with current detection circuits or voltage type inputs, optionally with active polarization network powered internally from voltages not less than 5 V. Typical examples of those inputs are digital inputs circuits of PLC controllers, test rigs' input circuits, galvanically isolated interfaces and so on. Typical threshold voltages lie in the range of 5-20 V with significant hysteresis. Typical threshold current is about 1-5 mA. Inputs with polarization circuits can be directly interfaced to the pulse output. See drawing below:

Figure 3. Connecting of the external inputs with polarization circuit: Circuit diagram illustrating the connection of the PEM-1000 pulse output to external inputs with a polarization circuit. The output terminals (5/6) are connected to inputs that either use a pull-up resistor or a current detection circuit, powered by an external voltage source (+V).

Inputs without polarization circuit should be interfaced with the help of external power supply. See drawing below:

Figure 4. Connecting of the external inputs without polarization circuit: Circuit diagram showing the connection of the PEM-1000 pulse output to external inputs that do not have a polarization circuit. The output terminals (5/6) are connected to a PLC input, requiring an external 24V power supply unit.

Please note, that some of the proposed circuits may require power supply of about 24 V. It could be any accessible 24 V source, e.g. local 24 V mains, power supply of the low voltage version of the flowmeter etc., due to galvanic insulation on the flowmeter's side. Also, typical industrial-grade PSU can be used, advisably as small as possible, because current drawn by these circuits are very small.

Passive inputs with low detection threshold are typically intended to be driven by a NPN outputs. Their maximum operating voltage is about 30 V, but the threshold voltage is about 1 V. Those circuits cannot be directly interfaced to the pulse output of the flowmeter, but can be used in “PNP" mode. Simple interfacing circuit resolves this issue. See drawing below:

Figure 5. Connecting of the passive input with low detection threshold for NPN drive: Circuit diagram for connecting passive inputs with a low detection threshold (intended for NPN drive) to the PEM-1000 pulse output. The output terminals (5/6) are connected to a low-voltage input through a resistor (R), utilizing a 24V power supply.

Value of R resistor is not critical, it should be anywhere in the range 2,2-22 kΩ. The higher the resistance, the smaller current drawing from power supply, but also the lower noise immunity of the circuit.

The most troublesome are low-power, low-voltage active digital inputs intended only for potential-free contacts such as relay, reed or open collector ones. Typical examples of those inputs are low power devices such as LCD pulse counters, heat meters or other devices based on low voltage logic. In this case the following circuit should be used:

Figure 6. Connecting of the low-voltage active discrete inputs intended for potential-free contacts: Circuit diagram for interfacing low-voltage active discrete inputs designed for potential-free contacts (like relay or reed contacts) with the PEM-1000 pulse output. This setup uses a Solid State Relay (SSR) connected to a counter input, powered by a 24V PSU.

The circuit uses solid state relay (SSR). For this task only versions with low on-state voltage drop, low output leakage current and low minimum working current are suitable, because input circuits of low voltage devices typically source currents in range of 1-2 mA and detection threshold voltage is less than 1 V. ELCO SSR20-524C is well suited for this task. The parameters of this device are shown in table below:

Resistor R is optional in this circuit (used only for high sensitivity SSRs). Any equivalent SSR of similar parameters can be used. Please note, that this SSR has polarized output and non-polarized input. Observe polarities for equivalent parts. For complete part list of this circuit see Appendix A. Part list for output interface circuit for reed contact inputs. For SSR equivalents see Appendix B. SSR equivalents.

Some versions of low-power low-voltage digital inputs cannot be driven even by a SSR relay because of extremely low driving current (in the range of µAmperes) together with low detection threshold and high input impedance. Typical examples of those inputs are battery-powered low power devices, such as LCD counters and energy meters (e.g. Zenner multidata series). Fortunately, these inputs can often be controlled by an active low voltage source. Driving of these inputs requires slight modification of the PNP schematic:

Figure 7. Connecting of the discrete inputs with extermely low driving current (in the range of microampers): Circuit diagram for connecting discrete inputs with extremely low driving current to the PEM-1000 pulse output. This configuration uses resistors R1 and R2 to form a voltage divider, protecting the low-voltage input from an excessive voltage from a 24V source.

Resistors R1 and R2 form a voltage divider limiting maximum output voltage of the driver. Calculations of values of these resistor for some typical cases are as follows (E series values are used, recommended tolerance is 1%):

For other values of these parameters, follow the formula below:

R2 / (R1 + R2) ≤ max. input voltage / max. power supply voltage

3. CONFIGURATION OF THE PULSE OUTPUT

The pulse output of the flowmeter must be properly configured to achieve the best resolution and avoid errors stemming from too high pulse frequency. The most important parameters are pulse length and volume per pulse. These parameters are self-explanatory – for each complete volume unit of “volume per pulse” one pulse of chosen length is issued. Please note, that PEM-1000 flowmeter uses “volume per pulse" approach, but some flowmeters or pulse detection devices use “pulses per volume” approach, those parameters are reciprocals of each other. That is why using round numbers as a volume per pulse parameter (or checking its reciprocal) is recommended.

Theoretical maximum instantaneous pulse frequency for pulse output is 500 Hz, because the shortest pulse is of 1 ms and the shortest gap is also of 1 ms. Such high frequencies should be avoided because of risk of pulses loss in case of temporary flow rate peak. Also, SSR frequency limits and external circuit frequency limits apply. Theoretical limits for volume per pulse value and pulse length result from the formula below:

Qmax / (volume per pulse [m³] * 3600) ≤ 1000 / (pulse time [ms] + 1)

where QMAX – maximum flow rate [m³/h].

Setting proper values of pulse output parameters requires some margins. That is why optimal output frequency for 100% of range should be no greater than about 200 Hz for 1 ms pulses.

The pulse length should be set to satisfy detection capabilities of external circuits. Practical values fit in the range 1 ms (fast inputs) – 100 ms (very slow inputs). Value used should be as low as possible, because it limits maximum achievable frequency. The longer pulse length, the lower the maximum theoretical frequency, for 10 ms pulses it drops to ~90 Hz.

Typical configuration of pulse output is as follows:

• Menu → Configuration → Outputs → Pulse output → On/Off = ON
• Menu → Configuration → Outputs → Pulse output → Mode = PULSE
• Menu → Configuration → Outputs → Pulse output → Pulse → Pulse time = 1[ms] (see notes above)
• Menu → Configuration → Outputs → Pulse output → Pulse → Mode = POSITIVE
• Menu → Configuration → Outputs → Pulse output → Pulse → Polar = POSITIVE
• Menu → Configuration → Outputs → Pulse output → Pulse → Volume / imp. = see explanation above.

Please note, that filtering setting "for pulse output” (“Menu → Configuration → Filtering → for Pulse out") regards frequency mode of the output. There is no filtering available for pulse output in pulse mode, because of self-averaging and high-precision nature of this mode of operation.

4. HISTORY OF REVISIONS

Appendix A. Part list for output interface circuit for reed contact inputs

Appendix B. SSR equivalents

Please note, that the equivalent of the SSR used in the interface circuit should be chosen with great care, most of the SSR's do not meet the requirements of low-power, low-voltage input circuits. SSRs based on thyristor / triacs or bipolar transistors with reverse polarity protection may not be usable due to their elevated voltage drop or minimum load current values. The following parts were chosen, according to their catalog specifications, but were not tested at the field. All of them are SPST-NO version, which is also the most popular. Packaging of these parts may not be compatible, please use relevant sockets for TH35 rail mounting.

Crydom DRA-CN024D24, 500Hz max. or CN024D24 (PCB mount mini version)

*) for CN024D24 PCB mount mini-SIP version

Phoenix contact OV-24DC/60DC/4 - 2982647, 100Hz max.

IDEC RV8S-L-D24-D24 or RV8S-S-D24-D24

Appendix C. Low voltage active driver calculations

Some low-voltage digital inputs can be driven by active voltage source, providing that maximum input voltage is not exceeded. Schematic of the connections is as follows:

Figure 7. Connecting of the discrete inputs with extermely low driving current (in the range of microampers): Circuit diagram for connecting discrete inputs with extremely low driving current to the PEM-1000 pulse output. This configuration uses resistors R1 and R2 to form a voltage divider, protecting the low-voltage input from an excessive voltage from a 24V source.

Resistors R1 and R2 form a voltage divider limiting maximum output voltage of the driver. Calculations of values of these resistor for some typical cases are as follows (E series values are used, recommended tolerance is 1%):

For other values of these parameters, follow the formula below:

R2 / (R1 + R2) ≤ max. input voltage / max. power supply voltage