Pitot Tubes

Pitot tubes are another type of differential pressure flowmeters. They are named after Henri Pitot who came with this invention in the year 1732. Pitot tubes are basically used to detect flow velocity of fluids. Pitot tubes have the potential to measure two pressures at the same time i.e. impact (dynamic) and static. In a Pitot-static tube, the kinetic energy of the flowing fluid is transformed into potential energy for measurement of fluid flow velocity. 




Other Flowmeters

Variable Area Flowmeter

Variable area flowmeters are a type of differential pressure flowmeters. These are simple and versatile devices which are used to measure the flow of liquids, gases, and steam. They work at a fairly constant pressure drop. In these types of flowmeters, a direct visual indication of flow rate is given by the position of the float, piston or vane. The position of their float, piston or vane gets changed as the rising flow rate opens a larger flow area to pass the flowing fluid. When the flow decreases, either the force of gravity or a spring is employed to return the flow element to its quiescent position. Gravity-operated meters also Rotameters must be installed in a vertical position, whereas spring operated meters can be mounted in any position. Each and every variable area flowmeter is available with local indicators. Moreover, they can also be provided with position sensors and transmitters i.e. Pneumatic, electronic, digital, or Fiberoptic types for linking to remote displays or controls. 

Mass Flowmeters

Open Channel Flowmeter

Any conduit or channel in which the fluid or liquid is flowing with a free surface open to the atmosphere is referred to as an “open channel”. For instance, tunnels, nonpressurized sewers, partially filled pipes, canals, streams, and rivers, all these are Open channels. There are various techniques which can be used to monitor an open-channel flow. However, depth-related techniques are the most widespread. These techniques are based on the conjecture that one can instantaneously determine the flow rate of liquid by measuring its depth (or head). Two most commonly used primary devices for measurement of open-channel flows are Weirs and Flumes.


See Mass Flowmeter

Flow Measurement

Flow measurement of liquids is one of the serious needs of modern industrial plants since it is critical to verify the amount of material purchased and sold. Accurate measurement of flow is so much significant in a number of operations, that it can create a whole lot difference between profit making and loss taking. Imprecise flow measurements or inability to take correct measurements can lead to severe results. In addition, flows throughout the process should be maintained near their preferred values with little variability. In such applications, good reproducibility is generally sufficient. Flowing systems require energy, usually provided by pumps and compressors, to create a pressure difference as the driving force, and flow sensors should bring in a small flow resistance, escalating the process energy consumption as slight as possible. Extraordinary considerations are obligatory for concentrated slurries, flow in an open conduit, and other process situations as compared to clean fluids flowing in a pipe.

CAS Data Clients

Make Field Data Available to your windows, SQL and enterprise applications

CAS has developed a number of Data Client products that provide field data to the enterprise applications for purposes of asset tracking, Preventative Maitenance scheduling, Trending, Logging … whatever you application. Now you do not need protocol knowledge / libraries etc to build you applications.

The CAS Data Clients are embedded on ruggedized embedded computers that do not use a fan for cooling. Multiple Ports (serial and Ethernet) are available. Device can also drive a monitor and can be used to local visualization of data. 




CAS2500-03 – ModbusTCP/RTU Data Client

Connects to ModbusTCP or ModbusRTU slaves to read data. The data and transactions are logged. The log files are available and can be transferred to other computers. Current data can be monitored by remote systems running applications that can issue HTTP or SOAP/XMP GET requests - such applications can be easily developed by end users. Of course, this data is available using an Internet Browser such as Internet Explorer or Google Chrome. Connection parameters, device parameters and data parameters are configurable.   

 
CAS2500-04 - BACnetIP Data Client

Connects to BACnet IP devices to read data. The data and transactions are logged. The log files are available and can be transferred to other computers. Current data can be monitored by remote systems running applications that can issue HTTP or SOAP/XMP GET requests - such applications can be easily developed by end users. Of course, this data is available using an Internet Browser such as Internet Explorer or Google Chrome. Connection parameters, device parameters and data parameters are configurable.

DUPLICATE MAC ADDRESSES/DEVICE INSTANCE NUMBERS

What would happen if you had two BACnet MSTP devices with different MAC address but the same BACnet Device ID (Node ID) on the same network and you send a Broadcast Who-is message.
This would be an illegal configuration (no two devices shall have the same Device ID) but the rules for address binding say the last one wins. If new information arrives saying "I-AM Device 7? the old information is replaced.

To understand this answer you must know a thing or two about a token ring networks.
Basically a token ring networks works by passing a token around the network, whoever has the token may send message out, and when they are done with the token they pass it on to the next person in line.

For Example:

• Device A has the MAC address of 0x03
• Device B has the MAC address of 0x04
• Device C has the MAC address of 0x05
• Device A has the token
• Device A responses to any messages it got since the last time it got the token, then sends any custom messages it may have in queue
• Device A sends the token to device B
• Device B has the token
• Device B responses to any messages it got since the last time it got the token, then sends any custom messages it may have in queue
• Device B sends the token to device C
• Device C has the token
• Device C responses to any messages it got since the last time it got the token, then sends any custom messages it may have in queue
• Device C has the token but there is no one with a higher MAC address then Device C so Device C sends it's token to Device A
• Repeat

If you have two different MSTP devices with the same MAC address your network will become unstable and unpredictable results will occur. Both devices will think that they received the token and both devices will attempt to talk at the same time. There will be collisions and you will not be able to determine what device will respond to what poll or what device has the token.

If you have two different MSTP devices with different MAC address and the same Node ID, the device next in line in the token ring will respond first to your requests. This happens because it will be the first device to be passed the token and the first device able to respond. When the token gets passed to the next device ready to respond it will also send a response. any values set by Device A will probably be over written by Device B.

Read more

CAS BACnet Watchdog

"Who changed the set point? Why did that device stop?"

The BACnet protocol does not support authorization or authentication. Use our Watchdog tool to monitor where the commands that changed set points/device states came from. Our tool allows you to monitor some or all devices/objects/properties.

The CAS BACnet Watchdog records MAC addresses, IP address, messages and other information useful in tracking down unauthorized/unexpected commands. It's easy to install and easy to use.

Key Elements of the COV Technology

COV Server :

A BACnet device that supports COV, accepts subscriptions and sends COV notifications messages to a COV Client.

COV Client :

A BACnet device, typically a SCADA or Logging application, which can subscribe for COV notification and which can process the notification messages.

Subscription :

Establishes a relationship between a COV Server and a COV Client.

Subscriptions have the following attributes :

• Subscription to an Object or to a Property BACnet provides two services for subscription. One subscribes to an object and the other to a property of an object.
• Identification of the Subscriber The server needs to know where to send the notifications.
• Object Identifier Eg. Analog Input 1 If subscribing to a property then the property must be identified too.
• Lifetime Is indefinite or a specific number of seconds. Values can be large.
• Notification Type Notification can be sent with/without requiring confirmation from the data client.
• COV Increment This parameter is only used in subscriptions to object properties. If not specified in the subscription the object uses its own increment.

Notification :

A message which reports the current value of the changed property as well as the current state of the objects Status Flag property if it exists. The notification also contains the number of seconds remaining to the subscription. Confirmed notifications require a response from the COV client.

6.0 - Change Criteria

The change criteria are based on the type of subscription

Subscribe to the object :

Either of these cases trigger notification:
1) If the status flags change at all
2) If the Present Value changes:

• Binary, Life Safty and Multistate Objects : any change to Present Value
• Analog, Loop and Pulse Objects: change by COV_Increment

Subscribe to a property :

Either of these cases trigger notification:
1) If the status flags change at all (if the object has status flags)
2) If the Property Value changes:

• Property is of type REAL : change by COV_Increment ( which may be defined in the subscription or may be the native increment defined in the device).
• Property is of some other type: any change to Present Value

FieldServer - Example

These are some of the examples you can produce with this system, the system is only limited by your imagination.

Simple Example

In this simple example we are only using the very basics of the template system, (display text and action buttons) but you can instantly see the power of this system, its easy to read, it’s easy to operate, it provides all the information that the user needs about the building.



Sleek and Pretty Example

In this example we put an information tag on an image of the factory; the loading doc has two tags associated with it, Lights and AC. You can see that the lights are on and the AC is off, the boarder is flashing red indicating a panic, the user then could click on this tag and get a screen similar to the one above where they can get more information and acknowledge the alarm.



Mirror example

In this example we copied the front display of the real device, anyone that has used the device before can quickly and easily navigate this interface.

FieldServer - How It Work?

FieldServer's Web Server

Did you know that a FieldServer can be used to serve web pages ? Did you know that the web pages can be animated with real time data collected by the protocol drivers talking to real field devices? Were you aware that we can serve sophisticated pages like this one, animated with data from a Canatal Air Conditioner?


This month we are featuring the FieldServer WebServer. Please explore the capabilities of the WebServer by reading our newsletter.

FieldServer Signal ltravoice

Beer and Vodka Can Help You Select a Terminating Resistor

Try this mnemonic if you are trying to remember the resistor color codes:

Bad				(0) Black
Beer				(1) Brown
Rots				(2) Red
Our				(3) Orange
Young				(4) Yellow
Guts				(5) Green
But				(6) Blue
Vodka				(7) Violet
Goes				(8) Grey
Well				(9) White
				(0.1) Gold
				(0.01) Silver

Note: If you're missing a tolerance band that implies that the tolerance is 20%. 

Which end do you start reading the color bands?

There are usually two ways:

1) If one of the bands at the end of the sequence is further apart then that is the tolerance band - start from the opposite end.

2) If all the bands are closer to one side of the resistor then start from that end - the tolerance band is the last one your read.

What should you carry with you to site? (for communication networks purposes)

For Terminations
Value		Tolerance
75	Ohms	5%
100	Ohms	5%
120	Ohms	5%

For Biasing
Value
10k	Ohms
4k7	Ohms
2k4	Ohms
1k	Ohms
560	Ohms
30	Ohms

How to Buy Resistor

RS485 on a Scope

Zone A, E and B

Zone C: Unless you have a very high speed scope and the scope can takes a huge number of samples you are unlikely to see each bit in the message. In this capture we can see the bytes (roughly speaking) but not the bits. We were forced to accept this compromise because the number of samples we could capture at a sampling rate high enough to see the bits would mean that we could only capture the 1st couple of bytes of the message and we would have to set the trigger to ignore zone B. Except with a very expensive scope you will be unlikely to be able to see the whole message and all the bits.

Zone D: We have finished transmitting but the transmitter is still enabled. Normally the device should disable its transmitter as soon as possible after transmitting the last stop bit of the message but since that can be difficult to achieve in the hardware, many devices run a timer to make sure they don't disable the transmitter too soon. The problem with this approach is:
1) The longer the time the more potential bandwidth is lost.
2) The receiving device may have already processed the message and try to send a response by enabling its own transmitter causing collisions.

 

Probes on plus and minus conductors. Segment connected to slave device.

 

Probes on plus and minus conductors. This chart represent the capture from a master device with no slaves connected to the network leaving the cable ends to float.

Determining Byte Order

The Modbus protocol itself is declared as a ‘big-Endian’ protocol, as per the Modbus Application Protocol Specification, V1.1.b:

    “Modbus uses a “big-Endian” representation for addresses and data items. This means that when a numerical quantity larger than a single byte is transmitted, the most significant byte is sent first.”

Big-Endian is the most commonly used format for network protocols – so common, in fact, that it is also referred to as ‘network order’.

Given that the Modbus RTU message protocol is big-Endian, in order to successfully exchange a 32-bit datatype via a Modbus RTU message, the endianness of both the master and the slave must considered. Many RTU master and slave devices allow specific selection of byte order particularly in the case of software-simulated units. One must merely insure that both all units are set to the same byte order.

As a rule of thumb, the family of a device’s microprocessor determines its endianness. Typically, the big-Endian style (the high-order byte is stored first, followed by the low-order byte) is generally found in CPUs designed with a Motorola processor. The little-Endian style (the low-order byte is stored first, followed by the high-order byte) is generally found in CPUs using the Intel architecture. It is a matter of personal perspective as to which style is considered ‘backwards’.

If, however, byte order and endianness is not a configurable option, you will have to determine the how to interpret the byte. This can be done requesting a known floating-point value from the slave. If an impossible value is returned, i.e. a number with a double-digit exponent or such, the byte ordering will most likely need modification.

The Importance of Byte Order

Voltage

Under / Over Voltage
   

Over Voltage is an increase in effective voltage to more than 110% for longer than one minute. Under Voltage is a decrease in effective voltage to less than 90% for longer than one minute. Take care with this definition because it tends to change from vendor to vendor.


Transient Voltages / Spikes / Surges
   

Refers to short duration (less than 1 cycle) events. Low frequency transients are often called "capacitor switching transients". High frequency transients are often called impulses, spikes, or surges. They can be caused when a discharged power-factor-correction capacitor is switched on across the line.

High frequency transients are caused by lightning, and by inductive loads turning off. Typical rise times are on the order of a microsecond; typical decay times are on the order of a tens to hundreds of microseconds. Often, the decay will be an exponential damped ringing waveform, with a frequency of approximately 100 kHz.

Extremely fast transients, or EFT's, have rise and fall times in the nanosecond region. They are caused by arcing faults, such as bad brushes in motors, and are rapidly damped out by even a few meters of distribution wiring. Standard line filters, included on almost all electronic equipment, remove EFT's.



Power Meter Solutions Using FieldServer











Convert Power Meters to BACnet.

    Includes configuration for a single meter
    Includes Free CAS BACnet Explorer license.
    Offer applies for certain meters only. Small extra fees apply for configurations which connect to multiple meters. Contact us for more info..

Real Power & Reactive Power

Sag / Swell or Dip / Surge

Duration is 0.5 cycle and greater. Voltage sags are the most common power disturbance. Voltage sags can arrive from the utility. In most cases, sags are generated inside a building. For example, in residential wiring, the most common cause of voltage sags is the starting current drawn by refrigerator and air conditioning motors.

Sags do not generally disturb incandescent or fluorescent lighting, motors, or heaters. However, some electronic equipment lacks sufficient internal energy storage and, therefore, cannot ride through sags in the supply voltage. Equipment may be able to ride through very brief, deep sags, or it may be able to ride through longer but shallower sags.

Power Meter

Power Factor and Phase

Power Factor is calculated: PF = Cosine ( phase angle in radians)

Power factor has no engineering units.

The value of PF ranges from -1 to 0 to 1 (lagging – none – leading)

Loads that only present a resistive load (no capacitance or inductance) have a PF of 1.

Inductive Loads
    Current phase lags the Voltage
Typical - Transformers and motors (wound conductors)

Capacitive Loads
    Current phase leads the Voltage
Typical - Buried Cables, capacitor banks

There is nothing ‘wrong’ with having a power factor that isn’t 1.0.

See also Apparent Power

Convert automation protocols

Get to know your Power Meter What is Real, Apparent and Reactive Power

First - a simple question
  

Right or Wrong? Power = Voltage x Current

That statement is correct for DC systems but there are two major complications for AC systems.

The value of current and voltage keeps changing. Which value do you use ?
The voltage and the current may not be in phase. Multiplying the current and the voltage when they are not in phase requires and adjustment to compensate for the phase. It is this phase shift that forces us to define Real, Apparent and Reactive Power. This phase shift occurs when a power source feeds an inductive or capacitve load". Most loads are either inductive (motors) or resistive (heaters) and therefore the phase shift is typically in one direction.
 

A motor has a winding. A wound conductor essentially defines an inductor. Thus the winding presents the resistance of the wound wire and the inductance resulting from the winding.

BACnet School

FLAVORS OF BACNET

Flavor	Application	Affects you?
IP	Uses the TCP/IP protocol Controller to contoller Controller to HMI Some field devices	On the up
Ethernet 802.3	Raw Ethernet Packets	Being displaced by IP
Point to Point	Modems and phone lines	Rare. Expected to disappear.
MS/TP	Field devices	Millions of installed devices.

Troubleshooting BACnet IP / Ethernet

Ethernet Cable Color Coding

The Difference Between Ethernet Cable Cat5 and Cat5e

There are two color coding standards. The color coding standard does not affect whether the cable is a cross-over or straight through patch cable. Color does not affect performance or use of the cable.

Standard 568A			Standard 568B This is the most commonly used for patch cables.
1	White-Green		1	White-Orange
2	Green		2	Orange
3	White-Orange		3	White-Green
4	Blue		4	Blue
5	White-Blue		5	White-Blue
6	Orange		6	Green
7	White-Brown		7	White-Brown
8	Brown		8	Brown

Modbus BACnet Protocol Gateway Store

What uses the MSTP bandwidth

• Do the devices support the “Read/Write Property Multiple” services or must each property be read in a separate message.

Find the answer to this question by reading the BIBs statement for each device or you could explore the device object of the device, find the property called BACnetServicesSupported and then look at the 14th item in the array to see if Read Property Multiple is supported and the 16th for Write Property Multiple. However, we have found that a large number of devices don’t display this information.

Obviously, if you can read a chunk of properties in one message you will be better off than if you can only read a single one.

• Can you use BACNet’s COV mechanism.

COV stands for Change of Value. When a device supports COV another device / application can subscribe to receive notifications when an object property changes. This means the data client doesn’t have to poll for data continuously but can wait passively to be notified of the change. This reduces the number of messages on a network dramatically.

• Some devices are slower than others.

BACnet allows up to 15 msec for a device to use the token. Since most messages on a MSTP network are token passes a device that uses the token in 5 msec will consume much less bandwidth than one that takes 15msec. (A number of vendors relax this requirement to allow for other vendors implementations. The more relaxed the more bandwidth is consumed doing nothing.)

How many BACnet MSTP devices on a trunk?

There are non electrical considerations to determine how many devices you put on an MSTP network.

The chart below illustrates (from one installation) how little of the bandwidth is used to transfer data. The APDU’s are application layer message that poll and respond with property values – they do work for us as data consumers. The rest is used to maintain the network – passing the token around and looking for new devices.

It’s not possible to provide a calculator to work out how many devices to install on a single network but the following list provides some help in assessing bandwidth considerations.

It takes approx 30 bytes to poll for a single property. It takes about 40 bytes to reply. A token is 8 bytes as is a Poll for master.

Assume that 50% of your bandwidth will be used by overhead (token, poll for master).

Divide the baud rate by 10 to get bytes per seconds.

Using a number like 30+40=70 as a best case and 100 as a worst case (obviously reading descriptions will take more) multiply by the number of objects and properties you are going to poll on a regular basis.

Here are some typical numbers assuming the device doesn’t support the ‘multiple’ services (see below).

Baud	38400	divide by 10
Bytes per sec	3840
Overhead use (token and poll for master)	50%
Byte per sec for payload	19200	a
Typical Poll and response for a single property	70	b
Number of properties that can be polled per sec	27.42857	=a/b
Typical number of props that will polled per object (pres value, status_flags, reliability, out of service)	4
Number of objects per sec	6.857143