Controls Tricks of the Trade

[joey791]

Hey everyone, how bout we start a thread to help out new people and old, anything from straight and simple to generic.


I saw someone post this already but

on a 4 to 20ma device you can use a 250 ohm resistor to make the device 0 to 5 VDC or 500 ohm for 0 to 10 VDC

Honeywell Modutrol Motors-Have a bunch of them that are potentiometer and you need a DC input instead? Honeywell makes a module that you can plug into the motor to change it to VDC, you can find the PDF on the module here
http://hbctechlit.honeywell.com/tech...0s/63-2235.pdf

Also Honeywell has the famous "Gray Book" you can find it here, its a big download
http://68.209.216.66/html/Training/H...llTraining.htm

[control_noob]

Thanks for the links, good stuff.

[wellhung]

your grey book link is dead.

[sysint]

This should just be a standard posting at the top of this section:

HONEYWELL ENGINEERING MANUAL of AUTOMATIC CONTROL for COMMERCIAL BUILDINGS

....Otherwise known as the Honeywell Grey Manual.

[codewriter]

http://www.kele.com/webservices/inde...h/trindex.html

[ctrlguy]

on a 4 to 20ma device you can use a 250 ohm resistor to make the device 0 to 5 VDC or 500 ohm for 0 to 10 VDC

Just a slight correction: 250 ohms gets you 1-5VDC. 500 ohms gets you 2-10VDC. Good ole Ohm's Law: 500*.004=2 500*.02=10

[joey791]

Just a slight correction: 250 ohms gets you 1-5VDC. 500 ohms gets you 2-10VDC. Good ole Ohm's Law: 500*.004=2 500*.02=10

It was 2 years ago, you can give me a break on the math

Also the Honeywell book in book form

http://store.hydronicpros.com/product.php?qcp=1028

[incontrol]

Why'd ya leave out the 1000ohm? That converts ma to volts directly. I've used AOs as constant DC sources before.

[wellhung]

in control, please explain about the 1000 Ohm set-up.

[codewriter]

1000ohm will turn 0-20ma to 0-20vdc
500ohm will turn 0-20ma to 0-10vdc
375ohm will turn 0-20ma to 0-7.5vdc
250ohm will turn 0-20ma to 0-5vdc
etc....
Use the formula (V = I x R)

Some controllers have a max load of 500ohms, etc... So you cant do this with everything.


Edit: almost forgot about this site -
http://www.csgnetwork.com/ohmslaw.html

Use the "To calculate the circuit voltage" (V = I x R)

So on the webpage, in the Amps field put .004 (4ma), put 500 in the Ohms field, click calculate and you get 2 volts,
etc...

[wellhung]

basic question------------what is the difference between a transmitter and a transducer?

[twisted pair]

A Transmitter would be a Field Device connected to a DDC Controller as an Analog Input Point, such as a static pressure transmitter or %RH.

A Transducer is connected to a DDC Controller on a Analog Output Point, which would modulate a signal to a Field Device. 0-10 vdc, 4-20 mA, 0-20 PSI, 0-135 ohms etc...

[codewriter]

They are all transducers in my mind. A transducer is an electronic device that converts "energy" from one form to another.

When it comes to pnuematics, I use the term transmitter though, it was just how I learned it back in the day.

I think a lot of people interchange this word, and most people will probably know what your talking about even if you do not use it so called "properly".

[twisted pair]

Agreed Code. To me a Sensor / Transmitter mean the same. Back when I started with Pneumatics the term "Transmitters" were used on all devices that measured a condition (Temp, RH etc..).

[codewriter]

twisted pair hit it on the head, easy to understand statement, follow his and you will be just fine.

[the_dude]

Meriam-Webster def'n:

Main Entry: trans·duc·er
Pronunciation: \-ˈdü-sər, -ˈdyü-\
Function: noun
Date: 1924
: a device that is actuated by power from one system and supplies power usually in another form to a second system <a loudspeaker is a transducer that transforms electrical signals into sound energy>

Another example:

"What is a pressure transducer?

A pressure transducer is a transducer that converts pressure into an analog electrical signal.

Although there are various types of pressure transducers, one of the most common is the strain-gage base transducer. The conversion of pressure into an electrical signal is achieved by the physical deformation of strain gages which are bonded into the diaphragm of the pressure transducer and wired into a wheatstone bridge configuration. Pressure applied to the pressure transducer produces a deflection of the diaphragm which introduces strain to the gages. The strain will produce an electrical resistance change proportional to the pressure. "

...so the device convert, or 'transduce' one type of signal into (generally) an electrical signal....

[codewriter]

Hell, I say Air Conditioning only. Unless a device is heating a substance other than air I do not say heating. People say they run their air conditioning in the summer and heating in the winter, but they really run their air conditioning year round. I laught at the trucks and companies who say "air conditioning and heating", because its conditioning air wether your adding heat, removing heat, adding humidity, removing humidity, filtering the air, etc... Its all the conditioning of air.

You will never hear me say transmitter unless its pnuematics, in DDC you will only hear me say transducer and sensor.

[phxurs]

Controls


When it comes to designing and operating a controls system, you want a building that is, above all else, reliable. This may mean that it is better to avoid some of the "smart" features and settle for dumb but dependable features.
The key is to keep controls design simple. After all, even though the controls designer may be a genius with three higher degrees in mathematics, it is very unlikely that the maintenance technician who will be working on the system in two years time will be at the same level. The result can be disastrous both in terms of comfort and energy efficiency.
With respect to monitoring, it's best to keep focussed on the things you really need to know, as a BMS can easily provide you with more information than you can cope with. You should resist the temptation to monitor everything, just because you can.
When it comes to control, it may be better to have a building that is dumb and reliable than a building that is smart but scatterbrained.
There is an old story about a developer who thought that he would build the world's smartest building. He asked his engineer "How many monitoring points does that building over there have?" His engineer replied about 2000. So he said "Well, give me 3000 points in this building!" Sadly, it's not that simple.
In reality controls can be the saviour or the death knell of a building in terms of service delivery and efficiency. A simple strategy to minimise risk in this respect is to keep the controls simple and robust rather than complex. After all, there will be a range of people working with the controls over time, and not all of these people will be Einstein.
A key temptation to avoid is the over burdening of the BMS with useless information. As a BMS is designed to monitor and control, it is sometimes assumed that almost every system event should be monitored by the BMS. This philosophy results in a huge amount of data being presented but the question must be asked, "Is it going to be useful to the building manager?" Data by itself is useless unless it can be associated with other data to provide some useful information. Indeed, unlimited data can cause enormous problems. There is a real building in Sydney with 7000 BMS points that had 15,000 alarm signals a day for the first 6 months of operation, and to this day has hundreds. The operators just ignore these.

kiss = KEEP IT SIMPLE STUBID