Calculating "absolute humidty" - PLC HW

[olle_hansen]

Hello,

I want to make a rather simple control for fans in a basement. The point is to evaluate the inside and outside temperatur, relative humidity and absolute humidity.

To do this I need a PLC and some sensors.
I'm thinking PLC Logo!8, due to the fancy display.
Humidity sensors are hard to figure out, since they can be very expensive. However, I think this will be suitable: http://www.temcocontrols.com/products/sensors/16/humidity-temp-transmitter.htm

Only a few humidity sensors have output for "Absolute humidty", hence they calculate the values themselves. I think it's better to have only temperature and relative humidity to the PLC, and then calculate the absolute humidity in the PLC.

Formula for absolute humidity:

Besides the inputs (temp+RH) the formular needs the natural logarithm "e".

Can the PLC Logo handle this formula?

Also, 4 analog inputs are needed. Instead would it be possible to use modbus?
Modbus communications port: RTU, 19.2/9600 Baud.

Any suggestions?

 

[2004sk4]

A modbus sensor for some of the inputs http://www.datanab.com/sensors/modbus_rth_lcd.php

 

[Tom Jenkins]

I've had success with humidity sensors from Omega Engineering.

If you are doing comfort cooling I don't see any need for absolute humidity - relative humidity would be adequate, or at worst dew point. If you look at the ASHRAE site they have some guidance.

e is a constant, = 2.71828

 

[olle_hansen]

Regarding modbus. The experience I'm lacking is the connectivity between a modbus device (temp transmitter) and the PLC. Is it plug'n'play, and easy to fetch the live values in the PLC?

Regarding the need for absolute humidity.
The outside humidity is often 80-99% in Denmark, at a temperature ranging from -10 to +25.
The basement will be 17-22 degrees at all times.

My point is to draw fresh air into the basement from the outside, but only when the outside air contains less water than the basement air.
This way the basement air will have increased humidity due to ventilation.

There are no dehumidifies. Nothing else to dehumidify the air.

The outside absolute humidity ranges from 3-12 g/m3.
The basement relative humidity should be no higher than 45-50% at 17-20 degrees. Than equals an absolute humidity of 6-8 g/m3.

If the outside AH is lower than basement AH, venting should start in intervals. No reason to vent constantly.
However if the basement RH is higher than 50, while the outside AH is still lower than basement AH, constant ventilation makes sense.

Of course, if the outside AH is higher than inside AH, ventilation should not be performed.

This is why I need the ABSOLUTE humidity.

 

[olle_hansen]

So...

Would you think that a Logo! PLC and Temp+RH sensors will match my needs?

 

[shooter]

To get it very simple, use outside a temperature sensor (i use Pt100 and Pt1000)
put a sock around it and you will see the wetbulb.
do same on inside.
put both signals on to a simple opamp 741. when the wetbulb out is higher start ventilation.

 

[olle_hansen]

To get it very simple, use outside a temperature sensor (i use Pt100 and Pt1000)
put a sock around it and you will see the wetbulb.
do same on inside.
put both signals on to a simple opamp 741. when the wetbulb out is higher start ventilation.

This doesn't fit my needs.

 

[Steve Bailey]

I googled "Siemens Logo function set" and after a quick review could not find that the Logo has an X to the power Y function. Perhaps someone with more experience can chime in to confirm or refute that.

 

[danw]

Regarding modbus. The experience I'm lacking is the connectivity between a modbus device (temp transmitter) and the PLC. Is it plug'n'play?

Modbus is NOT plug-n-play. Modbus is a project.

A modbus slave, the humidity sensor, is essentially passive, responding only to Modbus messaages addressed to it.

But getting a Modbus master porgrammed/configured is a recurrent theme on the forum.

Some vendors appear to be better than others in providing an example of how to set up a Modbus master.

And there are Modbus challenges. Like the A and B lines for 2 wire RS-485 are labeled one way by some manufacturers, the opposite by other mfgrs. Or the one offset in addressing; some vendors start counting at zero, others at one.

Modbus can work and work reliably, but getting it to that point is a project.

 

[mk42]

I googled "Siemens Logo function set" and after a quick review could not find that the Logo has an X to the power Y function. Perhaps someone with more experience can chime in to confirm or refute that.

I would be surprised if a smart relay had that level of floating point math functionality. You might be able to find a different smart relay that can, but you might need to upgrade to a micro PLC. If you like Siemens equipment, I'm sure the S7 1200 could handle that math. It doesn't have the built in display of the Logo though.

 

[marcius]

I think Logo doesn't support modbus rtu, in fact i think it doesn't support modbus at all (TCP or RTU).
It has only ethernet interface and can ve connected to other S7 controllers.

To check what function Logo supports you can download demo software here:
http://w3.siemens.com/mcms/programm...-module-logo/demo-software/pages/default.aspx

 

[boneless]

I recently bought this. Cheap and easy.

http://www.hobbyist.co.nz/?q=documentations/wiring-up-dht11-temp-humidity-sensor-to-your-arduino