Psychrometry Test

Just give the guy the answer -this is not an initiation

You are being way too harsh! I didn't read it word-buy-word, but I did scan the article of your link twice, and I didn't see any answer to the OP's question.

Clearly, a single reading may be misleading, and it is not likely that the OP needs the precision of a 1000-point matrix. somewhere in between is his answer...

Yes, you should sample several locations. It might be tricky to decide how many and what locations would produce a statiscally acceptable sample set. Don't prejudice by picking either the warmest or coldest, but try to find the most representative.

Normally, how many sample in practice shall be taken for a good statistic study of the psychrometry ?

The more the better, but on rough guess maybe 10 or so. Could be fewer if they aren't prejudiced, but that might be hard to assess.

It depends on the size of room and the importance or significance of your objective.

Take note that you are measuring air. A fart could easily be dispersed in a small room- bigger room needs bigger or numerous fart to impact. See if there are ventilation or air circulation if its a bigger room.

A smaller room basically needs not to have 100 readings. Would you agree, in the first place to take a thousand of samples for a 3mx2mx2m room? - to make it more likely a representative sample?

Depends on the degree of accuracy that you are aiming for.

nice, clever advice. I agree. Just run the trials using the fun, but not too near. Wind velocity at significant amount (PV=mRT), decreases temperature of the bulb, it might not be anymore representative sample of the room, might give erroneous readings.

The average temperature in an air conditioned room is usually taken at the return duct inlet....the relative humidity can be sampled at a few locations around the room...the dry bulb and wet bulb are taken to determine the relative humidity not the air temperature per se....which would be equal to the dry bulb reading...

https://www.ohio.edu/mechanical/thermo/Applied/Chapt.7_11/Chapter10a.html

Dear Mr.cckla,

The answer for your question is Yes. You have to measure in different corners as well as at 3 different levels namely at floor level,centre and at near ceiling level.

This is on account of my personal experience about 5 years back where there was a problem in the AC ROOM for the VFD for 1000 H.P. Motors - 4 nos, and it was found that though Air-Conditioners were installed to the required numbers, there was a variation in the Temp. when we measured at different corners and levels.

This has given a lesson for us and after re-orienting the AC unit, the problem was solved.

DHAYANANDHAN.S

Just being practical here. When you ask the question, just ask "when I measure the humidity of an air-conditioned room, do I have to make one measurement or several?" It means the same thing and is a lot easier to spell. Then you have to ask yourself "Do I need a really precise measurement? Is there likely to be a variation across the room? Does it matter if there is?" If the air-conditioning has been sensibly installed then it will generate a flow of air and mixing will occur. If all you are concerned about is user comfort, then a single sample will suffice, at most a sample at each user station. If you are wanting to sample different areas of the room for the sake of the statistical exercise, then you are going to have to decide your statistical null hypothesis. "The humidity at waist level is greater than at eye level - true or false" or "The humidity near the door is greater than that at the window - true or false" and design your sample site pattern accordingly, with at least 5 samples at each site. I would say that that would be a complete waste of time for practical purposes.

Psychrometric properties

Dry-bulb temperature (DBT)

The dry-bulb temperature is the temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation. The term dry-bulb is customarily added to temperature to distinguish it from wet-bulb and dewpoint temperature. In meteorology and psychrometrics the word temperature by itself without a prefix usually means dry-bulb temperature. Technically, the temperature registered by the dry-bulb thermometer of a psychrometer. The name implies that the sensing bulb or element is in fact dry.

Wet-bulb temperature (WBT)

The thermodynamic wet-bulb temperature is a property of a mixture of air and water vapor. The value indicated by a wet-bulb thermometer often provides an adequate approximation of the thermodynamic wet-bulb temperature.

The accuracy of a simple wet-bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings. Speeds up to 5,000 ft/min (~60 mph) are best but it may be dangerous to move a thermometer at that speed. Errors up to 15% can occur if the air movement is too slow or if there is too much radiant heat present (from sunlight, for example).

A wet bulb temperature taken with air moving at about 1-2 m/s is referred to as a screen temperature, whereas a temperature taken with air moving about 3.5 m/s or more is referred to as sling temperature.

A psychrometer is a device that includes both a dry-bulb and a wet-bulb thermometer. A sling psychrometer requires manual operation to create the airflow over the bulbs, but a powered psychrometer includes a fan for this function. Knowing both the dry-bulb temperature (DBT) and wet-bulb temperature (WBT), one can determine the relative humidity (RH) from the psychrometric chart appropriate to the air pressure.

Relative humidity

The ratio of the vapor pressure of moisture in the sample to the saturation pressure at the dry bulb temperature of the sample.

Dew point temperature

The saturation temperature of the moisture present in the sample of air, it can also be defined as the temperature at which the vapour changes into liquid (condensation). Usually the level at which water vapor changes into liquid marks the base of the cloud in the atmosphere hence called condensation level. So the temperature value that allows this process (condensation) to take place is called the 'dew point temperature'. A simplified definition is the temperature at which the water vapour turns into "dew"

Rotronic has some good technical notes...

http://content.rotronic-usa.com/rs/rotronicinstrumentcorp/images/Humidity%20Definitions%20WEBa.pdf

http://content.rotronic-usa.com/psych-charts

You might need to take a series of readings at the same place at different times as well.

It all depends on what you want the readings for.