Humidity calculation

Relative Humidity

try here Temp, Humidity, Dew Point ONA.

or here Engineer Tool Box.

You guys have got the right Idea just the wrong application, I dont know if it will make much difference, but what I am trying to do is control the humidity in an oven for cooking hams, bacon and sauages, for this process to work correctly for commerical cooking it is paramount that we keep the humidity at a set point, so that we do not loose too much moisture out of the product.

The thing that I dont understand with using the wet and dry bulb calculation is the pressure, we dont have any way of measuring pressure in the oven, so because of this and the fact that we are not introducing pressure, and we are acting at atmospheric pressure, I suppose that we can insert a value of one for the pressure (I am not sure)?

Thanks for your answers.

Cheers

Joe

Being that this is an oven, I might want to change my answer. I'm not sure of what easily available psychrometric data/charts exist for such high temperatures.

If your data are based on atmospheric pressure, that would be zero rather than one.

If based on absolute pressure, full vacuum equals zero, and atmospheric equals 14.68 psi, subject to altitude correction.

I'll try to look around for some high-temperature data; no promise of success, though.

As a practical matter, you could add a shallow pan of water to the oven, replenishing it as necessary. Now if we can just pin down the instrumentation.... And even then, you may need to do your own experimentation.

Hmmm, I'd say that your given application is a vital and missing note in your original post. In normal applications where humidity is concerned saturated air has a relative humidity of 100%. This of course does not apply to the generation of steam, which will be generated in an oven at cooking temperatures greater than 100C. At the temperatures experienced in an oven you will generate steam, which then is a whole different set of conditions where "humidity" does not apply. BTW, Psychrometric charts do not apply beyond 100C (the boiling point of water).

Hi, When cooking in industrial applications we never go over 100 degrees C.

Cheers

Joe

Hi, Pork Man,

With most smoke ovens that i have seen they only use the wet and dry bulb sensors, to calculate humidity. The wet one sits directly above a small vessel of water which has a soaked (water) cloth protruding from the water vessel and hung over the wet sensor.

The Dry sensor, does 2 jobs it measures the cabinet temperature (oven temp) and also is used as a reference for measuring the the temperature difference between the wet and dry bulbs.

We also know that if the values of the wet and dry bulb sensors are equal we have 100% humidity.

But the tricky part is because we are heating the oven at different temperatures and this temperature is varying all the time, we cannot use a simple calculation.

What we need to do is: depending on the cabinet temperature the cabinet can hold different amounts of humidified water ie; at a lower temperature the cabinet requires less water humidifed to acheive 100% humidity, but as the temperature rise's we will need more water humidifed to achieve 100% humidity.

I know that there is a formula that uses, these factors 1. cabinet temp 2. temp diff (Dry - Wet) with these values we then go to a reference chart and get a number.

But I cant remember where I came across this chart and I cant remember exactly how the calculation works.

The calculation is not required to be 100% accurate, +- 2% would be very accetable.

Also I dont think that it is necessarry to know which product you are cooking, in relation to the humidity as we are only controlling the humidity in the cabinet. The operator will select what humidity % he requires for the product he is cooking, depending on the weight loss that is accetable for that product, After all he may be doing products that require little or no humidity, so therefore it is up to the operator and his experience and his required result as to what the humidity should be.

As you have said Pork Man these is a bit of science and operator experience, that goes into cooking the product right, all operators have different idea's and different results.

Cheers

Joe

Brilliant reply...thanks for clearing up this mystery in the low-temperature application of humidity and how it applies to "cooking" in an "oven". Thumbs up!

1. Iam a marine engineer & can explain only certain things in my perspective.for measuring pressure inside oven u can use a pressure difference method.how much less or more than atmospheric pressure, using a u tube.
   

Hello Sailer,

I am not sure where you are going with your posting, I dont want to measure pressure, I want to measure humidity. I am just having troubles working out calculations using a very basic anologurtherm for the calculation of the humidity, problem is I cant find a data table that is workable.

Cheers

Joe

Or track these quantities on a psychrometric chart. (That's psychro-, not psycho-; I have never seen a psychometric chart, though it might be amusing....)

Is this more inline with what your looking for?

Mora.pdf

Check out these links

Humidity Calculations & More

http://www.vaisala.com/instruments/humiditycalculator.html

http://www.gorhamschaffler.com/humidity_formulas.htm

http://www.humidity-calculator.com/

http://www.usatoday.com/weather/wwincalc.htm

Much more information on the WEB. you can find if you try to google friend

greetings
their orders

Hi MrCarlos, the best one I found was the Vaisala site, the calculator is simple to use, but for my application if I wanted to calculate to the nearest degree, I would have to do about 9000 calculations and fit them in a data reference table, but that seems like to much work and use of data tables that I wanted to use for other applications in the software.

What I am trying to find is a simple calculation that uses the temperature (dry Bulb) and Dew Point (wet Bulb) and a simple look up table to calculate to finish the calculation.

I am sure that there is a calculation out there that goes somthing like this;

D - W = Tb

D x Tb = H%

Where:

D = Temperature

W = Dew Point Temp (Wet Bulb)

Tb = Look Up Table

H = Humidity %

I am not sure it my calcus is correct, I just have a feeling that I am working along the right lines.

Many Thanks to all that have are contributing to this forum.

Cheers

Joe

Joe

It is really easy, you could find a lot off information about it on Meteorology area try with the Psichrometer procedure.

Here is something in spanish but sure there are information in english to.

· Psicrometro

Mide el contenido de humedad del aire.
Consta de dos termómetros idénticos de mercurio. A uno de ellos se le recubre el bulbo con una gasa que se humedece con agua destilada, se le llama termómetro húmedo. El otro termómetro mide la temperatura del aire, se llama termómetro seco.
El bulbo húmedo mide una temperatura igual o menor al bulbo seco, debido a que el agua que se evapora de la gasa, absorbe calor. La diferencia entre ambas medidas está relacionada, para cada temperatura y presión atmosférica, con el contenido de vapor de agua de la atmósfera.
Cuando la humedad relativa es de 100%, la lectura de ambos termómetros es igual. Cuando hay poca humedad en el aire, la gasa evapora más rápido y se nota más diferencia entre los dos termómetros.

Pilar Valles

It's practical.

• Google "psychrometric chart", or
• see Kempe's Engineer's Yearbook, any edition, or
• see Perry, "The Chemical Engineer's Handbook", any edition, or
• any HVAC textbook.

RH is defined as the ratio of the mass of water vapour in a certain volume of moist air at a given temperature to the mass of water vapour in the same volume of saturated air at the same temperature. It is considered to be a more commonly understood measure of the degree of saturation of air. Psychrometer is used to measure R.H. It consists of dry and wet bulb thermometers fixed side by side on a wooden or metal plate. A dirty wick should be replaced from the wet bulb thermometer regularly. Before taking the readings the instrument should be whirled round rapidly for about a minute. R.H charts are available in the market. Once we know the DBT and WBT R.H cab be found from the chart.