AU3016599A

AU3016599A - Humidity probe

Info

Publication number: AU3016599A
Application number: AU30165/99A
Authority: AU
Inventors: Thomas Adamczak; John Kalitsis
Original assignee: BRI Australia Ltd
Current assignee: BRI Australia Ltd
Priority date: 1998-05-21
Filing date: 1999-05-20
Publication date: 1999-12-02
Anticipated expiration: 2019-05-20
Also published as: AU766454B2

Description

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AUSTRALIA

Patents Act 1990 BRI AUSTRALIA LIMITED

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT

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Invention Title: S9 Humidity probe The following statement is a full description of this invention including the best method of performing it known to us:- Field of the Invention This invention relates to a humidity probe, and to an apparatus for and a method for measuring the humidity in heated or cooled chambers such as ovens, particularly dryers, baking ovens, and furnaces or cryogenic and mechanical freezers.

Background of the Invention The moisture content in an oven or drier, will affect the overall heat and mass transfer to the product being baked and the final quality and appearance of the product. For this reason, it is desirable to be able to measure the humidity in a baking oven and, if necessary, change the humidity in the oven depending on the measured humidity. Baking ovens can operate at temperatures as high as 3500 C and in commercial baking ovens, the temperature generally reaches at least 250 0 C. Consequently it is necessary for any humidity probe to be able to operate at the same high temperatures.

Since the generally available commercial capacitive type humidity probes will only operate at temperatures up to 180 C, they are unsuitable for use in most commercial baking ovens. Thus, in commercial ovens the use of a zirconium oxide cell has been proposed.

Zirconium oxide cells operate based on the measurement of the oxygen content of the air which in indirectly fired ovens is related to the humidity in the oven, and this can then be related back to calculate the humidity in the o *oven.

25 However, in the baking industry both direct and indirect fired ovens are used. In direct fired ovens, oxygen is consumed to produce carbon dioxide and carbon monoxide, and this will affect the oxygen reading in the oven and therefore the calculated humidity. Thus, sensors based on zirconium oxide cells require extensive corrections to account for combustion products.

Thus there is a requirement for a method for measuring humidity and a humidity probe which can be used to measure the humidity of air in both indirect and direct fired ovens, especially those operating above 180f C.

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Summary of the Invention According to the present invention, there is provided a method for measuring the humidity in a remote, typically heated, chamber such as an oven or the like comprising the steps of:withdrawing air from the heated chamber; (ii) conditioning the withdrawn air by adjusting its temperature and transferring the conditioned air to a second chamber; (iii) employing a sensor located in the second chamber to directly measure the humidity of the conditioned air and using the measurement of the humidity and temperature of the conditioned air to determine the absolute humidity of the conditioned air.

The absolute humidity of the conditioned air in the second chamber at the sensor will be the same as that in the heated chamber.

Preferably the step of adjusting the temperature of the air withdrawn from the heated chamber includes the step of drawing the air through a heat sink. This step might either increase or decrease the temperature of the withdrawn air, depending on the temperature of the air in the chamber.

The temperature to which the air is adjusted is most appropriately a constant temperature, or within a predetermined range, which provides the advantage that the sensor is always exposed to air at a constant or substantially constant temperature which simplifies calibration of the sensor and reduces the possibility of errors.

The predetermined adjusted temperature is most appropriately one at S which condensation does not occur. The preferred temperature is 1000 C, 25 where under one standard atmospheric pressure, water is in a vaporous state and will not condense. However, temperatures other than 100 C could be used.

Standard psychometric and steam tables can then be used to calculate the humidity properties in the chamber if the dry bulb temperature in the heated chamber (baking oven) and chamber pressure are known.

Thus the present invention provides a system which can be used to accurately determine the humidity of a chamber operating substantially above 1000 C, the normal atmospheric pressure boiling point of water. This permits the use of a humidity sensor that is normally incapable of operating at the temperature inside the chamber of interest. Thus, for example, a capacitive type of humidity sensor which is normally operable only up to 1800 C, can be used to successfully measure the absolute humidity inside dryers, ovens or furnaces operating in excess of 2000 C, well above the operating limit of the capacitive sensor employed. This temperature is also far above the range at which the traditional wet/dry bulb psychrometers can operate.

The system also has the advantage over using zirconium oxide sensors since it measures humidity directly. In contrast, zirconium oxide sensors measure oxygen content in the chamber which is an indirect measurement technique and thus the use of those sensors to measure humidity, requires extensive considerations and corrections for combustion product gases which are normally present in ovens. Furthermore, the composition of these combustion gases can vary rapidly.

The method of the present invention may include the steps of using the measured humidity readings to regulate and control the conditions in 15 baking ovens, including tunnel ovens by, for example, controlling oven temperature, airflow through the oven, extractor fans, and the like. The readings may also be used to control the baking time.

The present invention also provides an apparatus for measuring the *.humidity in a remote, typically heated, chamber, such as an oven, comprising:means for drawing air from the heated chamber; means for conditioning the air withdrawn from the chamber by adjusting its temperature; and a humidity measuring means for measuring an indication of the 25 humidity of the conditioned air.

It is preferred that the means for conditioning the air is a heat sink made from a high density material with a large heat capacity and good thermal conductivity, such as copper or one of its alloys such as brass.

It is preferred that the heat sink incorporates a temperature sensor, such as a thermocouple and an associated temperature controller. Heating tape is preferably wound around the exterior of the heat sink. The heat sink is most preferably a brass rod having a central bore through which air can be drawn and a chamber located at one end in which the humidity measuring means is mounted.

_1 I~ It is preferred that the heat sink is maintained at a constant temperature so that the humidity measuring means is always exposed to air at a constant temperature, the preferred temperature being 1000 C.

The device preferably includes a control means for converting the reading to the absolute humidity, which will be the same in the heated chamber and at the humidity measuring device, to determine the humidity properties in the heated chamber by using the measurable dry bulb temperature in the heated chamber and readings of the chamber pressure.

Brief description of the Drawings A specific embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of apparatus embodying the present invention; 15 Figure 2 is an a enthalpy-humidity diagram for water vapour in air; Figure 3 is a graph of a comparison of the performance of a probe embodying the present invention with a zirconium oxide probe; Figure 4 is a graph of a comparison of the probes in an oven simulator; Figure 5 is a graph of a humidity profile of a travelling tray oven; and Figure 6 is a graph of a humidity profile of a tunnel oven *'.99 Detailed Description of a Preferred Embodiment Referring to the drawings, Figure 1 is a schematic illustration of equipment embodying the present invention. A baking chamber or oven is 25 shown schematically at 10. Air is drawn from the baking chamber through a tube 12 using a pump, not illustrated.

The air is drawn into a heat sink comprising a brass cylinder 14 and along a central hole 16 which passes through the centre of the brass cylinder 14. One end of the cylinder 14 defines an enlarged cylindrical cavity portion 18 which is connected to the central hole 16 into which air from the baking chamber passes. A tube 20 extends from the chamber 18 to a pump, not shown. Thus operation of the pump to draw air through the tube 20 draws air from the baking chamber 10 into the pipe 12 through central hole 16 where the air adjusts to the temperature of the heat sink 14 and thence into the chamber 18.

The temperature of the heat sink 14 is measured using a temperature sensor such as a thermocouple 22 and controlled with a proportional integral derivative (PID) temperature controller 24. The flow rate of air through the tube and the length of the tube should be such that the air reaches an equilibrium temperature with the tube at a temperature of 1000 C. Heating tape 26 is wound around the outside of the tube to assist in maintaining the tube at the temperature of 1000 C. The temperature is also controlled by adjusting the flow rate of the air through the system by controlling the pump.

A humidity probe 28 is located in the chamber 18. Capacitive type humidity sensors already exist that are capable of operating at temperatures of up to 1800 C. The embodiment described above utilises a Vaisala HMM 30C humidity transmitter (but any suitable humidity sensor could be used for the apparatus of the present invention).

In use, air is drawn from the baking chamber 12 by the pump into the 15 central tube 16 and thence into chamber 18 at which time the temperature of the conditioned air should be at 1000 C, with the temperature of the heat sink 14 being monitored and controlled by temperature controller 24. The actual temperature will depend on the flow rate of the air which can be controlled by controlling the pump, the length of the heat sink (which is not easily adjustable) and the temperature difference between the tube and the air.

Since the air is being cooled, it is desirable that the temperature at which the humidity reading is taken, be above the dew point of the air, otherwise condensation will occur rendering the sensor reading useless. Also the majority of humidity transmitters are calibrated for moisture versus 25 temperature relationships which do not apply at temperatures greater than 1000 C. Consequently, the temperature is preferably maintained at 1000 C to eliminate those problems.

A flow control unit controls the pump to limit the volumetric flow rate of the air so that the temperature of the air drawn from the baking chamber will equilibrate with the temperature of the heat sink.

The absolute humidity of the air in the chamber 18 is the same as that in the oven and standard psychrometric and steam tables (refer to Figure 2) then permit the humidity properties in the baking chamber to be calculated from measurable dry bulb temperature and pressure in the chamber.

The humidity sensor of the present invention could be connected to a control means for not only monitoring baking but also controlling baking in various types of oven including tunnel ovens. Humidity readings can be fed into a computer or micro-processor and used to control the baking time, oven temperature, air flow, extractor fans, and other oven control means and variables in baking.

For example when baking biscuits, the onset of gas (water vapour) production, indicates when the biscuit being baked has reached about 1000 C, and steam addition can safely be begun if needed for product character.

Cessation of water vapour production is an indication that the biscuits have reached the point that only bound water is being removed, which is the approximate "done" point for low moisture snacks and crackers. Thus knlowledge of the humidity in the oven can be used to determine what stage various baking operations have reached and when various products are baked.

The humidity probe of the present invention was tested in comparison 15 with the zirconium oxide cell and the results of the test follow and are fee.

illustrated in Figures 3 and 4.

Initial testing was completed using an indirectly heated type oven. In this experiment a zirconium oxide cell and the open end of the inlet 12 of the capacitive type of humidity probe embodying the present invention were placed in the same position in the oven. This was done to determine the accuracy of the humidity probe at higher temperatures. The oven used was an indirect fired oven so it was not necessary to correct the zirconium oxide cell for the presence of combustion gases. In Figure 3 it can be seen that the humidity measurement from the zirconium oxide cell and the reading from 25 the capacitive humidity probe were virtually identical.

Since the capacitive type of humidity probe measures the level of moisture directly it is unaffected by the combustion gases. Therefore the unit will work equally well in direct fired ovens as in indirect fired ovens.

Figure 4 illustrates the results of a similar test that was carried in the oven. A zirconium oxide cell was placed in the oven, along with the inlet 12 to the humidity probe system of the present invention, for a period of one hour and readings were taken at 60 second intervals. It can be seen that the readings taken from the humidity probe were almost identical to those taken from the zirconium oxide cell. Upon further investigation of Figure 4 there appears to be slight time lag of about 60 seconds for the zirconium oxide cell.

Figure 5 shows the humidity in a typical travelling tray bread baking oven. The object of this experiment was to determine whether the humidity probe could be used to measure and control the humidity in a travelling tray oven. The humidity was measured in the front of the oven by the stationary humidity probe. A fresh air damper was used to control the humidity. This damper controls the amount of air that is exhausted and the amount of fresh air that enters the oven. Initially the damper was set to position 0, ie. no fresh air. The humidity in the oven went to 16%. The damper was then put into position 2 which is about 50% open. The humidity dropped to about 10.5%. The damper was then opened to position 3 where it was 75% open and the humidity dropped to about In position 4 the damper was fully open and the humidity dropped to about 4%.

This experiment shows that with the aid of the humidity probe, it was possible to measure and control the humidity in the oven more efficiently.

This experiment was carried out on a soft bread roll type product. For this product it is important to note that it is desirable that the humidity be kept low. In the next experiment the humidity measurements were carried out on crusty rolls, since it is desirable to have a high humidity in the first region of the oven for this product.

Figure 6 shows the humidity in a tunnel oven. The oven was initially empty and the steam turned on. The oven was separated into four zones and steam injected into the first of these zones through a perforated tube. Since the zones were physically separated the humidity in the first zone rose to about 100% when there was no product in the oven. As the product entered 25 the oven the humidity dropped. This was because condensation of moisture occurs on the cool product. When there was a short break in the production and the oven partially emptied the humidity again began to rise. As the product entered the oven the humidity dropped. At the end of production the steam injection was switched off and the humidity quickly fell.

From the humidity experiments carried out in commercial ovens it has become obvious is that the cool product coming into the oven actually "pulls" some of the moisture out of the oven atmosphere. This is because the product surface temperature is below the oven dew point temperature so condensation will occur. The amount of condensation that will occur on the product surface is directly proportional to the oven humidity. From the standard psychrometric chart (Figure 2) showing the relationship of air 9 moisture and temperature it can be seen that some condensation will occur for oven humidity greater than The next fact that becomes obvious is that the bread will lose weight at a surface temperature of 100 0 C or greater. In between these two states it is uncertain exactly what happens. What is obvious is that in the first stage of the baking process condensation will occur on the bread surface, depending on the humidity in the oven. From then on there is a vapour liquid equilibrium where the surface of the bread will lose/gain moisture depending on the temperature of the oven, the temperature at the product surface, and the humidity in the oven.

Oven humidity has an effect on the properties and quality of the product. For soft rolls it is commonly accepted that the oven humidity must be low and in crusty rolls the humidity must be high.

It will be appreciated by persons skilled in the art that numerous r••o variations and/or modifications may be made to the invention as shown in 15 the specific embodiments without departing from the spirit or scope of the oooo invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

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Claims

1. A method for measuring the humidity in a remote, typically heated, chamber such as an oven or the like including the steps of:- withdrawing air from the heated chamber; (ii) conditioning the withdrawn air by adjusting its temperature and transferring the conditioned air to a second chamber; (iii) employing a sensor located in the second chamber to directly measure the humidity of the conditioned air and using the measurement of the humidity and temperature of the conditioned air to determine the absolute humidity of the conditioned air.

2. A method for measuring the humidity in a remote chamber as claimed in claim 1 wherein the step of adjusting the temperature of the air withdrawn from the heated chamber includes the step of drawing the air through a heat sink. 15 3. A method for measuring the humidity in a remote chamber as claimed ooo• in claim 2 wherein the temperature to which the air is adjusted is a predetermined constant temperature, or within a predetermined range,

4. A method for measuring the humidity in a remote chamber as claimed in claim 2 wherein the temperature to which the air is adjusted is about 100 0 C,

5. A method of regulating and controlling the conditions in baking ovens, including tunnel ovens by controlling oven temperature, or airflow through the oven, or extractor fans, baking times or the like based on and using measurements of humidity determined by a method as claimed in any one of 25 claims 1 to 4.

6. An apparatus for measuring the humidity in a remote, typically heated, chamber, such as an oven, comprising:- means for drawing air from the heated chamber; means for conditioning the air withdrawn from the chamber by adjusting its temperature; and a humidity measuring means for measuring an indication of the humidity of the conditioned air.

7. An apparatus as claimed in claim 6 wherein the means for conditioning the air is a heat sink.

8. An apparatus as claimed in claim 6 or claim 7 including a heat sink made from copper or a copper alloy having a central bore through which air can be drawn and a chamber located at one end in which the humidity measuring means is mounted.

9. An apparatus as claimed in claim 7 or 8 wherein the heat sink incorporates a temperature sensor, such as a thermocouple and an associated temperature controller. An apparatus as claimed in claim 9 wherein the controller is arranged to maintain the heat sink at a substantially constant temperature so that the humidity measuring means is always exposed to air at a constant temperature.

11. An apparatus as claimed in claim 10 wherein the preferred temperature is 1000 C.

12. An apparatus as claimed in any one of claims 6 to 11 further including :a control means for converting the humidity reading to give an absolute humidity value, to determine the humidity properties in the heated chamber by using the measured dry bulb temperature in the heated chamber and readings of the chamber pressure. S"13. A method of regulating and controlling the conditions in baking ovens comprising the steps of:- monitoring the humidity in the oven by: withdrawing air from the heated chamber; conditioning the withdrawn air by adjusting its temperature and transferring the conditioned air to a second chamber; (iii) employing a sensor located in the second chamber to directly measure the humidity of the conditioned air and using the measurement of 25 the humidity and temperature of the conditioned air to determine the absolute humidity of the conditioned air. recording, storing, or otherwise monitoring the changes in humidity in the oven; controlling one or more of baking time, oven temperature, airflow, extractor fans or other oven control means in response to the monitored humidity readings.

14. A method for measuring the humidity in a remote chamber as claimed in claim 13 wherein the step of adjusting the temperature of the air withdrawn from the heated chamber includes the step of drawing the air through a heat sink. 12 A method for measuring the humidity in a remote chamber as claimed in claim 14 wherein the temperature to which the air is adjusted is a predetermined constant temperature, or within a predetermined range,

16. A method for measuring the humidity in a remote chamber as claimed in claim 15 wherein the temperature to which the air is adjusted is about 100 0 C, Dated this twentieth day of May 1999 BRI AUSTRALIA LIMITED Patent Attorneys for the Applicant: SF B RICE CO oo* a a •o