CN101561406B - Calorimetric method and instrument for measuring activity of water - Google Patents

Abstract

The invention relates to a calorimetric method and an instrument for measuring activity of water of the solid or liquid material. A sample detection pool, an empty detection pool and a pure water detection pool of twinborn structures are arranged in a constant-temperature environment, a carrier gas with constant pressure and flux is used for passing the pure water detection pool, the empty detection pool and the sample detection pool uniformly and simultaneously, the water wherein is evaporated at a continuously stable rate; therefore, a compensation heat flow exists among the sample detectionpool, the empty detection pool and the pure water detection pool and the external constant-temperature environment, and the heat flow size needed to be compensated is directly proportional to the par tial pressure of the steam of the sample. By respectively measuring the calorimetric signals of the compensation heat flows of the sample detection pool, the empty detection pool and the pure water detection pool and subsequently comparing and computing the sample detection pool and the pure water detection pool with the calorimetric signals of the empty detection pool, the value of the water activity of the material is obtained. The method can consider the measurement precision and consumption time simultaneously and the water activity measurement instrument adopting the method has compact structure and simple and convenient operation.

Description

Measure the calorimetric method and the instrument of water activity

Technical field

The present invention relates to a kind of calorimetric method and instrument of measuring water activity, accurately and rapidly the water activity of measure solid or liquid material.

Background technology

Moisture is the key component of materials such as food, medicine, aquatic products and solution, and the state of its existence directly affects the biochemical process of material self and the breeding state of microorganism on every side.Water activity indicated water in the material state and with the combination degree of other components, be the important indicator of stability and security.Seeking a kind of convenience, water activity measuring method reliably, accurately and efficiently, is an urgent demand of relevant industries development, and important practical sense is arranged.

Under low pressure, cryogenic conditions, water activity can obtain through measuring the gas vapor partial pressure that balances each other with sample.Also can obtain through the relative humidity of measuring the gas that balances each other with sample, promptly formula (Aw=Pw/Pw*=%RH/100) is a theoretical foundation of measuring water activity.The measuring method great majority of existing water activity all are that this principle is derived and come, and promptly carry out through direct or indirect measurement partial pressure of water vapor or relative humidity.

Document (Zheng Yihua etc. the measurement of water activity in the food. modern instrument; 2006; 12 (2): 11-14) analyzed the significance of food water activity, provided and contained comprehensive measuring method, enumerated instrument commonly used; Pointed out measuring method, the instrument problem to be solved of existing water activity: 1. the equilibration time of sample and surrounding air is long, causes Measuring Time long (several hrs).Though the instrument for quick measuring device can be accomplished in a few minutes, all be cost with the sacrifice in measurement accuracy.2. the accuracy for guaranteeing to measure needs often surveying instrument to be proofreaied and correct.3. all there is the hysteresis (sensor etc.) of each link in the practical application of the whole bag of tricks and disturbs (temperature variation etc.), causes the error of actual measurement.4. when food contains more fugitive constituent or some component, can cause bigger measuring error.And can destroy sensor.5. the whole bag of tricks all has certain limitation, can have bigger difference in different measurement ranges and applicable situation result.

Patent of invention (Da Shikui. water activity detecting method and instrument, ZL 95113366.7) a kind of assay method and instrument of water activity having been proposed, except need carry out the measurement of material surface temperature and gas flow temperature, also to regulate airflow humidity.Because the insertion type that is measured as of material surface temperature and gas flow temperature is measured, meeting interference temperature field influences precision.And to the adjusting of airflow humidity, poor operability, wayward, be subject to external interference.

Summary of the invention

The purpose of this invention is to provide a kind of measuring accuracy and time consumption taken into account, reliable, as to be used to measure water activity calorimetric method and instrument.The principle of work of the method for the invention is as shown in Figure 1; Its technical scheme is such: the carrier gas of using level pressure, constant flow is evenly through sample detection pond 1, empty detection cell 2 and pure water detection cell 3; Keep moisture in wherein sample, the pure water with continual and steady speed evaporation, sky detection cell 2 is set, can omit the standard model correction link as reference; Effectively reduce error, eliminate background interference and noise.Because sample detection pond 1, empty detection cell 2 and pure water detection cell 3 are placed in the isoperibol; Then there is the compensation hot-fluid between they and isoperibol; Need the hot-fluid size of compensation to be proportional to the speed (enthalpy that water vapour is taken out of) that water evaporates; And the speed of water evaporation is related with the partial pressure of water vapor of material, thus gas velocity, water vapour take the partial pressure of water vapor that enthalpy, vapor partial pressure power and heat flow signal have reflected sample out of.Under the steady state (SS), the carrier gas air-flow is approximate saturated fully, and then compensating hot-fluid can be described as

$\frac{\mathrm{dq}}{\mathrm{dt}}=\mathrm{\ΔH}\·J_g\frac{P_i}{P_g}---\left(1\right)$

Wherein:

-compensation hot-fluid; Δ H-evaporation of water enthalpy; J _g-carrier gas molar flow; P _gThe partial pressure of-carrier gas; P _i-partial pressure of water vapor

Imposing a condition down, the partial pressure of evaporation of water enthalpy, carrier gas molar flow and carrier gas can be thought steady state value.Compensate hot-fluid so and be proportional to partial pressure of water vapor

$\frac{\mathrm{dq}}{\mathrm{dt}}\∝P_i---\left(2\right)$

When the sample detection pond, $\frac{\mathrm{Dq}}{\mathrm{Dt}}=V_1$ P _i=P _y

When empty detection cell, $\frac{\mathrm{Dq}}{\mathrm{Dt}}=V_2$ P _i=P _k=0;

When the pure water detection cell, $\frac{\mathrm{Dq}}{\mathrm{Dt}}=V_3$ P _i=P _c=P ^*

In sample detection pond 1, empty detection cell 2 and 3 of pure water detection cells be provided with thermal sensor 4 to obtain the calorimetric signal difference of 2 of sample detection pond 1 and pure water detection cell 3 and empty detection cells; And through PC the calorimetric signal is calculated and handles, obtain the value of water activity according to the convert formula (3) that obtains of the definition of water activity.

$A_w=\frac{P_w}{P_w^{*}}=\frac{P_i}{P^{*}}=\frac{P_y-P_k}{P_c-P_k}=\frac{V_1-V_2}{V_3-{\mathrm{<figure-callout\; id="2"\; label="V"\; filenames="H2009102035574E00013.png"\; state="\{\{state\}\}">V</figure-callout>}}_2}=\frac{\mathrm{\&Delta;}{V}_1}{\mathrm{\&Delta;}{V}_2}---\left(3\right)$

The invention has the beneficial effects as follows, a kind of dynamic calorimetric method of measuring water activity is provided, can effectively solve and evade contradiction and the defective between efficient and precision, its need not sample proofread and correct, anti-interference, the time is short, precision is high.In addition, the calorimetric measurement method is a kind of general measuring method, and it can wait through temperature, the temperature difference, hot-fluid and reflect, its measuring principle is simple, adaptability strong, be easy to through engineering approaches and miniaturization.

Description of drawings

Fig. 1 measures the calorimetric method principle schematic of water activity

Fig. 2 measures the process flow diagram of the calorimetric method embodiment of water activity

The structural representation of the calorimetric module that Fig. 3 is corresponding with Fig. 2

Embodiment

As shown in Figure 2, control valve 12, filtrator 13 is removed impurity, flowmeter 14 indication flows, carrier gas 11 is introduced by the level pressure constant flow under temperature constant state.The inlet thermal modules 16 then, and its structure is as shown in Figure 3, and it is made up of sample detection pond 1, empty detection cell 2, pure water detection cell 3, thermal sensor 4 and a constant temperature calorimetric piece 5.Three detection cells adopt twin structure, and structural parameters are in full accord, wherein, place sample in the sample detection pond 1, and empty detection cell 2 is vacant, place the long-pending pure water of constant volume in the pure water detection cell 3.Sample detection pond 1 and pure water detection cell 3 all with empty detection cell 2 as reference.Through adopting twin structure and reference method, can effectively eliminate interference, and need not to use standard model to proofread and correct.The outside of sample detection pond 1, empty detection cell 2 and pure water detection cell 3 contacts with a thermometric end face of thermal sensor 4 and is surrounded by thermal sensor 4, and thermal sensor 4 is semiconductor refrigerating/heater element (TEC1-12704 that series and parallel uses; 127 pairs of Bi/Te thermopairs are formed), its electric signal mutual superposition, so sensitivity is very high, can detect very faint thermal change.The signal of its output is input in the computing machine 19 through amplifier 17 and A/D converter 18 and handles, and obtains the value of material moisture activity.Another thermometric end face of thermal sensor 4 contacts with constant temperature calorimetric piece 5 and by its encirclement; Constant temperature calorimetric piece 5 is processed by quality very big (thermal capacitance is very big) and thermal conductivity good metal; And mass and size is much larger than the mass and size of sample; Behind its temperature stabilization, its ambient temperature and inner sample variation of temperature all can be to not causing interference, and its constant temperature stability is higher.Constant temperature calorimetric piece 5 to guarantee signal stabilization, improves precision by thermostat 15 constant temperature of outside.

Claims (7)

1. calorimetric method of measuring water activity; It is characterized in that; Sample detection pond (1), empty detection cell (2) and pure water detection cell (3) adopt the consistent twin structure of parameter; And be in the same isoperibol; The carrier gas of using level pressure, constant flow is simultaneously evenly through sample detection pond (1), empty detection cell (2) and pure water detection cell (3); The moisture of keeping wherein evaporates with continual and steady speed, then has the compensation hot-fluid between sample detection pond (1), empty detection cell (2) and pure water detection cell (3) and outside isoperibol, and the hot-fluid size that need compensate is proportional to the partial pressure of water vapor of sample; Measure the calorimetric signal of reflection sample detection pond (1), empty detection cell (2) and pure water detection cell (3) compensation hot-fluid then respectively through thermal sensor (4), the value of water activity is the calorimetric signal difference of the calorimetric signal difference of sample detection pond (1) and empty detection cell (2) divided by pure water detection cell (3) and empty detection cell (2).

2. calorimetric instrument of measuring water activity; Be made up of carrier gas (11), variable valve (12), filtrator (13), flowmeter (14), thermostat (15), calorimetric module (16), amplifier (17), A/D converter (18) and computing machine (19), it is characterized in that, carrier gas (11) is introduced calorimetric module (16) by level pressure, constant flow under temperature constant state; Calorimetric module (16) is made up of sample detection pond (1), empty detection cell (2), pure water detection cell (3), thermal sensor (4) and a constant temperature calorimetric piece (5); Calorimetric module (16) places thermostat (15), and sample detection pond (1), empty detection cell (2) adopt the consistent twin structure of parameter with pure water detection cell (3), and are in the same isoperibol; Wherein, Place sample in the sample detection pond (1), empty detection cell (2) is vacant, places the long-pending pure water of constant volume in the pure water detection cell (3); Empty detection cell (2) is placed between sample detection pond (1) and pure water detection cell (3); Sample detection pond (1), empty detection cell (2) and pure water detection cell (3) contact with a thermometric end face of thermal sensor (4); And surrounded by thermal sensor (4); Another thermometric end face of thermal sensor (4) contacts with constant temperature calorimetric piece (5) and is surrounded by constant temperature calorimetric piece (5); Constant temperature calorimetric piece (5) by quality very big and heat conductivility good metal process; Its mass and size is much larger than the mass and size of sample, and constant temperature calorimetric piece (5) is by said thermostat (15) constant temperature of outside, and the output signal of thermal sensor (4) is input to computing machine (19) through amplifier (17) and A/D converter (18) and handles.

3. the calorimetric instrument of measurement water activity according to claim 2 is characterized in that, said carrier gas (11) is an inert gas.

4. the calorimetric instrument of measurement water activity according to claim 3 is characterized in that, said inert gas is nitrogen or helium.

5. the calorimetric instrument of measurement water activity according to claim 2 is characterized in that, said thermal sensor (4) is the sheet thermoelectric pile with two thermometric end faces, is realized by single or a plurality of semiconductor refrigeratings/heater element series and parallel.

6. the calorimetric instrument of measurement water activity according to claim 2 is characterized in that, said thermostat (15) is the thermostat of semiconductor refrigerating/heated constant temperature mode or the thermostat of constant temperature water bath mode.

7. the calorimetric instrument of measurement water activity according to claim 2 is characterized in that, said amplifier (17), A/D converter (18) and computing machine (19) can be substituted integrated realization data processing function by single-chip microcomputer.

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