Thermocouple Psychrometry

Is there a direct way to measure ψS

No

Solutes depress the amount of

water

vapour above a solution.

Pure water (no solute):

relative humidity

100%

Dissolved solute

depresses the relative

humidity

 RH < 100%

This method involves measuring the amount of water vapour above a solution, via determining the

dew point depression of the air.

The magnitude of dew point depression in the air space above the solution is related to the

[solute] in the solution

Pure H2O(l) does not have a

dew point depression

Add solute to pure water →

dew point depression

We care about how much H2O(g) is in the air above

the solution, compared to about much H2O(g) is in

the air when the liquid is

pure water

Warm air can hold

more water vapour (H2O(g)) than cold air

The dew point

 temperature at which H2O(g) starts to condense

if you lower the temperature by even a fraction of a degree →

start seeing

condensation

Consider 25°C and RH = 100%. The dew point is

25°C

Raoult’s Law

the partial pressure of H2O(g) above a solution (PH2O(g)) is a function of

the mole fraction of solute (Xi) in the solution.

PH20 equation

equation fo Xi

 If we have pure water, then:

If there is dissolved solute, then:

Greater amounts of

dissolved solute leads to

lower PH2O(g) and a

lower dew point.

the dew point reflects the PH2O(g) at

the original temperature

the PH20(g) reflects

the [solute], or more specifically ψS, of the solution

Thermocouple

Two metal wires of different composition (different metals, different alloys) are

joined at each end (at the “junctions”).

 Seebeck Effect If the junctions are at different temperatures (ΔT), there will be a

voltage

Seebeck Effect The larger the ΔT, the larger the 

voltage

Seebeck Effect The reference junction has a known

temperature

Seebeck Effect The measuring junction is thus a

thermometer

Seebeck Effect Lots of digital thermometers are based on

thermocouples

The original thermocouple thermometers put the reference junction in an

ice bath (0°C).

Modern thermocouple thermometers use an

electronic reference junction.

Peltiers effect In this case the thermocouple loop is not

passive, as in measuring temperature.

Peltier cooling

If you pass a current through the loop, one junction becomes warmer and the other

junction becomes cooler

Seebeck effect used as a _ and Peltier effect used as a _

thermometer, cooling device

cannot use both effects at the same time but they can be used

sequentially

what do we need to measure PH2O(g)/dew point? 2

• A sample in a closed space with a small air volume.

• A thermocouple that can be used for both cooling (Peltier effect) and as a thermometer (Seebeck effect).

Procedure 4

1) Use the Peltier effect to cool the junction.

2) Sufficient cooling will lead to water condensation on the junction.

3) Turn off the cooling, and use the Seebeck effect to measure junction temperature as

it warms up.

4) Junction passively warms up. At the dew point water (H2O(l)) starts to evaporate;

→ plateau in the warming trend, due to enthalpy (latent heat) of vaporization (energy

is required to convert H2O(l) to H2O(g))

procedure of thermocouple psychrometry is the same mechanism for

how evaporation of sweat from the body

leads to cooling

what is actually happening inside the psychrometer

Old school thermocouple psychrometers generated the entire trace in

hard copy, and

users would then generate the data manually.

New school:

things are a lot more automated 

Quiz - For the two examples below:

a) what are the relative values of PH2O(g)?

b) what are the relative dew points?

c) what can you say about the relative humidity in each of the chambers?

a) Pure water has the higher \(P_{H_2O(g)}\).

b) Pure water has the higher dew point.

c) Pure water has 100% relative humidity, while the water + solute chamber has a relative humidity below 100%.

The Höfler diagram shows the relationships between

the various components of ψ

 think about that plant tissue in an environment that has a very high [solute]. And the [solute] keeps getting higher: what happens to ψ, ψS and ψP?

• What happens to cell size?

a) psis becomes more negative.

b)psip decreases because the cell loses water and turgor pressure drops.

c) psi decreases overall because:

d) Cell size decreases because water moves out of the cell by osmosis.

think about what happens if the external [solute] keeps getting lower? All the way to zero [solute] (= pure H2O).

• What happens to ψ, ψS and ψP?

• What happens to cell size?

a) psis becomes less negative as the external solute concentration decreases.

b) psip increases because water enters the cell and turgor pressure rises.

c) psi increases overall and moves closer to 0.

d) Cell size increases as water moves into the cell.