EN - How an aspirator box works

    

Question

There is an aspiratox box installed.

How does the aspirator box work?

 Answer 

The greenhouse temperature can be measured by an aspirator box.

Inside the aspirator box there is a PT500 sensor. This sensor is made of Platinum (PT) and has a resistance of 500 ohms at 0 °C.

Every degree increasement gives 2 ohm more resistance.

So at 20 °C the resistance will be 540 ohms.

The above picture  shows an "aspirator box" of 50 years ago.

The left thermometer measures the greenhouse temperature. The bulb at the bottom of the thermometer is dry. That's why we call this the dry bulb temperature.

The thermometer at the right is used to determine the relative humidity (RH).

The bulb at the bottom of the sensor is covered with a wick. The wick is hanging in a reservoir with deminerilised water. So the bulb is wet. That's why we call this the wet bulb temperature.

The water is evaporating on the bulb. That makes the wet bulb colder than the dry bulb. The dryer the greenhouse air, the colder the wet bulb.

The grower has to look to the table to find the RH which belongs to the measured temperature and temperature difference.

 

Nowadays the aspirator box with dry and wet bulb is also working like the principle described above.

Simulation old aspirator box

You can simulate the old aspirator box with a PT500 sensor

 

1.    Connect a multimeter to the PT500 sensor

2.    What is the resistance? …………………….Ω

3.    Calculate the temperature  …………………….°C

4.    Slide the (wet) wick over the PT500 sensor

5.    What is the resistance now? …………………….Ω

6.    Calculate the temperature …………………….°C

7.    Read the relative humidity in the table below …………………….%

 

	Temperature difference wet bulb
Dry bulb	0	0,5	1	1,5	2	2,5	3	3,5	4	4,5	5	5,5	6	6,5
16	100	95	90	85	81	76	71	67	63	58	53	50	46	42
17	100	95	90	85	81	77	72	68	64	60	55	51	47	43
18	100	95	90	86	82	77	73	69	65	61	56	53	49	45
19	100	95	91	86	82	78	74	70	66	61	57	54	50	46
20	100	96	91	87	83	78	74	71	66	62	58	55	51	47
21	100	96	91	87	83	79	75	71	67	63	60	56	53	49
22	100	96	92	87	83	80	76	72	68	64	61	58	54	50
23	100	96	92	87	84	80	76	72	69	66	62	59	55	52
24	100	96	92	88	84	80	77	73	70	67	63	60	56	53
25	100	96	92	88	84	80	77	74	70	67	63	61	57	54

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