Technical Guide

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Level Switches

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Level Switches

These devices equip electrodes to detect liquid levels. They have been widely used in water works and sewers for buildings and housing complexes, industrial facilities and equipment, water treatment plants and sewage treatment facilities, and many other applications.

Overview of Level Switches

Floatless Level Controllers (61F) are electronic liquid level detectors used in a wide range of applications such as water and sewer services for office and apartment buildings, industrial applications for iron and steel, food, chemical, pharmaceutical, and semiconductor industries, and liquid level control for agricultural water, water treatment plants, and wastewater plants. When the electrodes are in contact with liquid, the circuit is closed (the liquid completes the path for electricity to flow) and the electrical current that flows in this circuit is used to detect the level of the liquid. A variety of conductive liquids can be controlled using this method. Detecting the resistance between the electrodes and comparing it to see if it is larger or smaller than a reference resistance is used to detect the surface of the liquid.
 

Operating Principle

The operating principle is explained using a case where water is supplied from the water mains.

Office and apartment buildings normally have a ground tank and an elevated tank. Water is supplied from the water mains into the ground tank, pumped up to the elevated tank, then distributed to each floor.

When the water level in the elevated tank is low, water is pumped up from the ground tank to supplement it. When the water level reaches a certain level, the pump stops. (See figure 1.)

Elevated tanks are controlled in this manner to maintain the water level within upper and lower limits as shown below.

Figure 1. Water Supply Control

Pump Control According to Water Level (Two-pole Method)

1. When electrode E₁ is not in contact with the conductive liquid as shown in figure 2, the electrical circuit is open, and no current flows between electrodes E₁ and E₃. Consequently relay X does not operate and the contact remains at the b side.

2. When electrode E₁ is in contact with the conductive liquid as shown in figure 3, the circuit closes due to the conductive fluid completing the circuit between E₁ and E₃. Relay X operates and switches to the a side.
   By connecting the relay contacts to a contactor, the pump can be turned ON and OFF.
   However in practice, with only two electrodes, ripples on the surface of the liquid cause the relay to switch rapidly. This problem can be solved by forming a self-holding circuit. (The configuration shown in figures 2 and 3 can be used as water level alarms.)

Figure 2. Low Water Level

Figure 3. High Water Level

Liquid Level Control with Self-Holding Circuit (Three-pole Method)

An extra electrode E₂ is added, and E₁ and E₂ are connected via contact a2 as shown in figure 4. When electrode E₁ is in contact with the conductive liquid (as in point 2 of previous section), relay X operates and switches to the a side. Even if the liquid level falls below E₁, the electrical circuit made through the liquid and the electrodes is retained by E₂ and E₃, as long as contact a2 is closed.

This kind of circuit made from electrode E₂ and a contact is called a self-holding circuit.

When the liquid level falls below E₂, the circuit made through the electrode circuit opens, which de-energizes relay X, thus closing the NC contact of X. This enables control of relay X to be switched ON and OFF between E₁ and E₂.

Figure 5 shows the timing chart of this mechanism.

Operating as simply as it does, possible applications of the Floatless Level Controller other than liquid level control include applications as leakage detection, and object size discrimination.

Figure 4. Self-holding Circuit

Figure 5. Timing Chart

Note: Non-conductive liquids, such as oil, cannot be controlled using this method.

Level Controller Selection Criteria

Categories (Reference Information)

Categorized by Fluid Types

Categorized by Installation Conditions of Electrodes

Selection Criteria for 61F Level Controllers

Specific Resistance and Model Selection Criteria

The limit for specific resistance of liquid that can be controlled with a generic Level Controller is 30 kΩcm when using a PS-3S Electrode Holder within a submersion depth of 30 mm. For any fluid with specific resistance higher than this value, use a high-sensitivity Level Controller (H type). (See note.)

Table 1 and Table 2 shown upper and Table 3 below show specific resistances for typical liquids. Use these when selecting a model.

Note:
1. The high-sensitivity Level Controllers may suffer from resetting problems when used with certain types of water. In some cases it cannot substitute for the standard Level Controllers or Low-sensitivity Level Controllers. Be sure to select the model appropriate for the application.

2. The circuit configuration of the High-sensitivity 61F-[ ]H Level Controller is designed so that the relay is reset when there is water present between the electrodes. When power supply voltage is applied, the internal relay switches to the NO contact and, when there is conductivity between electrodes E1 and E3, the relay is reset to the NC contact.

This contact operation is reversed for models other than the high-sensitivity models. Although the internal relay operates (and operation indicator turns ON) simply when the power supply voltage is applied, this operation is normal. (The relay in the 61F-[ ]NH energizes when there is water present between the electrodes.)

Note: For the ultra high-sensitivity variable 61F-HSL Level Switch, malfunction due to electric corrosion may occur in the DC electrode circuit. Be careful not to use the product where current constantly flows between electrodes.

Table 1: Specific Resistance of Water (General Guideline)
 

Type of water	Specific Conductance
Tap water	5 to 10 kΩ · cm
Well water	2 to 5 kΩ · cm
River water	5 to 15 kΩ · cm
Rainwater	15 to 25 kΩ · cm
Seawater	0.03 kΩ · cm
Sewage	0.5 to 2 kΩ · cm
Distilled water	250 to 300 kΩ · cm min.

Table 2: Detectable Specific Resistance (Guideline)

 

Type of water	Specific resistance (recommended value)
Long distance (4 km)	5 kΩ · cm max.
Long distance (2 km)	10 kΩ · cm max.
Low sensitivity	10 kΩ · cm max.
Two-wire	10 kΩ · cm max.
General-purpose	10 to 30 kΩ · cm
High-temperature	10 to 30 kΩ · cm
High-sensitivity (COMPACT plug-in type)	30 to 200 kΩ · cm
High-sensitivity (base type)	30 to 300 kΩ · cm
Ultra high-sensitivity	100 kΩ to 10 MΩ · cm

Note: The specific resistance of liquids are those that can be controlled using the PS-3S when the submersion depth is 30 mm or less.

Conductance

Conductance is a scale describing how easily current can flow. The relationship of Conductance and resistance is defined by the following equation.

Table 1 can be modified to contain the corresponding conductance as shown in Table 1A.

Table 1A: Specific Conductance of Water (Guideline)

Type of water	Specific Conductance
Tap water	100 to 200 μS/cm
Well water	200 to 500 μS/cm
River water	67 to 200 μS/cm
Rainwater	40 to 67 μS/cm
Seawater	33,300 μS/cm
Sewage	500 to 2,000 μS/cm
Distilled water	3.3 to 4 μS/cm max.

Table 3: Specific Resistance of Various Liquids

 

Type of liquid	Temperature (°C)	Concentration (%)	Specific resistance (Ω· cm)
Beer (Company A) Port wine (Company K) Whisky (Company T) Barium hydroxide Ba (OH)2	12 12 12 12	--- --- --- ---	830.0 966.0 14,608.0 1,743.0
Silver nitrate AgNO3	18	5.0 60.0	39.5 4.8
Barium hydroxide Ba (OH)2	18	1.25 2.5	40.0 20.9
Calcium chloride CaCl2	18	5.0 20.0 35.0	15.6 5.8 7.3
Cadmium chloride CdCl2	18	1.0 20.0 50.0	181.0 33.5 73.0
Cadmium sulfate CdSO4	18	1.0 5.0 35.0	240.0 68.5 23.8
Nitric acid HNO3	18 15 15	5.0 31.0 62.0	3.9 1.3 2.0
Phosphoric acid H3PO4	15	10.0 60.0 87.0	17.7 5.5 14.1
Sulphuric acid H2SO4	18	5.0 30.0 97.0 99.4	4.8 1.4 12.5 117.6
Potassium bromide KBr	15	5.0 36.0	14.5 2.9
Potassium chloride KCI	18	5.0 21.0	14.5 3.6
Potassium chlorate KClO3	15	5	27.2
Potassium cyanide KCN	15	3.25 6.5	19.0 9.8
Potassium carbonate K2CO3	15	5.0 30.0 50.0	17.8 4.5 6.8
Potassium fluoride KF	18	5.0 40.0	15.3 4.0
Potassium iodide KI	18	5.0 55.0	31.4 2.4
Potassium nitrate KNO3	18	5.0 22.0	22.1 6.2
Potassium hydroxide KOH	15	4.2 33.6 42.0	6.8 1.9 2.4
Potassium monosulfide K2S	18	3.18 29.97 47.26	11.8 2.2 3.9
Copper sulfate CuSO4	18	2.5 17.5	92.6 21.8
Ferrous sulfate FeSO4	18	0.5 3.0	65.0 21.7
Hydrogen bromide HBr	15	5.0 15.0	5.2 2.0
Hydrochloric acid HCl	15	5.0 20.0 40.0	2.5 1.3 1.9
Hydrogen fluoride HF	18	0.004 0.015 0.242 298.0	4,000.0 2,000.0 275.0 2.9
Mercuric chloride HgCl2	18	0.229 5.08	22,727.0 2,375.0
Hydrogen iodide HI	15	5	7.5
Potassium sulfate K2SO4	18	5.0 10.0	21.8 11.6
Sodium chloride NaCl	18	5.0 25.0	14.9 5.6
Sodium carbonate Na2CO3	18	5.0 15.0	22.2 12.0
Sodium iodide NaI	18	22.2 12.0	33.6 4.7
Sodium nitrate NaNO3	18	5.0 30.0	22.9 6.2
Sodium hydroxide NaOH	15	2.5 20.0 42.0	9.2 2.9 8.4
Sodium sulfate Na2SO4	18	5.0 15.0	24.4 11.3
Ammonia NH3	15	0.1 4.01 3.05	3,984.0 913.0 5,181.0
Ammonium chloride NH4Cl	18	5.0 25.0	50.5 2.5
Ammonium nitrate NH4NO3	15	5.0 50.0	16.9 2.7
Ammonium sulfate (NH4)2SO4	15	5.0 31.0	18.1 4.3
Zinc chloride ZnCl2	15	2.5 30.0 60.0	36.2 10.8 27.1
Zinc sulfate ZNSO4	18	5.0 30.0	52.4 22.5

Selecting Electrode Material According to Resistance against Corrosion

To get the most out of the electrodes, refer to Table 4 to select the best material.

Table 4: Resistance to Corrosion of Electrode Material

 

Aqueous Solution			Electrodematerial
Type	Concentration (%)	Temperature (°C)	SUS 304	SUS 316	Titanium	HAS B	HAS C
SulphurousacidH2SO3	6	30	E	C	A	B	B
Sulphuric acid H2SO4	1	30	A	A	A	A	A
	1	BP	E	D	E	B	C
	3	30	B	A	A	A	A
	3	BP	E	E	E	C	C
	5	30	D	B	D	B	A
	5	BP	E	E	E	D	D
	10	30	E	C	E	A	A
	10	BP	E	E	D	C	E
	20	30	E	E	C	C	B
	20	BP	E	E	D	D	E
	40	30	E	E	D	B	B
	40	BP	E	E	D	E	E
	60	30	E	E	D	B	C
	60	BP	E	E	D	C	D
	70	30	E	E	D	B	B
	70	BP	E	E	D	C	D
	80	30	E	E	D	B	B
	80	BP	E	E	D	D	D
	90	30	E	E	D	B	B
	90	BP	E	E	D	D	D
	95	30	E	D	D	B	B
	95	BP	E	E	D	D	D
Hydrochloric acid HCl	1	30	E	D	B	B	A
	1	BP	E	E	E	D	C
	3	30	E	E	B	B	A
	3	BP	E	E	E	D	C
	5	30	E	E	C	C	A
	5	BP	E	E	E	E	D
	10	30	E	E	E	C	C
	10	BP	E	E	E	E	E
	15	30	E	E	E	C	C
	15	BP	E	E	E	E	E
	20	30	E	E	E	C	D
	20	BP	E	E	E	E	E
	37	30	E	E	E	C	E
	37	BP	E	E	E	E	E
Chromium oxide CrO3	10	BP	D	C	A	B	C
	20	30	C	B	A	B	B
	36.5	90	E	E	C	C	C
Nitric acid HNO3	10	30	B	A	A	D	A
	10	BP	B	B	B	D	C
	20	290	B	B	C	D	D
	65	175	C	C	B	E	E
	68	30	C	C	A	D	D
	68	BP	D	D	B	E	E
	90	80	E	E	A	E	E
Hydrogenfluoride HF	5	30	E	E	D	D	C
	100	30	E	D	C	C	C
PhosphoricacidH3PO4	10to85	RT	B	B	C	B	C
AceticacidCH3COOH	5to50	RT	A	A	A	A	A
	100	RT	A	A	A	A	A
	100	BP	C	B	A	A	A
Formicacid H·COOH	All	BP	D	D	D	A	A
Acetone CH3·CO·CH3	All	RT	B	B	A	A	A
Alum	All	RT	E	E	D	B	B
Aluminum sulfate	50	BP	D	C	B	C	A
Ammonium chloride NH4Cl	5	BP	D	D	A	B	B
Ammonium nitrate NH4NO3	All	BP	A	A	A	B	B
Ammonium sulfate (NH4)2SO4	5	RT	E	D	B	B	C
	10	BP	E	E	B	B	C
Ammonia NH3	100	100	C	C	A	B	B
	10	BP	C	B	B	B	C
	28	60	C	B	A	B	B
Potassiumhydroxide KOH	25	BP	B	A	C	B	C
Sodiumhydroxide NaOH	30	60	A	A	B	A	B
	50	65	B	A	C	A	C
Sodium carbonate Na2CO3	25	BP	B	B	B	B	B
Potassium carbonate K2CO3	20	BP	B	B	B	B	B
Zinc chloride ZnCl2	50	150	D	C	B	B	C
Calcium chlorideCaCl2	25	BP	C	C	A	A	A
Sodium chloride NaCl	25	BP	C	B	A	B	B
Ferric chloride	30	RT	E	E	A	E	B
Copper chloride	30	RT	E	E	A	E	B
Sea water		RT	C	C	A	B	A
Hydrogenperoxide H2O2	10	RT	B	B	B	B	B
Sodium sulfite	10	RT	B	B	A	B	B
Citric acid	All	RT	B	A	C	A	A
Oxalicacid CO2H·CO2H	All	RT	B	A	D	B	B
Sodium hypochlorite	10	RT	E	D	A	C	C
Potassium dichromate	10	BP	C	B	A	B	C
Magnesium chloride	30	RT	C	B	A	A	A
Magnesium sulfate	10	RT	B	B	A	A	A

Note:

1. RT: Room temperature; BP: Boiling point

2.

A: Adequate resistance to corrosion
B: Resistive to corrosion, erosion rate is less than 0.8 mm/year
C: Low resistance to corrosion, erosion rate is less than 1.8 mm/year
D: Highly corrosive, not usable
E: No resistance to corrosion, not usable

3. The table above is used for reference when selecting the electrodes. Even if the material has adequate corrosion resistance, it doesn't mean that it is not subject to corrosion.
Check regularly once a month to see if corrosion is occurring. If it is, replace the electrodes.

Reference

When selecting an Electrode Holder, make sure that you consider the corrosion resistance of the material of electrode holders as it may be exposed to the liquid inside the water tank.
 

Level Switch Glossary »

Recommended Products

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