Advantages of Thermal Conductivity Water Flow Sensors

over Plastic Spinning Water Flow Sensors

Xiang Zheng Tu

 

Reference to the above figure, a thermal conductivity water flow sensor is made up of two thermopiles, which is used as the sensing temperature difference element and operated in conjunction with a resistive heater element for thermoelectric sensing. The fabrication of such sensors is more complicated since less conventional materials are utilized for fabrication of thermopiles but CMOS (complementary metal oxide semiconductor) compatible processing is possible. The Seebeck effect of thermopiles enables higher sensitivity and unbiased output voltages with no offset or drift.

The thermopiles are constructed with thermocouples in series and so the output voltages due to temperature difference change is summed and increased over that of a single thermocouple. Since the thermal conduction between hot and cold junctions of the thermopiles and Johnson noise increases with increasing number of thermocouples, a high thermal isolation structure is desired in order to maximize temperature difference between hot and cold junctions.

The water mass flow (m) passing through the thermal conductivity sensor is calculated on the basis of the measured temperature difference (T_hot - T_cold) between the hot and cold junctions of the thermopile, and the thermal conductivity (C_p) coefficient (k), electric heat rate (q), and specific heat (C_p) of water, as follows:

m = kq/(C_p(T_hot – T_cold)                                    (1)

The electromotive force, or emf (V) created by the thermopile is directly proportional to the differential temperature (T_hot - T_cold) between the two junctions as

Emf_AB = nS_AB (T_hot - T_cold)                                (2)

Where n is the number of thermocouples of a thermopile and S (V/K) is called the Seebeck coefficient.

Still reference to the above figure, a plastic spinning water flow sensor has a rotor, a bearing, and a shaft, which are mounted in housing. The rotor spins as water passes over it. The measured flow rate is proportional to the rotational speed of the rotor. A variety of methods are used to detect the rotor speed, including a mechanical shaft and an electronic sensor.

Plastic bearings must be lubricated, not only to reduce friction and wear, but, in the case of plain bearings, to prevent them from seizing the shaft which they support. Self-lubricating plastic bearings contain a mix of dry lubricants. In operation, movement between shaft and bearing causes microscopic abrasion of the dry lubricant, filling and smoothing the shaft surface to reduce friction. The resulted micron particles will enter the water flow which is harmful to human health.

Most plastic bearing materials expand when exposed to heat and moisture. This factor is more significant when the running clearance between the bearing and shaft is less than 0.001 in. Plastic bearings and shafts are fabricated by injection molding process which has typical accuracy within 0.005 in. As a result, excessive wear or seizing of the shaft occurs very often.

Bubbles inevitably form as air is entrained in the water during the pouring process. The formed bubbles can create many problems in plastic spinning water flow sensors, such as:

● decreasing lubricity caused by an air emulsion,
● reduction of thermal conductivity,
● higher noise emission, and
● decrease water output efficiency.

Compare with plastic spinning water flow sensors, the thermal conductivity water flow sensors have the advantages as:

1.     Thermal conductivity water flow sensors have no moving parts, in which there are no any mechanical failures to take place.

2.     Thermal conductivity water flow sensors are MEMS devices with small size, higher sensitivity, higher reliability, low power consumption, ease of fabrication, and low cost.

3.     Thermal conductivity water flow sensors calculate mass flow rather than volumetric flow and do not require temperature or pressure correction, which means there is no additional expense for the purchase and installation of additional equipment.

4.     Thermal conductivity water flow sensors provide excellent accuracy and repeatability over a wide range of flow rates using bypass flow tube design. The sensor is placed in a bypass around a restriction in the main pipe and is sized to operate in the laminar flow region over its full operating range.

It should be emphasized that the thermal conductivity water flow sensors are not influenced by the air bubbles entrained in the water. The effect of the bubbles can be added to the series conductivity by using conductivity of the air-water mixture for the water conductivity. The thermal conductivity of continuous water phase with entrapped air bubbles can be calculated using Maxwell’s model as

k_m= k_c (k_d + 2k_c – 2p_d [ k_c –k_d])/(k_d + 2kc + p_d [k_c –k_d])            (3)

Where:

K_m = conductivity of the mixture,

K_c, k_d = conductivity of continuous and disperse phases, respectively, and

P_d = volume fraction of the disperse phase.

Replacing equation (3) into equation (1), the water mass flow rate measured by the thermal conductivity water flow sensor should be

 m = {(k_d + 2k_c – 2p_d [ k_c –k_d])/(k_d + 2kc + p_d [k_c –k_d])}q/(C_p(T_hot – T_cold)     (4)                        

It can be seen that the measured water mass flow rate does not contain the air bubbles entrained in the water.