Mass flow meter with chip-type sensors

#1. Field of the Invention

#This invention relates to mass flow meters, and more particularly to chip-type temperature sensors and four-sensor bridge circuits for mass flow meters.

#2. Description of the Related Art

#FIG. 1 is a simplified sectional view of a capillary tube MFM in accordance with the invention;

#FIG. 2 is a schematic diagram of a four-sensor bridge circuit that provides a MFM output in response to the sensors of FIG. 1;

#FIG. 3 is a graph illustrating the linear relationship of the temperature between the upstream and downstream sensors to the bridge output;

#A capillary tube MFM in accordance with one embodiment of the invention is illustrated in FIG. 1. A minor portion of a fluid flow, either gas or liquid, through a main conduit 2 is diverted to a capillary tube 4; the main conduit and capillary tube structure can be conventional. The cross-sections of the main conduit and capillary tube are precisely machined in a conventional manner to assure their fluid flow rates are equal. However, instead of the platinum wire windings previously provided around the capillary tube, a pair of chip-type temperature sensors U1 and U2 are provided at an upstream location along the tube, and another pair of chip-type sensors D1 and D2 at a downstream location along the tube. The sensors of each pair are preferably positioned symmetrically at 180° intervals, on opposite sides of the tube. This allows for a more accurate sensing of the fluid temperature within the tube, in case the fluid temperature varies slightly from one side of the tube to the other. For example, if the sensors were located along a horizontal portion of the capillary tube rather than a vertical portion as shown, rising heat would make the upper sensor detect a slightly higher temperature, and the lower sensor a slightly lower temperature. The symmetrical sensor placements tend to cancel out such discrepancies, and also to enable a more uniform introduction of heat into the conduit. The capillary tube wall has a high thermal conductivity, so that the fluid temperature within the tube is accurately transmitted to the sensors; stainless steel is commonly used for this purpose.

#FIG. 2 illustrates a four-sensor bridge circuit used to sense the mass flow rate of a fluid through the capillary tube of FIG. 1. The bridge circuit is organized into left and right branches, with each branch having upper and lower sections. The left branch includes a pair of the upstream and downstream sensors, with the upstream sensor U2 in its upper section and the downstream sensor D2 in its lower section. The right branch also includes a pair of up and downstream sensors, but their relative positions within the branch are reversed; the downstream sensor D1 is in the upper section, while the upstream sensor U1 is in the lower section.

#An actuating current from a current source I1 is fed into the top of the bridge, with current flowing out of the bridge through a resistor R to a ground reference. The bridge outputs are the voltages Vo1 and Vo2 at the connections between D1 and U1 in the right branch, and between U2 and D2 in the left branch, respectively.