T. K. Davies and H. Hodapp
University of Victoria Victoria, B.C., Canada
I
I I
Thermal, Flow, and Coolant Level Switch
There arises in every laboratory the need for a control dcvicc to turn off an apparatus or initiate corrective measures when an overheating condition exists. Typically these are for reflux and uv photolyzing condensors, vacuum diffusion pumps, and liquid lev& such as liquid nitrogen. There are a number of controllers that,have been described in the literature to do with these functions, however, each is specialized for one type of cont~ol and some are difficult to set-up and move from one apparatus to another. Described below is a switch which can be used for most coolant flow or level and over-heating problems. Thc heart of the device is a Schmidt Trigger circuit
which is activated by a voltage derived from a thermistor potential divider. If the temperature of the thermistor probe is greater than that set on the control dial, the circuit will trip off any connected load of up
Pods not specified in Figure 2 Rl
T
Figure 1.
A, Liquid nitrogen level probe; 8, flow pmbe.
K1 T1 H1 SKI
3.3K 1 ' 1 W (41 135K @ 25-C Thermistor (Allied Electronics 791-0714) or 500K @ 25'C (for LN1) Thermistor (Allied Electronics 791 -0801 1 2500 ohm15.8 mA RELAY (Allied Electronics 886-1071) Triad F94X Transformer (Allied Electronics 954-0794) 15 W in. heating codridge. (Hatwan Co. 6932) Cord edge connector Amphenol 225.21 41 1-401
'Ir
Volume 49, Number 6, June 1972
/
443
t t
I B Figure 3.
Prit*led circuit board:
A, foil ride; 8, component siide.
to 250 W until manually reset in the case of over temperature or until the normal temperature of the probe is re-established by a remedial measure initiated by the controller. The type of control required determines which probe is to be used. For over-heating a simple thermistor
444
/
Journal o f Chemical Education
probe is used. For flow and level control see the probe illustrations in Figure 1, ( A ) liquid nitrogen level probe and (B) flow probe. The sensors for liquid and flow are essentially probes which respond to changes in heating capacity of the environment in which they are placed. If the "heat sinking" ability of the liquid around or flowing through the sensor decreases, the device trips. The advantages are obvious from the control standpoint for one switch will do many jobs. It is particularly easy to construct electronically and mechanically (Fig. 2). The probes can be fabricated from commonly available materials, and electronics is simplified by the use of a 3l/8 X 31/rin. printed circuit board. (For layout and scale positive from which the board can be reproduced, see Fig. 3). The details of construction of the liquid nitrogen probe are few aside from that shown in Figure 1. The resistors which make up R l are four '/%-W resistors mounted with epoxy glue, two either side of the thermistor disc, and are connected in series. The flow probe is constructed of common copper tubing sizes, however, the '/&n. tube must be enlarged with a number 44 drill. The three tubes are soft soldered in the configuration shown. The connection to the controller is made with a 5 pin socket (SI(2) giving access to the necessary connections for the various probes, that is, (a) the thermistor for the temperature probe, ( b ) the thermistor and resistor for level control and (c) the thermistor and heater for flow control. For liquid nitrogen connections gas valves and/or heaters for the refilling of the apparatus must be added. Two switches are used; one to reset the controller when it is used as protector unit, and the other to over-ride the controller for manual liquid nitrogen handling. This device has proven its worth in many applications and seems to be limited in its application by it,suser ingenuity.
