«TH 



»P 



R TH = THERMISTOR : RESISTANCE 91.4, 



AT 25°C (VECO TYPE 21A2) 

 «S =49.2* 

 Rp =5533* 



Z 



>- o 



< s 



"> o 



10 15 20 



TEMPERATURE CO 



Fig. 2. 



Thermistor compensation network and 

 graph of salinity correction as a func- 

 tion of temperature for water of salinity 

 of 35-3 parts per thousand. 



which was designed to have a resistance tempera- 

 ture relationship similar to sea water. 2,7, 13 

 In Fig. 2 a typical circuit and its compensation 

 curve are shown. The errors between 5°C and 

 20°C are usually quite small "but outside these 

 limits increase quite rapidly as shown. In some 

 cases, depending on the characteristics of indi- 

 vidual thermistors, errors can "be as low as 

 +0.05 parts per thousand in salinity from 0°C to 

 25°C. However, thermistors have a number of 

 serious disadvantages . They are non-uniform in 

 characteristics and are sometimes unstable. The 

 compensation accuracy is generally inadequate as 

 shown in Fig. 2. 



Compensating Cell 



A theoretically ideal method of temperature 

 compensation using sea water itself as the com- 

 pensating element was experimented with by the 

 author and others at the Woods Hole Oceanographic 

 Institution. This technique utilized Copenhagen 

 standard sea water in a small sealed platinum- 

 electrode glass conductivity cell. The cell was 

 fitted with a flexible membrane so that the 

 sealed sample of standard sea water in the cell 

 was at the same pressure as well as temperature 

 as measured sea water sample. The earlier 

 experimental units had a thermal response time 

 of 0.8 seconds. However, their long term sta- 

 bility is poor, at least in the cells made to 

 this date. They are also fragile and difficult 

 to fabricate. 



Platinum Resistance Thermometer Bridge Circuit 



A compensation circuit consisting of a double 

 bridge circuit incorporating two precision 

 platinum resistance thermometers has been studied. 

 Two variations of this scheme are shown in Fig. 3. 

 Even though the temperature coefficient of a 

 platinum resistance thermometer is only about 



RA1=RA2 = R B1=«B2 



Rfi=Rf2 

 Rti=Rt2 



R t , ti. R T2 ARE THE RESISTANCE 

 THERMOMETERS 



— 'I 



RF1 = «F2 



RT1 = R T2 



M =E 2 ='/jEi 



N, =N 2 



io = h-12 



R T1 & R T2 ARE THE PLATINUM 



RESISTANCE THERMOMETERS 



Fig. 3- Double Wheatstone bridge compensation 

 circuit (above) and double transformer 

 compensation circuit (below) . 



one-sixth that of sea water at 15°C and of 

 opposite sign, it can be shown that the circuits 

 shown in Fig. 3 can be made to have a tempera- 

 ture coefficient closely matching that of sea 

 water from to 25°C. 



An examination of a simple Wheatstone bridge 

 with a resistance thermometer in one arm will 

 show that the relative temperature coefficient 

 of the ratio of the output short circuit current 

 to input voltage, (l/l ) (dI /dT), becomes pro- 

 gressively larger as the bridge approaches 

 balance and reverses sign on the other side of 

 the balance point. However, an analysis of the 

 simple bridge shows that if the bridge is 

 adjusted to give accurate compensation at 15°C, 

 the temperature coefficient of the bridge 

 rapidly deviates from that of sea water at higher 

 and lower temperatures. However, a detailed 

 analysis of the double transformer bridge cir- 

 cuits shown in Fig. 3 leads to the results shown 

 in Table I. For the various values of R s the 

 bridge resistors, R F1 and R F2 , were adjusted so 

 that the temperature coefficient of the bridge 

 exactly equalled that of sea water at 15°C. 

 Where R g is large the temperature coefficient of 

 the bridge circuit closely matches that of sea 

 water over a wide range. In Fig. k a plot is 

 shown of the salinity errors (for S = 35 parts 

 per thousand) with R s at a very large value. 

 The salinity errors are quite small for tempera- 

 tures below 22.5°C and due to the stable charac- 

 teristics of well designed platinum resistance 

 thermometers these errors are quite stable and 

 can be allowed for in the final analysis . 



PRESSURE COMPENSATION 



The effect of hydrostatic pressure on the 

 conductivity of sea water^-^ i s very considerable 

 as shown in Fig. 5. The figure also shows that 

 the relationship between pressure and conduc- 

 tivity is not linear and that at pressures as 



21 



