Counterflow: The Same Area Does More When the Streams Run Opposite

/*
  hx_logger.ino -- log the four temperatures of a tube-in-tube heat exchanger.

  Four DS18B20 waterproof probes on ONE data wire: hot in, hot out, cold in,
  cold out. Libraries (Library Manager): "OneWire" and "DallasTemperature".
  Wiring: all probes' red -> 5V, black -> GND, yellow (data) -> D2.
          Pull-up between D2 and 5V of about 4.7 k: two 10 k resistors in
          parallel give 5 k, which works on short leads.
  First run prints each probe's address: label the probes in that order.
*/
#include <OneWire.h>
#include <DallasTemperature.h>

const int PIN_BUS = 2;
OneWire bus(PIN_BUS);
DallasTemperature sensors(&bus);
DeviceAddress addr[4];
const char* NAMES[4] = {"hot_in", "hot_out", "cold_in", "cold_out"};
int found = 0;

void printAddress(DeviceAddress a) {
  for (int i = 0; i < 8; i++) {
    if (a[i] < 16) Serial.print('0');
    Serial.print(a[i], HEX);
  }
}

void setup() {
  Serial.begin(9600);
  sensors.begin();
  found = sensors.getDeviceCount();
  if (found > 4) found = 4;
  for (int i = 0; i < found; i++) {
    sensors.getAddress(addr[i], i);
    sensors.setResolution(addr[i], 12);
    Serial.print(F("# probe ")); Serial.print(i); Serial.print(F(" = "));
    printAddress(addr[i]); Serial.print(F(" -> ")); Serial.println(NAMES[i]);
  }
  Serial.println(F("seconds,hot_in,hot_out,cold_in,cold_out"));
}

void loop() {
  sensors.requestTemperatures();          // ~750 ms at 12-bit resolution
  Serial.print(millis() / 1000);
  for (int i = 0; i < found; i++) {
    Serial.print(',');
    Serial.print(sensors.getTempC(addr[i]), 2);
  }
  Serial.println();
  delay(4000);
}

/*
  The heat each side carried, from the logged steady values:
     Q_hot  = m_hot  * 4186 * (hot_in  - hot_out)    watts, m in kg/s
     Q_cold = m_cold * 4186 * (cold_out - cold_in)
  They should agree within a few per cent. If Q_hot is much larger, heat is
  leaking out of the jacket to the room: insulate it.
*/