in a constant temperature liquid circulating apparatus , in order to determine parts and locations requiring maintenance work and maintenance time intervals before the occurrence of an irregular operation , so that any downtime in operation of a constant temperature liquid circulating apparatus is made as short as possible , a system is provided in which respective components of the apparatus are provided with sensors for use in monitoring various kinds of condition amounts in a continuous manner . especially , among such components , a three - way proportional valve is provided between the outlet of a heat exchanger and an inlet to a holding tank , in a cooling liquid circulating circuit which supplies a constant temperature cooling liquid to a load , and a sensor is provided which detects a degree of valve opening of the three - way proportional valve . a trend line of the three - way proportional valve opening is monitored continuously , and when the trend line shows signs of rising above a certain valve opening percentage , such a tend is taken as an indication that the cooling capacity of the apparatus may be decreasing , and preventative measures are undertaken to investigate the cause of such irregular operation .