The PTC 310F & 313F Surface Thermometers use a unilateral Thermal Sensor to measure the temperature of the Contact Surface. These Instruments are Fully Enclosed to protect the Bimetal from dirt to dust. The Temperature Range is 0-300F +/- 2% & 5-500F. 310.7 Optional Spring Holder The PTC 314F Dual Magnet Mount Surface Thermometers are designed to be used on Horizontal or Non-Horizontal Ferrous Surfaces. The Range is 50-750F +/- 2%.