The Lawler Model LB-76 liquid baths are programmed such that the sample cooling rate equals the cooling rate experienced by a sample when immersed in the constant low temperature air chamber. The bath can also be programmed to cool at a rate of 1°C per minute from ambient to as low as -55°C as per CEC-L-18A, ISO 9262, and IP 267 for the measurement of European low temperature Brookfield viscosity.
Operating temperature range of the programmable liquid baths is +20° to -55°C. Temperature control is within ±0.1°C. Windows based PC proprietary software is capable of controlling two baths simultaneously. This allows operating baths at two independent temperatures with independent start, stop, and duration times. The turntable holds 10 test samples. After the 16 hours conditioning time, the turntable is manually rotated until the proper test cell is under the viscometer. The spindle is connected while the sample is still in the liquid bath. This allows the measurement of viscosity without removing the sample from the temperature controlled environment. Far superior viscosity data repeatability is obtained using this procedure as opposed to the air chamber procedure.
Duplicate samples at -40°C were measured in a Model LB-76 Liquid Programmable Bath. Of the 56 samples, the average variability between the duplicate samples was ±1.13% of mean. This compares very favorably to the method’s stated precision of 11.8%. The key factor in this improved precision is the superior temperature control of the sample as its viscosity is being measured.