Bi-Metal Thermostat FAQs
We get lots of questions on the temperature choices of bi-metal thermostats. I’ll try to answer a few of those here:
How accurate are the bi-metal thermostats?
± 5° Tolerance
Senasys has two general specifications on the thermostat temperatures. One style uses a plus or minus some tolerance, typically 3-7ºF per temperature. So, if we say the device opens at 95ºF it will open between 90 and 100ºF (with the most common +/- 5º setting).
Our other style is called a min/max and for those units we just specify the outer temperatures with a 10 or 15 degree hysteresis. For example as 125-115 min/max open on rise switch will open by 125 and close by 115. Generally these would open around 122 to 125 and reset by 118 to 115. However, they may change at 122 and 120 giving a small hysteresis as well.
Custom tolerances are easily available to help manage your thermal systems. Also, for all of our units these temps are accurate to 0.5ºF degrees and are calibrated to NIST standards.
What happens to the system if the temperature is off by a few degrees?
If failure to get the temperature right means the system will be damaged severely (frozen pipes) add some gap between the danger zone and the thermostat setting. The thermodynamics of systems can change with air flow, humidity and other factors so from a simple design standpoint it may never get to the danger zone but in the field odd combinations of conditions might cause problems.
Why doesn’t the temperature I select work the way I want it to?
Sometimes the temperature sensor is placed in a central location but there may be hot or cold spots in the system that aren’t the same temperature. In these cases make sure to add a couple degrees to your specs to account for that thermal difference. If you are protecting pipes from freezing there may be an elbow or other feature that gets extra cold compared to the rest so the thermostat may see 35 degrees but down the pipe a bit is a cold spot that will still freeze before the thermostat is triggered.
For accurate measurement metal to metal contact over a large surface area is generally best. Air to metal contact is good if there is some airflow around the thermostat, the more airflow the better. If the thermostat is measuring air that is not moving there will be more lag between when the air gets to your target temperature and when the thermostat gets to that temperature. If you have a system element that is changing the air temp that you are then measuring there will be a lag and a gap in temp between the system element and the thermostat.
How small can the hysteresis be?
For these types of switches 10ºF is about the smallest range they can maintain cost effectively and consistently.
For complex systems actual testing is usually pretty important. We support this process by making it easy to test multiple package styles and disc temps in sequence or all at once just give us a call.
Senasys designs and manufactures snap disc thermostat switches. Senasys switches are UL and CUL Recognized. The majority of our switches are made in the United States and our direct customer service would be happy to help you find the right switch for your application. Senasys snap disc thermostats are single pole, single throw, fixed thermostat which may be used in applications such as industrial and HVAC equipment.
To view our standard temperature options visit our online store.
For more information about our bi-metal thermostats, contact our direct technical support at 715-831-6353.