Hoffer Flow Controls has recently supplied two flowmeters for use in the snowmaking process at Whitetail Ski Resort located in Mercersburg, Pennsylvania.
One of the flowmeters is a 2" gas turbine meter that will be used to monitor air during the snowmaking process. The other meter is a 1½" *STAR* flowmeter that will be used on water flows.
Whitetail Ski Resort is located in southern Pennsylvania almost in the center of the state. Mike ad Brian are "Snow Technicians" responsible for managing the snowmaking process at Whitetail. Mike advises that the "window of weather opportunity" during which they can make snow is often very short due to their location. Therefore, when the temperature drops below freezing, they must move quickly and efficiently to make as much snow as possible. They have 280 snow guns that may be moved around the 17 ski trails. They draw from a 100,000,000 gallon water reservoir to make the snow. When the conditions are right for snowmaking,Mike reports they are drawing approximately 5400 gallons per minute.
The Hoffer 1½" *STAR* flowmeter along with a Model 46 totalizer/rate indicator is used to monitor the water from the impoundment to the various individual snow guns. The Hoffer 2" gas flow-meter is used to monitor and control the air to the snowguns. The water to air ratio is 280 snowguns line the slopes critical to make "good" snow. If this ratio is off, the snow can be too heavy or wet. With the information obtained from the Hoffer meters, Brian and Mike can be sure that the snow guns are operating at peak efficiency and in the proper ratios.
The Model 46 units are battery powered by an internal lithium battery to provide approximately 3 to 5 years of field life. Power, therefore, is not required by the Hoffer Flow systems which makes installation easier and more cost effective. The Model 46 units are directly mounted to the flowmeters and located outdoors, therefore, a wide operating temperature range for both the electronics and the flowmeters was a must. The operating temperature range of the Hoffer flowmeters and Model 46 totalizer easily met this challenge.
Hoffer is pleased to provide accurate flow measurement systems to help meet the criteria of another challenging application. If your winter recreation plans include a little snow skiing in central Pennsylvania, you may want to try Whitetail. If the weather is right for snowmaking, you can be assured that the snowmaking capabilities of Mike and Brian, along with accurate flow measurement provided by the Hoffer systems will provide you with some of the finest man made ski conditions along the East Coast. For all you ski buffs, Happy Skiing! For those of you with challenging flow applications, call us and let us help.
Friday, February 5, 2010
Wednesday, January 20, 2010
NANMAC NOTES : THE STEM EFFECT
All contact type of temperature sensors such as thermocouples, RTD's, thermistors, bi-metallic thermometers, etc. are subject to an error we call the "STEM EFFECT' (SE). When a probe is immersed into a gas or a liquid environment, a new thermal conductive path is created by the probe's stem. Thus, heat is conducted away from the sensing tip via the probe's stem to the outer atmosphere. This results in the sensing tip reading a temperature which is lower than the surrounding gas or liquid.
Several years ago, NANMAC conducted an experiment to determine the magnitude of the errors produced by the "Stem Effect". The experiment consisted of a container of hot water surrounded by a reservoir of cold water. The hot water was kept hot by an immersion type heater and the cold water was kept cold by a simple circulating pump and cooling radiator. A gradient of about 80 deg. F was thus maintained over extended time periods.
The temperatures in the hot and cold reservoirs were continuously monitored by laboratory thermometers with an accuracy of + .5 deg. F. The carbon steel hot water tank had a dimension of 5" O.D. with a wall of 1/2" thickness.
Four thermocouples, each with a different style of thermal junction,were installed in the wall of the hot water tank and aligned flush with the inner surface of the tank. All thermocouples were in direct contact with the hot water. All units had iron/constantan elements and the outputs were monitored by the same pyrometer through a thermocouple selector switch. In addition, all thermocouples were made from the same lot of wire and resistances were matched.
Thermocouples T-1, T-2 and T-3 are conventional type thermocouples, and T-4 is the "Right Angle" unit with ribbon elements. The conventional style thermal junctions produced errors of from 10% to 66% in the observed temperature changes while the "Right Angle" thermocouple showed no measurable errors due to conduction,
The Patented "Right Angle" thermocouple features a thermal junction at right angles to the longitudinal axis of the probe. The thermal elements consist of ribbons electrically welded together at the sensing tip. These ribbon elements are brought out to the opposing sides of the probes along the interface of a rod and cone assembly made out of high temperature insulation. The ribbons in the vicinity of the thermal junction are parallel to the plane of heat flow. Thus both the thermal junction and the ribbons are heated simultaneously and conduction errors are minimized. The distance from the junction to the corners of the ribbon elements divided by the junction thickness produces a ratio of 20 to 1. This ratio is the reason why the ribbon thermocouples have an insignificant error caused by the "Stem Effect". Ribbon elements also have response times in the low milliseconds to transients.
For more information on the NANMAC "Right Angle" thermocouples contact TALON Technical Sales.
Tuesday, January 19, 2010
Discussion of Error Band as it Relates to Pressure Sensors
A Whitepaper from PMC
Pressure sensor manufacturers commonly group the errors associated with pressure measurement into two groups, specifically Static Error Band and Total Error Band. While certain industries and application areas may occasionally list other errors, the two above are by far the most common and can be found in marketing literature from almost any sensor Manufacturer. Sensor errors are invariably referred to as %FS (percent of Full Scale).
In the simplest terms, Total Error Band (TEB) is the maximum deviation from “perfect” performance under any combination of environments to which the sensor may be subjected. Therefore, as long as the sensor is operating within the parameters defined by the manufacturer’s specification, the user should always be able to rely on the sensor output being accurate within the TEB limits.
For most modern sensor technologies, the largest contributor to TEB is thermal error. Thermal error may be as much as 90-95% of the total error, particularly when the sensor is expected to operate over a wide temperature range.
By far the most prominent error term referenced in marketing literature is Static Error Band. Most sensor companies define Static Error Band as the combined effects of nonlinearity, hysteresis, and repeatability errors. However, this definition can be (and is) misleading at times, specifically because it does not include two important but often ignored error sources, specifically the zero setting and span setting errors.
Users often assume that readings from pressure sensors at room temperature will always be accurate within the static error band limits. When zero set and span set are not included however, large errors often occur. For example, some manufacturers list 0.25% static error band, but separately list 0.5% zero set error and 0.5% span set error. In this example, the user may rely on a reading being within 0.25% of the true value, but in reality the reading could be in error by as much as 1.25%.
Standard practice at STS-Sensors is to include these setting errors in the Static Error Band definition. Under the STS definition, where Static Error includes the combined effects of non-linearity, hysteresis, repeatability, zero-set and span-set errors, the user can be assured that readings taken at room temperature will indeed be within the stated 0.25%.
Because the setting errors are generally large relative to the non-linearity, hysteresis, and repeatability errors, an STS sensor specified as a 0.25% device is usually equivalent to a 0.1% device from other manufacturers, and an STS 0.1% device is about equivalent to 0.04% from others.
The following definitions further explain these errors, all of which are expressed in Percent of Full Scale (%FS):
Hysteresis – The difference in sensor output at any given test pressure, depending on whether the test pressure was achieved by increasing or decreasing pressure. A perfect sensor with zero hysteresis would retrace the same line as the pressure is increased or decreased. In reality, hysteresis error causes the output signal to form a loop as pressure is increased and decreased.
Linearity (non-linearity) – The deviation of the electrical output from a perfect straight line as the pressure is changed from zero to full scale, or from full scale to zero. Always expressed as %FS, but several methods are used, including BSL (Best Fit Straight Line), TSL (Terminal Straight Line), or forced zero BSL. BSL is by far the most common, and FZ-BSL is rare. In the BSL method, a Best fit Straight Line derived by a linear regression analysis is used as the reference line for error measurement. In the TSL method, the reference line is draw between the two endpoints of the output curve.
Repeatability – The difference between consecutive readings taken at exactly the same pressure, when the test pressure is attained from the same direction.
Span – The change in the output of the sensor when the pressure is increased from zero to full scale
Zero-setting error – Sometimes called zero balance, this is defined as the deviation of the output from the correct value, when zero pressure is applied.
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