Heat meter measurement and detection

Measurement accuracy

The heat meter is divided into three levels of accuracy, namely: primary, secondary and tertiary. The first thing to note is that the accuracy level of the heat meter cannot be described by a fixed error number, such as 2% or 5%, etc., because even if the heat meter of the same accuracy level has different working conditions, the error requirement is also different.

1. Measurement accuracy of integral heat meter

Since the metering components of the integrated heat meter are logically inseparable, their accuracy must be given by the standard device at one time, and its error limits are given by the following formulas:

First class table: E = (indicating value - standard value) / standard value * 100%

Secondary table: E = (indicating value - standard value) / standard value * 100%

Third-level table: E=(indicator-standard value)/standard value*100%

Where: E - relative error limit, %

Δtmin - minimum temperature difference, °C.

Δt - the temperature difference within the range of use, °C.

Qp - common traffic, m3/h.

q - the flow within the range of use, m3/h.

2, the measurement accuracy of the split heat meter

The measurement accuracy of the split heat meter is determined by the three parts of the heat meter: the metering accuracy of the flow meter, the temperature sensor and the totalizer, and the error limit is the arithmetic sum of the errors of the above three components (that is, The sum of absolute values).

In the split heat meter, since the flowmeter accuracy is divided into three levels, the measurement accuracy of the split heat meter is also divided into three levels.

Detection method

In principle, the heat meter verification should be carried out as much as possible to simulate the actual working state. However, the actual state of the heat meter is determined by any combination of two parameters of flow rate and temperature difference, and it is difficult to simulate all actual states. Therefore, the following method is usually used for detection.

1. Overall verification method

The integral heat meter is preferably verified by the overall detection method. The specific method is to set a flow rate and temperature difference respectively by the standard verification device. The standard value of the heat is directly given by the standard device, and the heat value of the heat meter to be inspected is By comparing the standard values ​​of the standard devices, the error of the heat meter to be tested can be obtained. Only this type of verification method is a true detection of the heat meter. However, this method is extremely demanding for the verification device. At present, there is no such verification device in China.

2, split test

The split test method uses different devices to verify the three components of the heat meter, the flow meter, the temperature sensor and the totalizer respectively. After obtaining the error of the three parts, their arithmetic sum is considered to be the heat meter. The overall error, and no new errors are generated. The specific approach is:

Flow sensor verification: It only detects the performance of the flow meter in terms of flow metering. Its nature is like detecting a water meter. However, for the flow meter of the heat meter, it is also necessary to detect the metering characteristics of the hot water at different temperatures. The general practice is to comprehensively investigate the error of the flowmeter to be tested according to the rated flow rate Qn of the flowmeter to be tested, and set different flow points (flow rates) and different temperature conditions on the standard device. The flow point is set as follows:

Factory inspection is divided into three points: 1.1qmin, 0.1qp, qp

The type test is divided into six points: 1.1qmin, 0.1qp, 0.3 qp, 0.5 qp, qp, 0.9 qp,

The above flow points were measured once at room temperature, 55 +/- 5 ° C, and 85 +/- 5 ° C, respectively.

The resulting measurement results are calculated as follows:

E=(indicator-standard value)/standard value*100%

Among them, the standard device usually adopts three methods: volumetric method, weighing method and standard table method. The volumetric method is greatly affected by temperature changes and gasification of the medium, so it is rarely used. The current popular practice is to combine the weighing method with the standard table method, that is, to use a standard table to ensure the automation of the operation, and to use weighing to ensure accuracy.

Temperature sensor verification: If the temperature sensor and the totalizer of some integral meters are fixed together, the error of the temperature sensor and the error of the accumulator will be added together. Otherwise, the local temperature sensor is separately Verification. The method is to put the temperature sensor into the thermostat, and to check the error between the temperature and the standard temperature at different temperature points. It should be noted that for the temperature sensor not only a single test, but more importantly, the pairing error is also detected.

Verification of the totalizer: Since the design principles of the totalizer are different, it is best to use the corresponding verification method for their respective principles. Specifically, the temperature difference signal and the flow rate signal are input to the totalizer through the simulation device, and then the error between the calculation result and the theoretical result is examined.

3. About the first verification:

As a measuring instrument, the heat meter must be checked by the relevant national authorities for the first time before installation. The first verification is different from the production verification or type verification in the detection method, because the first verification of the heat meter is a pre-use verification as a commodity, and the verification method cannot affect or even damage the product itself, which means It cannot be verified by the method of split verification. This requires the heat meter to output high data accuracy under the use condition, which is not possible for the reed switch and the Hall principle heat meter, because their flow data can only be 1 liter minimum. That is to say, such a heat meter cannot be first verified, and thus its quality standard cannot be guaranteed.